Minesite Water Management Handbook

Minesite Water Management Handbook

1997

Copyright © 1997 Minerals Council of Australia Inquiries should be addressed to the publishers. Minerals Council of Australia PO Box 363, Dickson ACT 2602 Telephone: 61 262793600 Facsimile: 61 262793699 Email: [email protected] First Edition 1997 Minesite Water Management Handbook ISBN 0 909276 73 0 In 2008 the first edition was transcribed into electronic format, without consideration of the accuracy or currency of the content. Users should note that in some areas of the book, more recent publications (post 1997) provide updated technical information. Every effort has been made to contact the copyright holders of material used in this book. However, where an omission has occurred, the publisher will gladly include acknowledgment in any future editions. Disclaimer This Minesite Water Management Handbook (the Handbook) has been prepared by the Minerals Council of Australia in the interests of encouraging excellence in environmental management. However, the Minerals Council of Australia accepts no liability (including liability in negligence) and takes no responsibility for any loss or damage which a user of the Handbook or any third party may suffer or incur as a result of reliance on the Handbook and in particular for: (a) any errors or omissions in the Handbook; (b) any inaccuracy in the information and data on which the Handbook is based or which is contained in the Handbook; (c) any interpretations or opinions stated in, or which may be inferred from, the Handbook.

Contents 1. Introduction

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2. Statutory Requirements

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3. Planning and Principles

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3.1 INTRODUCTION

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3.2 THE HYDROLOGIC CYCLE AND MINESITE WATER BALANCE

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3.3 SITE DESCRIPTION

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3.3.1 Climate 3.3.2 Geology and Geomorphology 3.3.3 Topography 3.3.4 Catchment Characteristics 3.3.5 Site Water Requirements 3.3.6 Vegetation and Fauna Assessment 3.3.7 Aquatic Ecology 3.3.8 Heritage Values 3.3.9 Downstream and Offsite Users 3.3.10 Monitoring

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12 12 12 12 12 12 13 13 13 13

3.4 SITE PLAN

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3.5 MONITORING AND DATA MANAGEMENT

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Water Chemistry

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4.1 CHEMISTRY OF NATURAL WATERS

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4.1.1 Introduction 4.1.2 Dissolved Versus Particulate and Total Constituents 4.1.3 Difference Between Organic Acid and Carbonate Water Systems 4.1.4 Load Versus Concentration 4.1.5 pH 4.1.6 Alkalinity 4.1.7 Hardness 4.1.8 Conductivity 4.1.9 Salinity 4.1.10 Solids 4.1.11 Turbidity 4.1.12 Oxygen Demand (Dissolved Oxygen, BOD and COD)

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4.1.13 Anions and Cations 4.1.14 Metals (Trace Metals, Heavy Metals, Metal Speciation) 4.1.15 Nutrients 4.1.16 Oils, Greases and Hydrocarbons 4.1.17 Organics, Natural Organic Matter, Dissolved Organic Carbon 4.1.18 Colour 4.1.19 Cyanide 4.1.20 Odour and Taste 4.1.21 Radionuclides

24 25 25 26 26 27 27 28 29

4.2 BIOLOGICAL ASPECTS OF WATERS 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5

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Micro-organisms Algal Blooms Toxicity and Ecosystem Health Factors Influencing Bioavailability and Toxicity of Contaminants Bio-monitors, Bio-accumulation and Bio-amplification

30 31 31 32 32

4.3 NATURE OF WATERS

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4.3.1 Beneficial Use 4.3.2 Assimilative Capacity 4.3.3 Receiving Waters

33 33 33

5. Water Sampling and Flow Measurement

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5.1 INTRODUCTION

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5.2 PRINCIPLES AND PURPOSE OF MONITORING

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5.3 COMPLIANCE MONITORING

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5.3.1 Ambient, Point Source and Non-point Source pollution 5.3.2 Mixing Zones

36 36

5.4 DATA COLLECTION - QUALITY

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5.4.1 Monitoring Design 5.4.2 Identification of Key Monitoring Parameters 5.4.3 Initial Screening Program 5.4.4 Sampling Locations 5.4.5 Sampling Frequency 5.4.6 Sampling Techniques and Design 5.4.7 Sample Transportation 5.4.8 Sample Analysis 5.4.9 Data Management 5.4.10 Laboratory, Pilot Plant and Leach Tests

36 37 37 37 37 38 39 39 40 40

5.5 DATA COLLECTION - QUANTITY

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5.5.1 Rainfall Reading 5.5.2 Flow Recording

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5.6 GROUNDWATER 5.6.1 5.6.2 5.6.3 5.6.4

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Groundwater Mapping Testing and Monitoring Groundwater Parameters Prediction of Groundwater Characteristics and Responses

42 43 46 46

5.7 REVIEW OF MONITORING DATA

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6. Water Supply

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6.1 SURFACE WATER

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6.1.1 Catchment Yield 6.1.2 Recycling of Water

48 49

6.2 GROUNDWATER

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6.2.1 Sources of Supply 6.2.2 Security of Supply

49 49

7. Exploration

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7.1 SURFACE WATER

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7.1.1 Surface Water Data Collection 7.1.2 Access Tracks 7.1.3 Exploration Sites

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53 54 54

Open Cut Mines

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8.1 SURFACE WATER RUNOFF 8.1.1 8.1.2 8.1.3 8.1.4 8.1.5

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Flood Mitigation Methods of Flood Mitigation In-Pit Drainage Interception Drainage Around Pit Sediment Containment

56 57 59 60 61

8.2 GROUNDWATER

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8.2.1 Groundwater Inflow 8.2.2 Managing Groundwater Inflow

63 63

8.3 WATER QUALITY

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8.3.1 Pit Water Disposal 8.3.2 Acid Drainage 8.3.3 Salinity

65 65 66

8.4 PIT CLOSURE

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CONTENTS

9.

Underground Mines

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9.1 SURFACE DRAINAGE AWAY FROM HEAD WORKS

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9.2 GROUNDWATER INFLOW

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9.2.1 Managing Groundwater Inflow

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9.3 WATER QUALITY

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9.3.1 Treatment and Disposal of Underground Mine Water

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10. Heap Leach Processes

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10.1 INTRODUCTION

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10.2 PLANNING FOR HEAP LEACHING

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10.2.1 Baseline Evaluation 10.2.2 Rainfall Events, Acceptable Risk, Contingency Planning 10.2.3 Baseline Groundwater Monitoring 10.2.4 Closure Planning

70 70 71 71

10.3 SOLUTION CONTROL DURING OPERATIONS

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10.3.1 Maintenance of Drain and Pond Capacity 10.3.2 Integrity of the Pad or Liner 10.3.3 Integrity of Piping and Valves

72 72 72

10.4 WATER MANAGEMENT ON CLOSURE

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10.4.1 Criteria for Long-term Leachate Quality 10.4.2 Residues and Long-term Contaminated Site Management

72 72

11. Waste Dumps

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11.1 WASTE DUMP CONSTRUCTION FOR WATER MANAGEMENT

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11.2 SURFACE WATER

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11.2.1 Location of Waste Dumps 11.2.2 Erosion on Waste Dumps 11.2.3 Interception Drainage Around Waste Dumps 11.2.4 Sediment Containment Around Waste Dumps

73 73 74 75

11.3 GROUNDWATER

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11.3.1 Infiltration to Groundwater 11.3.2 Monitoring

75 76

11.4 WATER QUALITY

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11.4.1 Acid Drainage 11.4.2 Salinity 11.4.3 Suspended Solids 11.4.4 Leachate and Other Constituents 4

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CONTENTS

12. Tailings Water Management

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12.1 DISPOSAL METHODS

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12.2 CHARACTERISTICS AND MANAGEMENT OF TAILINGS WATER

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12.2.1 Nature of the Water 12.2.2 Management

80 80

12.3 SEEPAGE MANAGEMENT

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12.3.1 Seepage Control 12.3.2 Monitoring 12.3.3 Water Control

80 81 81

13. Mine Infrastructure

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13.1 PROCESS PLANT

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13.1.1 Characteristics 13.1.2 Containment and Treatment Technologies

82 82

13.2 INDUSTRIAL AND WORKSHOP AREAS

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13.2.1 Containment and Treatment Technologies

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13.3 HAUL ROADS

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13.3.1 Environmental Issues 13.3.2 Surface Water Drainage 13.3.3 Groundwater Drainage

84 84 84

References

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Glossary

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List of Tables Table 2.1: Typical State and Commonwealth Legislation

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Table 4.1: Typical Conductivity Range of Waters

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Table 5.1: Key Planning Steps for Water Monitoring

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Table 5.2: Selection Criteria for Establishing Sampling Sites

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Table 5.3: Advantages and Disadvantages of Using Numerical Models

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Table 6.1: Sources and Uses of Recycled Water

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Table 10.1: Suggested Minimum Design Event Criteria for Heap Leach Operations

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Table 11.1: Prevention and Remedial Strategies for Acid Drainage

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CONTENTS

List of Figures Figure 4.1:

Species of the Carbonate System as a Function of pH

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Figure 5.1:

Typical Groundwater Surface Map

42

Figure 5.2:

Relationship Between Piezometric Level and Groundwater

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Figure 5.3:

Typical Piezometer Installation

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Figure 5.4:

Diagram of a Piezometer Dip Meter

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Figure 8.1:

Calculating the Lowest Cost Flood Mitigation Scheme

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Figure 8.2:

Types of Constructed Embankments

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Figure 8.3:

Conceptual Drainage Around an Open Pit

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Figure 8.4:

Idealised Pit Inflow

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Figure 8.5:

Effects of Barriers to Groundwater Flow

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Figure 8.6:

Effects of Dewatering Around a Pit

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Figure 8.7:

Channel Dewatering

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Figure 8.8:

Water Flows in Open Voids

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Figure 11.1: The Soil Capillary Zone

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Figure 11.2:

Monitoring Network Around a Waste Rock Dump

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Figure 12.1:

Seepage Paths from a Tailings Storage Facility

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Figure 13.1:

Drainage Considerations on Haul Roads

85

Fact Sheets Fact Sheet No. 1:

Field Record Data Sheets

93

Fact Sheet No. 2:

Estimation of Surface Runoff

97

Fact Sheet No. 3:

Understanding Event Probability

101

Fact Sheet No. 4:

Open Channel Drains

103

Fact Sheet No. 5:

Construction of Small Earth Embankment Dams

105

Fact Sheet No. 6:

Culvert Crossings

110

Fact Sheet No. 7:

Acid Drainage

112

Fact Sheet No. 8:

Erosion Control and Sediment Containment

115

Fact Sheet No. 9:

Bioremediation Technology

121

Fact Sheet No. 10: Hydrological Data for Design Purposes

122

Fact Sheet No. 11: Groundwater

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Fact Sheet No. 12: Numerical Modelling

125

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Acknowledgments

The Minesite Water Management Handbook has undergone a considerable gestation period and many individuals have assisted in its production. It is with much appreciation that the Minerals Council of Australia acknowledges the contributions of these people, all experts in their individual fields, who gave freely of their time: Raj Aseervatham, Denis Brooks, Michael Cox, Geoff Day, Tom Farrell, Kurt Hammerschmid, Gavin Murray, Pamela Ruppin, Peter Roe, Ian Wood, and Ray Woods. The comments of many other individuals on earlier drafts were invaluable in efforts to treat such a broad range of material as fully and accurately as possible. The Minerals Council of Australia would also like to acknowledge the companies and organisations for whom the individuals work. All input has been most valuable.

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1. Introduction

which include both theoretical and practical topics relating to mine water management. The first five chapters provide an overview of the regulatory requirements, management planning and principles, basic water chemistry and the principles of sampling and flow measurement. Chapters 6 to l3 describe the major water-related technical issues relevant to all areas of a mining operation. They include generic guidelines for:

In the course of mining and mineral processing, landscapes are altered and soils, rock and water are subject to physical and chemical change. These changes must be managed to ensure that any resulting impacts are minimised, do not jeopardise future land and water uses, and do not breach any regulatory requirements. Failure to manage these impacts in an acceptable manner will result in the mining industry finding it increasingly difficult to obtain community and government support for existing and future projects. The Minesite Water Management Handbook provides practical guidance, based on scientific principles and leading industry practice, on how to investigate and manage surface and groundwater during exploration, mining and mineral processing. The information is sourced from industry, government(s) and research organisations, consultants and individuals actively participating in the minerals industry. This handbook has been prepared as a companion document to the AMIC (now the Minerals Council of Australia) Rehabilitation Handbook (AMIC 1990). The handbook has been developed for those who are not familiar with the fundamentals, processes and requirements (both technical and legislative) of water management for mining purposes, and for those site personnel with limited or no experience or training in water management from an environmental perspective. It also provides an indication of what the minerals industry sees as its prime objectives and directions with regard to water management. The handbook is divided into 13 main chapters

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• the design, construction and maintenance of site surface water drainage; • issues associated with erosion and sediment control; and • management and monitoring of surface and groundwater quality: Specific topics, for example acid drainage, are presented as fact sheets. Both theoretical and practical aspects of each issue are discussed. A glossary of terms is included and, finally, a reference list which is designed to direct the reader to a greater level of detail than is provided in this handbook.

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2. Statutory Requirements

Environmental management of mining and mineral processing requires consideration of both State and Commonwealth legislation, although most minerals industry operations are subject only to State environmental law.

Typical State and Commonwealth environmental legislation relevant to water management in the Australian minerals industry is shown in Table 2.1 This legislation is frequently supported by regulations which provide more detail on how the legislation is to be implemented and complied with. For example, regulations under a Clean Waters Act may contain limits for physical, chemical and biological parameters which cannot be exceeded in effluents.

Legislation relevant to water issues within the mining industry is passed by both State and Commonwealth governments. These laws are usually enforced by the relevant State Environmental Protection Authority or Department, State Department of Mines or the Commonwealth Department of the Environment.

TABLE 2.1: Typical State and Commonwealth Legislation

State Legislation

Commonwealth Legislation

• Mining Act • Environmental Protection Act

• Environment Protection (Sea Dumping) Act 1981

• Local Government Act

• Great Barrier Reef Marine Park Act 1975

• Clean Waters Act

• Petroleum (Submerged Lands) Act 1967

• Groundwater Act

• Protection of the Sea (Prevention of Pollution from Ships) Act 1983

• Pollution of Waters by Oil Act

• Seas and Submerged Lands Act 1973

• Environmental Protection/Marine (Sea Dumping) Act

• National Parks and Wildlife Conservation Act 1975

• Marine and Harbours Act • Petroleum (Submerged Lands) Act

• Environment Protection (Alligator Rivers Region) Act 1978

• Coastal Protection Act • Soil Conservation Act

• Environment Protection (Impact of Proposals) Act 1974

• Dangerous Goods Act • Radiation Control Act

• Industrial Chemicals (Notification and Assessment) Act 1989 • World Heritage Properties Conservation Act 1983

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S TAT U T O RY R E Q U I R E M E N T S

Most operations involving water, either supply or disposal, will be licensed under the relevant act. Licences are issued for a defined period, typically one year, and have conditions attached to them. These conditions may specify the monitoring which is required to ensure compliance, the limits which apply, and specific procedures which must be followed in order to reduce the environmental impact of the discharge. As a minimum, every operation should ensure that its facility fully complies with the relevant State and Commonwealth acts, laws, regulations and licences. Therefore, systems need to be established and maintained to track compliance with these statutory requirements and to report this compliance on a regular basis.

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3. Planning and Principles

3.1 Introduction

3.2 The Hydrologic Cycle and Minesite Water Balance

Water respects no boundaries, and drought and floods are events beyond our direct control. The industry’s role in water management is one of stewardship, not ownership, and therefore our operating philosophy should be based on the following concepts:

The hydrologic cycle is the primary model for the input and output water management elements in any site development. These elements include:

• efficient use of water; •

implementation of the reduce, re-use, recycle concepts;

•

avoid or minimise contamination of clean streams and catchments;

• recognise and protect downstream beneficial uses (for both surface and groundwaters); and •

rainfall; surface runoff; evaporation; groundwater flow; seepage; site and process water uses; site and process water outputs; offsite discharges; and on-site discharges.

Assigning values to the parameters of the hydrological cycle will identify the water surplus or deficit nature of the site. This process is referred to as the water balance.

on relinquishment of title, the quantity and quality of drainage from the site should not prejudice the productive use of the land.

Implementing these concepts requires considerable planning, based on a clear understanding of the project and the hydrological, geochemical and processing regimes in which it operates. This section sets out the principles, while subsequent sections will provide the tools to prepare a detailed water management plan for a site.

The minesite water balance is a central component in the minesite water management system. Through the water balance, we can gain a clearer understanding of the principal water management issues of supply, protection, containment and discharge. The principal data required for a water balance include: • •

determining the appropriate timestep for the flow detail being assessed (hourly, daily, monthly or yearly); and defining the inputs, demands and outputs.

The results obtained from the water balance present data that provide definable benefits in developing the components and systems for effective water management.

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P lanning and P rincip L E S

contour intervals are dependent on the level of investigation and the type of structures - the more advanced the project the closer the contour intervals and the greater the accuracy. Typical values are 0.5 to 1.0 m (+/- 0.25 to 0.5 m) intervals for detailed design and 2.5 to 5.0 m (+/- 1 to 2 m) intervals for preliminary investigations. More detailed survey data may be required in particular cases.

Various tools are available for the water balance including: spreadsheets for analysis; commercial software such as AWBM and RORB for rainfall/ runoff analysis; and customised software to suit the circumstances of a particular site.

3.3 Site Description

3.3.4 CATCHMENT CHARACTERISTICS

Basic information about a site is necessary so that a workable water management strategy or plan can be developed. Many of the components and processes in this description are required for other site assessment purposes. However, each topic should be considered in terms of the information needs required to address potential water management issues at the site. Not all topics will need to be researched intensively for every site.

A characterisation of the site for parameters relevant to the surface and groundwater hydrology is essential for the planning, design and operation of site water management systems. Storm and volume runoff coefficients, times of concentration for peak runoff, storage parameters, erosion potential, sedimentation characteristics and hydraulic coefficients such as Manning’s “n” are relevant for surface characterisation. Hydraulic conductivity and permeability, sub-surface water zones and aquifers and storage and yield characteristics are typically required for an understanding of the groundwater system. Monitoring systems are necessary to obtain site-specific data and to confirm calculations.

3.3.1 CLIMATE The essential climatic parameters are rainfall and evaporation. To a lesser extent, temperature, relative humidity, wind speed and direction and solar radiation are also required. Prior to resource development, daily records generally form the basis of data collection systems. Because longterm historical data are central to optimising water management studies and design, the earliest possible installation of real-time continuous data recording equipment is advised when a nearby weather station is not available. Once a project is undergoing detailed feasibility studies, climate monitoring systems which provide more frequent and specially targeted records may be required.

3.3.5 SITE WATER REQUIREMENTS It is important to understand what are the site water demands and how they may vary with time. A dynamic water balance is frequently a great asset in establishing and maintaining a water management program. Short-term benefits in reducing water use and cost should not jeopardise future opportunities for expansion of the operation. 3.3.6 VEGETATION AND FAUNA ASSESSMENT

3.3.2 GEOLOGY AND GEOMORPHOLOGY

The purpose of this assessment is to provide a clearer understanding of the catchment characteristics for rainfall runoff assessments, and to highlight sensitivities to the implementation of the various water management strategies.

The data compiled here will assist with an understanding of the groundwater and surface water movement characteristics and likely responses to mine induced changes in flow or water quality. 3.3.3 TOPOGRAPHY A site plan showing the geographic setting, contours and the land systems at the site is required. The 12

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P lanning and P rincip L E S

changes to the water budget may be accommodated. The planning process should consider:

3.3.7 AQUATIC ECOLOGY The impact of the various strategies must recognise the type and diversity of species and relevant conservation values. Opportunities to utilise natural systems, eg. local wetland species in water treatment schemes can be highlighted in this assessment.

•

identifying the locations of potential sources and probable yields (including surface water yields from rainfall and groundwater);

•

identifying the locations of potential users of water and their likely demands;

•

sizing and positioning of dams and other water control structures to cater for local demands;

•

preventing degradation of water quality by identifying and separating “clean” and “dirty” streams;

Identification of the potential offsite impacts from the changes to the existing water patterns is required. The operator should assess the constraints, the target quality and quantity parameters required and where any benefits of the mine water management systems might pass to downstream users.

•

optimising the flexibility of the water system by linking components in the water circuit (using gravity drainage where possible);

•

focusing excess water to down-gradient control dams of adequate size and at key locations to control offsite discharges;

3.3.10 MONITORING

•

implementing recycling schemes to reuse water wherever possible; and

•

implementing monitoring systems to quantify water entering the circuit, moving within the circuit and exiting the circuit.

3.3.8 HERITAGE VALUES A comprehensive assessment, listing and plan of archaeological, heritage, historical values and the visual character at the site will enable proper planning and locating of water management structures. 3.3.9 DOWNSTREAM AND OFFSITE USERS

Monitoring will be required during the various phases of mine development: baseline, feasibility, construction, operations, decommissioning and active rehabilitation. The monitoring systems must be established with a view to understanding the catchment responses to the proposed site activity and verifying licence requirements, and for corroborating design data. Such systems need consistency through all phases of the project.

Frameworks of water management systems derived in this way may be used to assess the impact of future changes in the water budget. This may be achieved by modelling the response of the mine water circuit to these changes commonly referred to as the minesite water balance. Models may be written using computer programming languages or developed using conventional spreadsheets.

3.4 Site Plan

Models should include:

Mine water management is a long-term process which may be simplified by: •

planning for the energy-efficient storage, transport and use of water; and

•

modelling to quantify present and future water budgets.

The thoroughness of the initial planning process will determine the ease with, and extent to which future 1

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flexibility to alter quantities of source and demand water;

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flexibility to alter water transport rates;

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flexibility to alter dam sizes;

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flexibility to add or delete water transport routes; and

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P lanning and P rincip L E S

• ensure that Commonwealth, State and local statutory requirements are observed and incorporated into the monitoring plan;

• ‘calibration’ checks against monitored quantities where appropriate. Planning and modelling of site water budgets will allow any imbalances between water supply and demand at the site to be quantified and accommodated efficiently. The quality of water may also need to be considered in such an analysis.

• ensure that sufficient data are collected over time in order to enable accurate assessment of the physical and chemical properties of all point source, diffuse source, industrial and domestic wastewater streams; and

The site plan must also address the final land use and the use of the water management infrastructure for the site after mining is finished. This will be a constant reference for ongoing planning of the water management systems.

• collect representative samples of the medium being measured and an adequate number of duplicate and quality control samples. Data management forms an important part of the monitoring system. The following points should be considered when designing a monitoring system:

Where quantitative data are collected as part of the site description they should be compiled and stored on an appropriate water management database for reliable reference and review. Where possible, qualitative data arising from this compilation should be stored on the same database.

• samples must be collected according to a site-specific protocol, established to fulfil the objectives of the monitoring program; • all samples should be analysed using NATA registered methods;

3.5 Monitoring and Data Management

• all data collected using electronic loggers must be validated and calibrated against physically measured data wherever possible;

Within the resources industry, the basic principles of water monitoring are to:

•

• identify the receiving waters or natural resources which require protection from the existing or proposed mining and processing development; • establish water quality objectives for these resources; • collect and evaluate site specific data such as local climatic conditions, permeability of soil and underlying bedrock, any potential pathways for the migration of contaminants;

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• all water quantity and quality data should be stored in a database designed specifically for the site’s requirements; data should be able to be retrieved rapidly and systematically; • water information should be reported regularly to site management (ie. actually used for management purposes); and

• prepare and implement a monitoring program for the region prior to the commencement of mining. Collect rainfall data, background flow and water quality data for all surface waters (especially up and downstream of the operation), groundwater, estuarine and coastal waters that may be affected by the development;

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calibration procedures must be established at the earliest possible stage in a monitoring program and the calibration of equipment should be checked periodically;

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• data should be regularly reviewed and interpreted to ensure that the beneficial uses (eg. ecological, recreational) of regional watercourses are protected in accordance with appropriate guidelines for receiving water quality in the region (eg. ANZECC 1992). Further information on the establishment of site monitoring programs can be obtained in EPA (1995). M

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4. Water Chemistry

4.1 Chemistry of Natural Waters

• units in which the parameter is commonly measured and reported;

4.1.1 INTRODUCTION

• sources (what activity can contribute to the levels of these parameters); and

Water quality is a generic term and is usually determined by the levels of various indicator substances. These indicators are generally selected on the basis of the type of waterbody in question (eg. stream, estuary, groundwater, potable water) and the water use (eg. the quality of water required for drinking is higher than that required for recreation).

• environmental significance of the parameter being determined. However, before discussing individual physico-chemical parameters, several terms and concepts common to most aquatic and geochemical parameters will be introduced.

Impacts on surface and groundwater water quality can occur during exploration, construction and operation of mines, as well as at abandoned and rehabilitated minesites. Uncontrolled drainage from mines can contribute potentially harmful materials to local waterways and may degrade the water’s suitability for domestic, agricultural or industrial uses, or be harmful to the ecology of the receiving environment. Government authorities are placing tighter controls on site discharges and many sites throughout Australia now operate under a zero discharge policy.

4.1.2 DISSOLVED VERSUS PARTICULATE AND TOTAL CONSTITUENTS Definition The distinction between dissolved, particulate and total constituents is one of the most important definitions used in water quality assessment. An element can move between the dissolved and particulate phase depending on physicochemical conditions such as temperature, pH or the presence of some other element or compound. This is often referred to as “partitioning”. Discharge licences generally relate levels of a certain element to either the dissolved, particulate or total concentration. The following example depicts the relationship between the three phases.

It is important to understand the characteristics associated with the various types of water sources and discharges likely to be encountered. While the quality of the source or discharge at any given site is dependent on the geochemistry, mineralogy and geographical location of the operation, there are general characteristics associated with the water that may be encountered in Australia. This section includes a general overview of some of the common physicochemical parameters and includes for each:

Consider a one litre bottle of a water sample collected for the analysis of cadmium. The sample contains both dissolved and particulate forms of cadmium. The dissolved cadmium concentration is the cadmium in the sample after it has been filtered through a 0.45µm pore size filter. The particulate cadmium in the sample is what remains bound to the material on the filter.

• definition and alternate names;

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Total concentration can be determined either directly or by calculation from the dissolved and particulate results. It is not simply a summation of the two concentrations as the suspended solids concentration has to be taken into account. For example, the dissolved cadmium concentration was found to be 3µg/L, the particulate concentration of cadmium was determined as 250µg/g (or mg/kg), and the suspended sediment concentration was 6540 mg/L (0.650 g/L). Therefore the total cadmium concentration is:

4.1.3 DIFFERENCE BETWEEN ORGANIC ACID AND CARBONATE WATER SYSTEMS The main aquatic geochemical processes throughout most of Australia's inland fresh waters are dominated by one of two general geochemical systems. In the context of this handbook, these will be termed: • carbonate water (water in which the carbonic acid equilibrium plays the dominant role in governing water chemistry); and • organic acid water (water with natural high levels of dissolved organic matter).

0.650(g/L) x 250 (µg/g) + 3 (µg/L) = 154.5 µg total Cd/L.

Waters in which the primary control is the carbonic acid system have pH values ranging from 6 to 8.5 and electrical conductivities up to many thousands of mS/cm. Organic acid systems generally have a pH less than 6 and much lower electrical conductivity.

Alternatively, the total cadmium concentration may be measured directly by digesting (using acid) and analysing a sub-sample of the original one litre sample.

Carbonate Waters

The definition of “dissolved” using a 0.45 µm filter is purely operational and has no direct biological rationale. Precise definitions may be found in APHA (1994). The classifications of total, particulate and dissolved concentration are used widely when setting discharge permits and water quality criteria. Generally, dissolved criteria are more often used for the protection of aquatic ecosystems. This is because most toxicity data show that it is the dissolved phase of pollutants which is bioavailable to aquatic organisms and thus potentially toxic. Total concentration criteria are generally used for recreation, livestock and human health water quality criteria, given that separation of the particulate load prior to either swimming or drinking raw water is unlikely to occur. In addition, the acidic nature of the human gut means that many pollutants can remobilise into the dissolved phase and therefore become more bioavailable.

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The carbonate, or carbonic acid, system describes water in which carbonate species in solution control or influence aquatic geochemical processes. The principal components of the carbonate system include carbon dioxide (CO2), carbonic acid (H2CO3), bicarbonate (HCO3-) and carbonate (CO32-)The reactions involving these species are very important in surface waters, groundwaters and in the atmosphere. Carbonic acid in water can be derived from several sources, the most important of which are: 1.

the weathering of carbonate rocks via:



CaCO3  Ca2+ + CO32-



CO32- + H+  HCO3-



and

2.

uptake of CO2 from the atmosphere via:



CO2 + H2O  H2CO3



H2CO3  H+ + HCO3-

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Almost all surface partitioning and adsorption processes involving natural sediments are mediated to some degree by organic matter of this type. Rivers draining regions where little or no carbonate is present, and where bedrock is resistant to weathering, tend to have a low pH and low conductivity. Soils developed in these areas are frequently organic-rich because the bedrock is resistant to breakdown and therefore contributes little mineral to the soil. As water percolates and circulates through the organic rich soil, cations that are present in solution (Ca, Mg, Na, K) are exchanged for H+ in the soil organic matter.

The species of the carbonate system that is present depends on the pH of the solution (see Figure 4.1). Below pH 6.4, carbonic acid (H2CO3) is the dominant species in solution whereas above pH 6.4 bicarbonate (HCO3-) is the dominant species. The greater the total concentration of the carbonate species, ie. HCO3- plus CO32-, the greater the buffering capacity of the water, ie. the greater the ability of the water to resist change from either acidic or basic inputs. The amount of carbonate produced from reaction 2 is far less important than that derived from the weathering process of rocks. Generally, carbonate system rivers have a higher conductivity, due not to the presence of bicarbonate but rather the co-cations in solutions which are also weathered as a part of the same process that liberates the bicarbonate.

As the H+ accumulates in solution, the pH decreases. As the pH decreases, organic compounds are leached from the surface litter, into solution. Organic acids are also synthesised by soil organisms and excreted by plant roots.

Organic Acid Waters The particular organic acids which control the second major system of aquatic geochemical processes occurring in Australian freshwater rivers and streams are derived from what is loosely termed humic and fulvic material or dissolved organic matter (DOM). DOM is derived from the breakdown products of organic matter and comprises a wide range of complex molecules.

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These waters also originate from areas of high rainfall where peat deposits are common, eg. the western highlands of Tasmania. 4.1.4 LOAD VERSUS CONCENTRATION In determining water quality, the distinction between load and concentration must often be made. Concentration of the element compound

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is emphasised in systems where a threshold or regulatory level is desirable in the receiving water, eg. maintaining total suspended solid values below 100 mg/L, or dissolved oxygen above 9.5 mg/L. Concentration is usually expressed in terms of mass per unit volume, ie. µg/L, mg/L, g/L or %. There are other situations where total load or flux (ie. the total amount - mass or volume - of substance per unit time) may be of more concern, eg. nutrient loading into lakes and rivers to avoid algal blooms or the spread of nuisance weeds and phytoplankton. Loadings are usually expressed in terms of mass per unit time (g/day, tonnes/year), mass per unit area (kg/ ha), or mass per unit area per unit time (kg/ha/year). 4.1.5 pH

pH is an indicator of the intensity of the acidic or basic character of a solution (APHA 1994). Units of Measurement pH is a dimensionless parameter and is represented on a logarithmic scale of 1 to 14. A pH value of 1 indicates a highly acidic solution, 7 is neutral and 14 is strongly basic, or alkaline. The technical definition is: pH = -1/log10[H3O+].

Sources and Environmental Significance One of the greatest causes or contributors to the production of acidic water is from sulphide oxidation of iron sulphide minerals such as pyrite (FeS2) in the presence of oxygen (air) and water. The oxidation reactions are bacterially mediated, primarily by Thiobacillus ferrooxidans. Acid generating conditions can occur in damp mine workings, in exposed waste rock dumps, tailings dams and in washeries. Fact Sheet No.7 discusses acid drainage in greater detail. As the water moves through the acidic material, oxidation of reactive sulphides occurs, generating acidity which initially can be neutralised by alkalinity in the groundwater. If more acid is 18

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The bacteria that catalyse the acidity producing reactions thrive only under acid conditions so that, once acidity is initiated, acid production becomes more rapid and the problem increases rapidly. A phenomenon only recently identified in Australia is natural acidification of water as a result of acid sulphate soils. These waters have developed in tidal swamps, wetlands and estuarine environments along coastal regions where iron rich silts and muds have mixed with accumulated organic matter. Bacteria thrive in these anaerobic conditions, creating pyrite. When these soils are exposed to air, as occurs with disturbance due to coastal development, sulphuric acid is produced due to oxidation of the pyrite. Potential acid sulphate soils occur in most coastal regions from north of Sydney to Onslow in Western Australia. Any mining development which potentially affects such soils could also result in acid drainage.

Definition and Alternative Names



generated than the initial alkalinity of the water, the alkalinity will be consumed and acid water will result. If sufficient oxygen is present, the amount of acidity generated is determined by the amount of reactive sulphides in the material. In the absence of mining, acid waters are uncommon because dissolved oxygen in the groundwater is insufficient to produce acidity greater than the alkalinity of the groundwater. During mining, gaseous oxygen is introduced as the rock is broken up, and water movement through the system is accelerated.

T

In most natural streams where acid drainage is not present, pH levels range between 5.5 and 8.5. Extremes to these levels are usually the results of high loads of natural organic acids (DOM) or high carbonate concentrations. Another effect of mixing acid water with receiving waters high in carbonate is the formation of CO2 which affects the respiration of aquatic biota. When pH values fall below 4, most aquatic biota will be severely stressed. In contrast to the low pH water produced by acid rock drainage, many mineral processing facilities

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•

require water with an elevated pH (9 to 11) which is normally achieved through the addition of lime. Problems of scaling in pipes and ecosystem stress brought about by high pH waters are no less serious than the problems associated with low pH waters.

• dissolved metal solubility and bioavailability (toxicity) to aquatic organisms; • foaming, scaling and metallurgical problems; and • dissolution of bicarbonate and carbonate, causing liberation of CO2 and corrosion.

Treatment Options Several approaches can be adopted to raise or lower pH including:

4.1.7 HARDNESS

Definition and Alternative Names

• addition of an alkali or acid;

Hardness is commonly associated with a waters ability to lather or foam soap. The harder the water the more difficult it is to lather the soap. The principal components of hard water are calcium and magnesium ions (Ca2+ and Mg2+).

• activated carbon or ultra-violet irradiation to remove DOM; and • bubbling with CO2 to manipulate the carbonic acid equilibrium. 4.1.6 ALKALINITY

Total hardness is defined as the numerical sum of the calcium and magnesium concentrations, expressed as calcium carbonate. When hardness is numerically greater than the sum of carbonate and bicarbonate alkalinity, that amount of hardness equivalent to the total alkalinity is called “carbonate hardness”; the amount of hardness in excess of this is called “non-carbonate hardness”. When hardness is numerically equal to or less than the sum of the carbonate and bicarbonate alkalinity, all hardness is carbonate hardness and non-carbonate hardness is normally absent.

Definition and Alternative Names Alkalinity refers to the acid neutralising capacity (pH buffering) of water, ie. its ability to reduce changes in pH brought about by the addition of an acid. The higher the alkalinity, the more acid is required to reduce the pH. Alkalinity is generally due to the presence of inorganic anions including carbonate (CO32-), bicarbonate (HCO3- ) and hydroxide (OH-); however alkalinity may also result from the presence of borates (B4O72- ), phosphates (P0 3-) and silicates (SiO 2-). 4

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Units of Measurement

Units of Measurement

Hardness is reported in the same units as alkalinity, ie. mg (CaCO3)/L.

Alkalinity is expressed in the units of:

mg of calcium carbonate per litre of water (mg CaCO3/L).

There are two methods for determining hardness. The first is by calculation from the Ca2+ and Mg2+ concentration in solution, the other is by titration.

The reported results for alkalinity are influenced by the method of the determination and depend on the pH end-point used in the analysis. Analytical methods are documented in APHA (1994).

Hardness may range from zero to several hundred mg/L, depending on the source and any prior pre-treatment of the water.

Sources and Environmental Significance

Sources and Environmental Significance

The main sources of alkalinity are the soluble salts of the anions listed in Section 4.1.13.

Hardness usually occurs throu­gh dissolution of minerals containing calcium, magnesium, and silica compounds, typically calcium and magnesium carbonates, sulphates, chlorides or

Alkalinity is known to influence several aquatic geochemical processes including: 1

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pH and effects from acid drainage;

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Units of Measurement

nitrates. Because of the inverse solubility of these compounds with temperature, at high concentrations they precipitate out of solution in, for example, boilers and hot water pipes. There are no reported human toxicological consequences of elevated hardness; however, high hardness waters are generally unpalatable.

Conductivity is usually determined by measuring the resistance where: Conductivity =

1 Resistance.

The SI1 unit for conductivity is mS/m (milliSiemens per metre); however µS/cm is still in common use, and many conductivity instruments use the units µmhos/cm, where 1 mS/m = 10 µmhos/cm.

Treatment Options Treatment options for water with high hardness comprise mainly precipitation of the Ca2+ and Mg2+ ions using a mixture of lime (Ca(OH)2) and soda ash (Na2CO3). In this process, the Ca2+ and Mg2+ ions precipitate as CaCO3 and Mg(OH)2. As this process occurs at high pH, subsequent pH adjustment may be required. This can easily be achieved by the addition of either H2SO4 or by the bubbling of CO2 through the softened solution.

Sources and Environmental Significance Conductivity is used to monitor several different processes, some of which include: • determination of amounts of ionic reagent needed in certain precipitation and neutralisation reactions; and • estimation of total dissolved solids (TDS) in mg/L and salinity in a sample by multiplying the conductivity in mS/m by an empirical factor. For TDS this factor may vary from 0.55 to 0.90 depending on the soluble components of the water and the temperature of the measurement. In the absence of a site-specific relationship, a factor of 0.68 is commonly assumed. Similarly, an estimate (in milliequivalents per litre) of either anions or cations can be derived from the conductivity measurement.

4.1.8 CONDUCTIVITY Definition and Alternative Names "Conductivity" is a measure of the ability of water to conduct an electric current. Factors affecting conductivity include temperature and the type, concentration and valency of ions present (eg. Na+, Ca2+, Cl- and SO42-). The higher the concentration of conducting solutes, such as salts, the higher the conductivity (see Table 4.1).

4.1.9 SALINITY Definition and Alternative Names

TABLE 4.1 Typical Conductivity Range of Waters

Salinity is an indirect measurement of the total amount of soluble salts in solution. These salts include sodium chloride as well as various calcium and magnesium salts of chlorides, sulphate and nitrates.

Conductivity Range (mS/m)

Water Freshly distilled

0.5 - 2

Potable waters

50 - 1 500

Seawater

Units of Measurement

40 000 - 50 000

Groundwater

Salinity is generally expressed as parts per thousand (ppt or 0/00).

up to 50 000

1 Systéme lnternationale = International System of Units 20

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industry has standardised on a range of filters from various manufacturers all with a similar nominal pore size of around 1.2µm. In Australia, perhaps the most widely used is the Whatman glass-fibre filter GF/C After the water sample is filtered through the GF/C filter, the filtrate is evaporated to dryness at 1800C and weighed; the TDS is calculated from this result.

The only direct method of measuring absolute salinity is to analyse the individual chemical components. Given the time and costs associated with individual analyses, indirect methods such as conductivity are normally favoured. Conductivity measurements can be made in the field or laboratory with a meter and probe which has temperature compensation. Total dissolved solids is also an approximate measure of salinity

It is important not to confuse dissolved solids, which are filtered through the GF/C type filters, with the dissolved component of metals. Dissolved metals refers to that portion of the total metals in a sample which pass through, or are not retained on, a 0.45µm filter membrane.

Sources and Environmental Significance Dry land salinity is a major problem in certain areas of Australia, caused primarily by the widespread clearing of native vegetation. Replacement of deep­ rooted perennial native vegetation with shallow rooted annual pastures which use much less water, allows the water table to rise, bringing dissolved salts to the surface where they are concentrated by evaporation. Similarly, the storage of acid and saline mine water in dams can pollute high quality groundwater reserves. Hypersaline groundwater, with salinities well in excess of seawater, is used as process water in the goldfields of Western Australia. Release of this water into the environment can cause death of vegetation and land degradation.

TSS may also be referred to as non-filterable residue (NFR) or suspended particulate matter (SPM). This parameter measures the amount of solids suspended in a water sample which can be separated from the water and dissolved solids phase by filtration through a filter of fixed pore size. TSS can be related to the turbidity of a water sample. With careful site-specific calibration, and where the sediment source is relatively constant and homogenous, turbidity can be used to calculate TSS (see: Section 4.1.11). However, extreme care must be taken in developing this relationship.

Criteria for salinity pertaining to various livestock, irrigation and domestic uses can be found in ANZECC (1992) and DME (nd).

Units of Measurement Total solids and its constituent parts are reported as mg/L. In samples with very high concentrations the units may be expressed as %.

4.1.10 SOLIDS Total solids, as the name suggests, is a measure of all the substances associated with a water sample, other than the water itself. It can be further refined into its constituent parts, total dissolved solids (TDS) and total suspended solids (TSS), ie.

Sources and Environmental Significance The composition of total solids depends on the geology, land use, geochemistry and the environment of the catchment. Dissolved solids in water may result from the dissolution of materials exposed during mining, or from the addition of soluble chemicals during the processing of ores. High levels of TDS are often not suitable for potable water, mainly due to inferior taste. In addition, waters high in TDS are rarely suited for industrial applications.

TS = TDS + TSS. Definition and Alternative Names Another name for total solids is total residue. TDS or filterable residue is that portion of a sample (other than water) which passes through a filter of pre-defined pore size. This will obviously depend on the pore size of the filter used. For this reason, 1

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Suspended solids can result from erosion of unprotected ground surfaces, from wash water, or from stormwater mobilising solids deposited on the ground surface as a result of mining or processing activities. The TSS in water can affect the operation of biological and physical wastewater treatment processes. Samples high in TSS are also aesthetically unsatisfactory and affect the partitioning and distribution of various contaminants in the aquatic system. Suspended solids reduce light penetration through the water column, affecting growth of aquatic flora and fauna as well as the aesthetic appeal of the water and its subsequent use for recreation. Under certain flow conditions, suspended material settles out and can smother benthic organisms and their habitats. Other problems with sedimentation include possible disruption to navigation. Since most pollutants can be carried by or adsorbed onto suspended solids, tight controls of TSS in a water management plan can also lower the flux or total load of pollutants entering watercourses. Adsorbed nutrients and organic matter are also a source of nutrients for algal blooms. Solids remain in suspension only when there is enough force or energy (turbulence) in the water column to keep them in suspension. Rivers with lower gradients and lower energy enable suspended sediments to settle out and become benthic sediment or bed load. The effect of increased sediment loads to a river system are numerous. High suspended sediment loads can effect the gills of fish leading to irritation and lesions. When suspended sediment settles, it can increase river bed elevation or aggradation which, as well as affecting aquatic organisms, may also lead to increased overbank flows and flooding. Sedimentation in water storage can reduce the life of a dam, or increase the costs of dredging as well as decreasing the quality of the retained water.

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Definition and Alternative Names "Turbidity" is an optical measurement of the sample’s inherent ability to scatter light. Turbidity measurements can be affected by the particle size of the suspended matter, its mineral content and its respective abilities to scatter and absorb light. In addition, fine colloidal material can have a major effect on increasing the turbidity (light scattering) of a sample but only have a minor effect or increase in the concentration of total suspended solids.

Units of Measurement The units of turbidity are generally reported in nephelometric turbidity units (NTU). It is possible to produce a calibration curve or regression curve of turbidity versus TSS at a given site; however, this must be repeated for each site, because of the likely changes in the characteristics of suspended solids between different geological regions.

Prevention of dust generation through control of processes and stockpiles, and erosion of 1

4.1.11 TURBIDITY

Optical right angled back-scatter nephelometers are generally used for low level turbidity measurements while forward scattering devices, which are more sensitive to the presence of larger particles, are generally used for in-stream analysis systems. Care must be taken in using optical devices, especially in tropical regions where algae and slime growth can rapidly affect the calibration of these instruments. Similarly, in waters with high suspended solids, abrasion of the optical surface can affect calibration of the instrument.

Treatment Options

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land through controls on clearing and prompt revegetation, are ways of reducing solids loadings to water. Sediment retention through the placement of sediment traps will lead to a reduction in the amount of sediment reaching natural watercourses. Sediment traps upstream of a storage dam are an effective means of prolonging the life of a relatively small dam. Treatment of water containing suspended sediments prior to use in a plant or for domestic potable water may require settling, screening, filtering or dosing with a flocculant.

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Flow rates can also affect particle size distribution and hence the relationship between turbidity and TSS.

Reduction in dissolved oxygen within natural aquatic systems can result from inputs of organic material to the system (eg. sewage, some mineral processing effluents) and also from algal blooms. Dissolved oxygen concentrations usually decrease with increasing water temperature.

Sources and Environmental Significance By world standards, Australian watercourses are quite turbid as a result of intense rainfall and flood events, and the erodibility of agricultural and arid soils. The aquatic ecosystems of many Australian watercourses have adapted to higher turbidity levels than existed prior to white settlement, but most probably at a cost of lower species numbers and diversity.

Biochemical Oxygen Demand (BOD) Definition and Alternative Names The BOD test is an empirical test in which standardised laboratory procedures are used to determine the relative oxygen demand of wastewaters, effluents and polluted waters. It is often referred to as the BOD5 test, referring to the biochemical oxygen demand over a five day incubation period.

Turbid waters normally require some form of treatment prior to their use as industrial or potable water. Treatment processes used to remove turbidity can include filtration, coagulation and settling. 4.1.12 OXYGEN DEMAND (DISSOLVED OXYGEN, BOD AND COD)

Units of Measurement The units of BOD5 are expressed in mg/L along with the incubation time.

Dissolved oxygen is a key water quality parameter required to sustain a healthy aquatic ecosystem. The presence of excess organic materials such as sewage sludge can significantly add to the oxygen demand of a system, consuming dissolved oxygen from the water as they decompose.

Sources and Environmental Significance The BOD test measures the oxygen consumed by biochemical degradation of organic material (carbonaceous demand) and the oxygen used to oxidise inorganic material such as sulphides and ferrous iron. It may also measure the oxygen used to oxidise reduced forms of nitrogen (nitrogenous demand), unless their oxidation is prevented by an inhibitor.

Dissolved Oxygen Definition and Alternative Names Dissolved oxygen refers to the oxygen molecules that are dissolved in water.

If BOD5 in effluent is high, then oxygen dependent organisms in the receiving waters may become stressed.

Units of Measurement Dissolved oxygen is usually expressed in parts per million or mg/L. For some natural systems, % saturation is also commonly used.

Chemical Oxygen Demand (COD) Definition and Alternative Names

Sources and Environmental Significance

The COD test is used as a measure of the oxygen equivalent of the organic matter concentration of a sample that is susceptible to oxidation by a strong chemical oxidant. For samples from a given location COD can be empirically related to BOD, organic carbon or organic matter.

For the protection of aquatic ecosystems, ANZECC (1992) recommends that dissolved oxygen should not normally be permitted to fall below 6 mg/L or 80-90% saturation, this being determined over at least one diurnal cycle.

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Units of Measurement Results are expressed in units of mg O2/L. Sources and Environmental Significance COD is a useful, but not commonly used, parameter in mine water management. Its usefulness stems from its measurement of the total oxygen demand, unlike BOD which measures oxygen demand available to bacteria over a five day period. As a result, COD concentrations will normally always be higher than BOD concentrations from the same sample. 4.1.13 ANIONS AND CATIONS Definition Anions are those elements with a negative charge (eg. Cl-, OH-, HCO3-, SO42-, CO32-, P043-) as opposed to cations which are positively charged (eg. Na+, K+, Ca2+, Mg2+). This discussion will be restricted to the common inorganic anions and cations. Inorganic Anions

Units of Measurement Anions are typically reported in the units mg/L. Values in natural and wastewaters range from zero to several hundred mg/L.

The sources of these anions is dependent on geology as well as prior treatment and uses of the water. Sources of chloride are salts such as NaCl and CaCl2 which are often present in high concentrations in groundwater. For example, the aquifers of the Hunter Valley of New South Wales contain high concentrations of salt as a result of deposition of sediments in a marine environment. The most common source of soluble SO42– from mine operations is from the oxidation of sulphide

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Definition Cations are those elements with a positive charge, such as sodium (Na+), potassium (K+), calcium (Ca2+), and magnesium (Mg2+). These are among the most abundant natural elements in the environment. Units of Measurement Cations are typically reported in the units mg/L. Values in natural surface and groundwaters and wastewaters range between zero to several hundred mg/L.

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The concentrations of these elements in natural waters depends on the geology and geochemistry of the host rock. Calcium concentrations in water from limestone areas are typically higher than for waters from non-calcareous areas. High concentrations of these cations are typically found in groundwaters and increase their hardness. They also affect the permeability and fertility of soils and, for this reason, their concentrations are closely monitored in the agricultural sector.

Sources and Environmental Significance

1

Inorganic Cations

Sources and Environmental Significance

Common anions associated with mine water quality management are chloride (Cl–), hydroxide (OH–), bicarbonate (HCO3–), nitrate (NO3–), sulphate (SO42–), carbonate (CO32–), and phosphate (PO43–)·

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minerals such as pyrite (FeS2). Phosphates (which are present in domestic and industrial detergents) and nitrates (from mine explosives and fertilisers used in mine rehabilitation) can also find their way to watercourses. If these nutrients occur in moderate to high concentrations they can readily stimulate the growth of algae and aquatic weeds.

Other sources of these cations include leachate from waste rock and tailings dams. The ratio of the specific major cations relative to each other is also an important factor in considering the implication of their respective concentrations in either feed, process or discharge water.

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The impact of a particular metal on water quality depends not only on the type and concentration of the metal, but also on its chemical form or speciation. The chemical speciation of a metal (eg. whether copper exists as Cu2+, CuCO3, Cu(OH)2, or Cudissolved organic matter complexes etc.) dictates how bioavailable it is and the extent to which it may enter the food chain, where it may accumulate to toxic levels. Generally, metals are most toxic in their soluble free ionic form (species) eg. Cu2+, Ag+ etc., compared to metals complexed with either inorganic or organic ligands (eg. CuCO3 or Cu-DOM) or in particulate form (associated with minerals). One exception is mercury which is more toxic in the methyl mercury (CH3Hg) species compared to the free (Hg2+) species.

4.1.14 METALS (TRACE METALS, HEAVY METALS, METAL SPECIATION) Definition Two terms are commonly used when discussing metals in water and environmental management. These are: • Trace metals, which commonly refers to: – metals at very low levels in the environment (trace analysis); or

–

trace elements which are either essential nutrients or serve some other necessary biochemical function. These include zinc, iron, copper, cobalt, sodium and potassium;

Further information on individual metals and their environmental Significance can be obtained from the various ANZECC guideline documents.

and • Heavy metals, which are generally thought to mean toxic metals. Strictly speaking the term refers to metals with an atomic weight greater than that of sodium (22.9).

4.1.15 NUTRIENTS Definition and Alternative Names

Units of Measurement

The term "nutrient" refers collectively to elements and compounds which are essential to sustaining adequate biological function. The most common nutrients which may affect the water management of a mining operation are nitrogen and phosphorus. There are various forms of nitrogen such as ammonia, nitrite, nitrate, and organic nitrogen. Phosphorus can be found in the form of orthophosphate, total phosphorus and organically bound phosphates. The form of the nutrient has an integral role in its function and fate in the aquatic environment. Biological productivity may be limited by the availability of either nitrogen or phosphorus, which are often referred to as the growth limiting nutrients. Silica has also been identified as a limiting nutrient in some aquatic systems.

The units are dependent on the metal and its concentration. Particulate metals are usually reported as µg/g or mg/kg. Dissolved metals are usually expressed in terms of µg/L or parts per billion. Other units in which metals are sometimes reported include mol, millimol or micromol per litre (mol/L, mmol/L, µmol/L). These units relate to the number of molecules of the metal that are present and are not influenced by the actual weight of the elements of concern. This unit is most commonly used in toxicological assessment. Sources and Environmental Significance In natural systems, most metals are only sparingly soluble in water, with higher concentrations usually associated with the particulate phase. The amount of a metal released from its particulate phase into solution is a function of pH, particle geochemistry, aquatic geochemistry, hydrologic factors, temperature, etc. Mobilisation of metals is frequently a secondary effect of acid drainage. 1

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Units of Measurement

micrograms of total phosphorus or total nitrogen per litre (µg TP/L or µg TN/L) and milligrams of ortho-phosphorus or nitrate nitrogen (mg Ortho-P/L or mg NO3-N/L). Sources and Environmental Significance

Oil and grease in water samples is commonly expressed in mg/L. Oil and grease in solid sludge is expressed as % of dry solids. Hydrocarbons are also expressed in this way.

Sources of nutrients in mining operations include:

Sources and Environmental Significance

• sewage or septic wastewater;

If present in high amounts, oil and grease can reduce the efficiency of water treatment processes by interfering with anaerobic and aerobic biological processes. Large quantities of oil and grease discharged in wastewater can cause surface films and deposits and result in the staining of riverbanks and coast lines. They can also affect oxygen exchange, oxygen demand and palatability.

• nitrogen based nutrients from explosives; • phosphorus based nutrients from process chemicals and industrial detergents; • fertilisers applied during rehabilitation works;

and

• degradation products of cyanide. Excessive concentrations of nutrients can promote and accelerate growth of aquatic plants and algae, including attached and floating macrophytes and dense suspensions of free-floating algae. These reduce light penetration and, upon decomposition, cause odours and loss of oxygen in the host ecosystem.

Treatment Options Treatment options available for the reduction of synthetic organics (fuels, oils, grease etc.) include simple oil-water separators through to expensive dissolved air flotation systems.

4.1.16 OILS, GREASES AND HYDROCARBONS

4.1.17 ORGANICS, NATURAL ORGANIC MATTER, DISSOLVED ORGANIC CARBON

Definition and Alternative Names

Definition and Alternative Names

The parameter “oil and grease” refers to a range of chemicals which can be extracted from a water sample into the organic solvent trichlorotrifluoroethane. The types of compounds collectively analysed by this method are primarily fatty components from animal and vegetable sources and hydrocarbons from petroleum products. While trichlorotrifluoroethane is used to extract the group of compounds of interest, there are three subsequent analyses which can be conducted depending on the make-up of the water being examined and the likely constituents. Oil and grease determination can also be performed on sludge samples.

The term organics refers to a broad group of chemical parameters, some of which are used in the resource development and mineral processing industries. In addition to manufactured organic compounds, there is a broad group of naturally occurring organic compounds which play an important role in aquatic biogeochemical processes. Collectively, these compounds are referred to as dissolved organic matter (DOM), natural organic matter (NOM), dissolved organic carbon (DOC), or humic substances (HS). Units of Measurement For the more general definition of synthetic organics, the units of measurement depend on the analysis being undertaken. Most commonly, they are reported in either mg/L or µg/L.

If required, total petroleum hydrocarbons (TPH) can be selectively analysed as a separate group by a modification of the oil and grease method.

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Units of Measurement

Naturally derived organic material is most commonly measured as DOC and expressed in units of mg C/L. DOC typically represents approximately 50% by mass of DOM.

Several methods exist for the analysis of colour, varying from the simple visual comparison, to techniques requiring sophisticated instruments and determination of the colour wavelength of the sample. The units of colour depend on the method of analysis but generally correspond to a “colour number” or code which is based on a visual comparison of the colour of the sample to that of a series of standards, usually made with a platinum ­cobalt solution. Alternatively, the colour can be measured by light transmittance through a special system of photoelectric cells and light filters. The final choice of measurement depends on the specific water quality to be determined. Regulatory authorities usually specify the parameters to be determined and the specific method of analysis.

Sources and Environmental Significance Process reagents such as collectors, frothers and flocculants are all synthetic organic-based compounds. Usually, the amounts of organic compounds used for mineral processing are small and any residual concentrations decay rapidly. DOM, NOM, DOC and HS refer to a generic group of compounds which are best described as the humic and tannin extracts of soil and plant materials which impart the characteristic tea colour of some natural waters. The organic compounds making up DOM are a group of weakly acidic molecules which, in high concentrations, are able to reduce the pH of the water.

Sources and Environmental Significance

Treatment Options

Colour may result from a number of sources including metallic ions (iron and manganese), dissolved organic material (humus and peat material), plankton and weeds. Highly coloured industrial wastes can also contribute to the colour of water.

The removal of natural organic material can be performed in many ways and is dependent on the amount of DOC present and the amount of water requiring treatment. Common treatment options include adsorption onto activated carbon, UV oxidation and ozone oxidation.

The environmental implications of colour depend on the element that is imparting the colour.

4.1.18 COLOUR

4.1.19 CYANIDE

Definition and Alternative Names

Definition and Alternative Names

The term colour can be divided into:

Cyanide (CN) is used widely throughout the mining industry to dissolve and complex gold and silver to separate them from the ore. In terms of water quality management there are three main forms or species of CN:

• True colour, ie. the colour of a sample from which turbidity has been removed; and • Apparent colour, which includes the colour and turbidity of the total sample.

• total CN - refers to all forms of CN and is usually determined by performing an exhaustive hot acid extraction whereby all the CN from both liquid and solid phases are dissolved and subsequently analysed as NaCN;

Apparent colour is measured on the sample prior to any treatment (except inversion of the sample to suspend all particulate matter) and true colour is measured after either filtration or centrifugation. Normally, unless otherwise stated, the term colour refers to the measure of true colour.

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• weak acid dissociable CN (WAD CN) includes only those CN compounds that are liberated

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•

under weakly acidic conditions, ie. it does not include all the CN present in the sample; and

complex formation with metals - CN forms complexes with metal ions which are common in mineral processing wastes. These complexes are usually resistant to biological uptake and are stable in the environment, although some may be readily broken down to their basic components, for example CuCN; and

• free CN (CN–) and hydrogen cyanide (HCN) are the most bioavailable forms of CN, the abundance of which is strongly dependent on pH. The lower the pH the greater the proportion of the total CN that exists as HCN. Units of Measurement The units in which CN is expressed depends on the form being analysed and from where it was collected. Samples from process waters containing CN will generally have total CN values in the mg/L range; however, after storage or treatment the values may realistically be in the very low µg/L range. Generally, for process waters using CN values are reported as: • mg total CN/L; • mg WAD CN/L; and • g free CN/L. Sources and Environmental Significance While CN can be formed naturally by nitrifying bacteria, the main source in the mining industry is waste streams from cyanidation processes. The mechanisms affecting the environmental fate of CN include:

4.1.20 ODOUR AND TASTE Definition and Alternative Names

• bacterial degradation - movement of CN through soils and sediments is thought to be restricted through biodegradation by soil organisms and adsorption to soil particle complexes; • atmospheric diffusion - at neutral and acidic pH, CN in solution occurs predominantly as HCN gas which readily diffuses into the atmosphere; •

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conversion to thiocyanate - free CN reacts with pyrite and pyrrhotite to form thiocyanate, which is relatively stable and non-toxic. Thiocyanate is also produced as a part of the natural detoxification and biodegradation of CN in biotic systems;

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• photochemical degradation - although complex ions such as ferro-CN and ferri-CN are thermodynamically stable, they can undergo photo-reduction to form free CN in the presence of UV light. In compacted and solid tailings dams, this is only a problem at the surface of the dam. Beneath the surface, away from the UV light, the CN remains as a stable metal-CN complexes. The conversion of CN complexes to free CN is affected by pH, temperature, pond geometry and the intensity of UV light incident on the pond. The concentration of total CN has been observed to drop from around 60 mg/L to less than 5 mg/L in just over 1.5 months (Smith &. Mudder, 1991).

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Both odour and taste are subjective tests which often depend on an individual’s personal criterion to determine the acceptability of the water or otherwise. The tests are usually based on a comparison with tasteless and odourless water samples. Flavour is more objective, and can be used instead. Documented procedures for flavour are available. Units of Measurement Taste and odour are generally reported as dimensionless descriptive numbers which relate to threshold detection limits where the sample is compared to a standard with no, or some definable taste or odour characteristics. The measurements include threshold odour number, flavour threshold number, flavour rating assessment and flavour profile analysis number.

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4.2 Biological Aspects of Waters

Sources and Environmental Significance Taste and odour may render the water unsuitable for human consumption and domestic use as well as tainting fish and other foods which inhabit the water. There is no single compound which causes odour. However, tests exist for the determination of several of the prime compounds which impart an odour in waters.

Mining and mineral processing operations rely on or influence the biological component of natural or artificial systems. These systems can include: • biological processes beneficial to the operation, eg. anaerobic and aerobic treatment ponds, artificial wetlands;

4.1.21 RADIONUCLIDES

• ecosystem protection, ie. limiting the physical and/or chemical parameters associated with mine discharges to levels suitable for ecosystem protection; and

Definition and Alternative Names The mining and milling of ore containing uranium may result in water and wastewater that contains variable concentrations of radionuclides present in the ore. The water that is retained or discharged from an operation should, as a minimum, be analysed for radium-226, thorium-230, lead210, uranium-238 and polonium-210.

• bio-monitoring, ie. using aquatic organisms to monitor the effects and effectiveness of water management practices.

ANZECC (1992) recommended four biological indicators to assess ecosystem condition or health. These indicators are based on the assumption that the extent to which the integrity of an ecosystem is being maintained can only be assessed when the characteristic biological communities of a region are known or, since this will rarely be the case in Australia, by comparison of the biological community at the site or sites of interest with unimpacted communities in similar habitats elsewhere in the region. Each of these indicators relies on a rigorous and statistically sound sampling scheme, which is able to distinguish between various population parameters between impacted and unimpacted sites. Of these biological indicators, two relate to biological community structure and two to community processes.

Units of Measurement The commonly used unit of measurement for radionuclides is the becquerel (bq). For water, the units are expressed as bq/L and for soil and sediment the units are expressed as bq/g. Sources and Environmental Significance Radionuclides can be found in wastewater arising from the mining and milling of radioactive ores. Typical streams are: • excess process water, which may be pumped to a tailings impoundment; • runoff from the mine pit, ore stockpiles, waste dumps, borrow areas, haul roads and plant area;

The biological indicators recommended are:

• seepage from the mine pit, tailings dam and evaporation ponds; and

Species Richness

• water from water supply bores and dams which has flowed through mineralised material.

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Measures of specific richness indicate the number of species present in a sample of organisms of given size. They differ from diversity measures which also incorporate the concept of species evenness. A decrease in richness is generally considered as an indicator of ecosystem stress. M

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major energy pathways is maintained, and that natural detritus-driven aquatic systems are not converted to autochthonous primary production driven systems, and vice versa.

Since different components of an ecosystem may respond differently to stress, it is important that all the major biological groups (eg. macroinvertebrates, fish) be evaluated. The ANZECC guideline specifies that the species richness as measured by a standardised index should not be altered.

Levels of Protection Two categories of aquatic ecosystems are identified within the national ANZECC guidelines:

Species Composition ANZECC (1992) has proposed a guideline that, in any waterbody, impacts that result in Significant changes in species composition compared to those in similar, local unimpacted systems should not be permitted. It is possible, although probably unlikely, that ecosystems could maintain species richness while still changing markedly in species composition. Primary Production

• Pristine ecosystems are not subject to human interference through discharges or activities within the catchment. For these ecosystems, now largely restricted to National Parks, it is appropriate for the existing water quality to be protected and preserved through strict management; and • Modified ecosystems include all those systems subject to human interference. Some modified ecosystems have been permanently altered physically, for example through stream channelisation or port construction. Others have been changed through long-term chemical toxicity caused by contaminated sediment or by changed river flow regimes.

Primary production forms the basis of most aquatic food chains. In any waterbody, net primary production should not vary from the levels encountered in similar local, unimpacted habitats, under similar light, temperature and nutrient loading regimes. Primary production is known to be sensitive to light (water clarity), temperature and nutrients, amongst other factors.

4.2.1 MICRO-ORGANISMS

Ecosystem Function

Micro-organisms play an important role in natural aquatic systems and in the treatment of wastewater. The greatest use of microbes in wastewater treatment is for the treatment of sewage using anaerobic and aerobic treatment systems. Other uses of micro­ organisms relevant to the minerals industry are:

In any waterbody, changes that vary the relative importance of the detrital and grazing food chains should be minimised. Production to respiration ratios should not vary significantly from those of similar, local, unimpacted systems. Some ecosystems, such as large standing waterbodies, have autochthonous primary production (produced within the waterbody) as their major energy source. Others, including forest streams and some wetland systems derive most of their energy from allocthonous detritus (produced from outside the waterbody and is transported to where it is used).

• treatment of cyanide waste streams generated from mining and mineral processing operations; • treatment of hydrocarbon contamination arising from spillage or leaks from storage tanks or pipes; and • remediation of high nutrient or sulphate waste­waters.

Aquatic systems should be managed such that the relative balance between these two

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4.2.2 ALGAL BLOOMS

The time required to kill 50% of the population is then used as an index of toxicity. Standard LC50 and LD50 tests are performed over 96 hours. The 96 hour duration is operationally defined and has no biological or biochemical foundation. It was established so that a test could be completed within one working week. It refers to a specific dose of a test compound and is usually expressed as a concentration of the test compound per mass of test organism body weight. Such information is usually used to calculate and assign a safe exposure limit or of recommended dose per person per day.

Problem algal blooms are usually the result of a number of factors and not generally the result of a single person or a projects activities. A bloom is usually an indication of widespread problems or stress throughout the catchment, as in the case of blue-green algal blooms along the Murray-Darling system. While localised algal blooms can occur on a site, they usually do not pose any great problems and can frequently be controlled. Algal blooms are usually short-term occurrences leading to a population explosion and normally result from a combination of high light penetration and water temperatures, slow flowing or stagnant water and high concentrations of nitrogen and phosphorous. Oxygen depletion and the release of toxic constituents from bluegreen algae are common problems that can develop when a bloom collapses and the algae decay.

Lethal concentration50 (LC50) is similar to the lethal dose but refers to a concentration. Therefore, this figure is more widely used to test aquatic organisms such as fish and invertebrates. Often, toxicity data are related to a time of exposure, eg. a value of 50µg/L is not to be exceeded more than once over any 12 month period. While such limits do take into account accidental spillages, they are assigned on a purely arbitrary basis and the toxicological information in relation to this value being exceeded is not absolute in nature.

4.2.3 TOXICITY AND ECOSYSTEM HEALTH In general, toxicity testing involves determining the effect of various compounds on test organisms under set conditions. The terms LD50 and LC50 are both acute measures of toxicity. However, toxicity can also be measured in terms of non-lethal, chronic parameters such as an organism’s growth rate, fecundity changes and behavioural response changes.

Chronic Toxicity This term refers to long-term toxicity as opposed to sudden death resulting from a test compound. Chronic toxicity is much more difficult to diagnose and relates to longer term exposure to a specific compound. Continued chronic exposure can include adverse responses such as changes to spawning, metabolism or growth rates, or appetite, behavioural or reproductive changes. Because chronic effects are harder to identify, minimal work has been performed to date on the chronic effects of most pollutants, except in the case of human health (mercury for example). Chronic toxicity is often more subjective than a measurement of acute toxicity or LC50 or LD50. However the chronic toxicity effects of pollutants are now becoming much more important to maintain long-term ecosystem health.

An extensive listing of toxicological data has recently been compiled within the ANZECC guidelines, which list the types of compounds and the range of toxicity data available. In general, toxicity evaluation is time-consuming and very expensive. Acute Toxicity This term refers to a relatively short-term lethal or other effect, usually defined as occurring within four days for fish and macroinvertebrates and less for smaller organisms. Lethal dose50 (LD50) refers to the dose of a test compound, which kills 50% of the test population.

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particulate phase compared to the aqueous phase, in addition to the pH and concentration of organic matter. The more the compound is associated with the particulate phase, the less bioavailable it will be. The partition coefficient is the term which defines the ratio of the amount of particulate bound pollutant to the amount in the aqueous phase.

4.2.4 FACTORS INFLUENCING BIOAVAILABILITY AND TOXICITY OF CONTAMINANTS The following factors play a major role in determining the fate of any waste discharge to the aquatic environment. • Carbonate equilibria and effect on metals speciation - The presence of carbonate enables the formation of inorganic carbonate-metal complexes, as well as buffering pH which can have a major effect on metal speciation.

4.2.5 BIO-MONITORS, BIO-ACCUMULATION AND BIO-AMPLIFICATION Definitions

• pH effects on speciation - The lower the pH (ie. the more acid the water), the higher the proportion of a dissolved metal which is bioavailable or in the free ionic or weakly complexed state. If there are significant quantities of particulate-bound metals in the waterbody, a reduction in pH can leach metals from the particles into solution and thus alter the distribution (partitioning) of the metal between the soluble and particulate phases. • Effects of organic matter on complexation and speciation - Natural organic matter in aquatic systems can consist of large polyelectrolytic molecules with numerous binding sites of different polarities. Consequently, on a single molecule, numerous sites are available for binding metals and pesticides. The degree to which organic carbon partitions between the solid and solution phase also influences pollutant partitioning. High concentrations of dissolved organic carbon (DOC) can increase the solubility of metals and pesticides by stabilising and complexing these compounds into soluble aqueous complexes. If high suspended solids are present, DOC also binds strongly with sediment particles, and consequently detoxifies the adsorbed contaminant. DOC is critical in assessing the environmental fate of effluent containing metal and organic wastes. • Partitioning between dissolved and particulate species - Bioavailability is dependent on whether a compound is associated with the 32

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Bio-monitors are organisms used to determine the extent of pollutant transport and the extent of biological uptake of a pollutant. Bio-accumulation refers to the increase in a contaminant concentration within a particular organism or group of organisms, eg. liver of fish, egg shells of birds of prey. Bio-amplification refers to the amplification of the bio-accumulated contaminant through the food web from one organism up the trophic order. Organisms such as bivalves (mussels, oysters etc.) are sometimes used as bio-monitors because they filter large volumes of water and any associated metals and organic pollutants, thus bio-concentrating the actual levels of a pollutant within the water column. At this stage bio-­monitors can only be used reliably as indicators of the presence of a pollutant. Further research is required before the significance of any relationships between biomonitor and ecosystem health can be established. Whether a compound will bio-accumulate depends on a number of physico-chemical parameters such as the class of compound (eg. metal, organic pesticide), its concentration, exposure frequency and duration. Bio-accumulation also depends on the target organism, the compound of concern and its fate within the target organism. Many organisms have the ability to regulate pollutant levels in certain

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4.3.2 ASSIMILATIVE CAPACITY

parts of their body. Therefore identification of key organs (kidney, liver, adipose or fat tissue) are important considerations when interpreting bio-accumulation data.

Assimilative capacity refers to a waterbody's ability to absorb or resist changes brought about by the addition of a particular parameter. An example is that of buffering capacity, where high alkalinity waters are able to assimilate additions of low pH water with no adverse changes.

Bio-amplification is an extension of bio­accumulation where a contaminant which has been taken up by one particular organism or trophic level is passed on to higher order organisms - such as the case of mercury in fish which are then consumed by humans.

4.3.3 RECEIVING WATERS The type of receiving water into which wastewater is discharged is an important factor in determining the effect and ultimate fate of discharged pollutants. For example, the fate of metals discharged into a freshwater lake will be different to that of an estuary or ocean. Physical characteristics such as temperature, flow, pH, salinity, dissolved oxygen and light penetration determine the behaviours of a specific pollutant in the aquatic environment. The capacity of the receiving environment to dilute and assimilate the effluent stream is also of primary importance. These considerations should be evaluated prior to an effluent stream being discharged to a receiving water. Effluent streams of significance emanating from mining operations include sewage treatment plants, stormwater discharges from haul roads, waste dumps, workshop discharges and machinery washdown discharges containing hydrocarbons, or surfactants.

4.3 Nature of Waters This section outlines a number of additional concepts which are pertinent to the complete understanding of the properties of water. 4.3.1 BENEFICIAL USE Beneficial use refers to the designated uses of a waterbody. Examples of beneficial uses include: •

ecosystem protection;

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recreation - swimming, fishing, aesthetics;

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domestic and potable water;

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livestock watering;

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commercial fisheries; and

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irrigation.

Dischargers to waterbodies will generally be required to identify and meet a designated beneficial use. This may include the designation of a mixing zone.

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5. Water Sampling and Flow Measurement

5.1 Introduction

loggers, portable computers and associated software. Standard and uniform sampling and preservation procedures need to be used. If this expertise is unavailable within the organisation, consideration should be given to using a reputable and experienced consultant.

Water monitoring can be a very expensive and time consuming exercise and therefore the monitoring plan must be well designed before the program is implemented. Suggested planning steps are shown in Table 5.1

3. Execution of the program - The type of sample collection (eg. automatic or manual grab sampling), frequency, number of monitoring sites and phase (exploration, feasibility, construction, operation, decommissioning and after site closure) of the project should be identified within the initial planning stage.

In addition to these key steps, specific requirements of the National Water Quality Management Strategy need to be considered and the ANZECC (1992) guideline documents also need to be reviewed.

5.2 Principles and Purpose of Monitoring

4. Budget - Sufficient financial resources must be assigned to meet the objectives of the program, or else the program needs to be modified. Ideally, staff and financial resources allocated to a monitoring program should complement the scope of the program, and the sensitivity of the local environment. In circumstances where financial resources are limited, it is better to:

The key issues that must be addressed before the commencement of sampling and flow monitoring are listed below. 1. Reasons for monitoring - The objectives and purpose of the monitoring program must be established. Monitoring programs are usually implemented for compliance with an operating licence, to meet company or corporate policy requirements, for project design input data or for a baseline survey. Data from monitoring will also provide valuable feedback and corroboration of design data adopted. The program should meet the defined objectives.

• ensure that the samples collected are representative in both time and space; • restrict sample collection to key locations (including controls); and • review previously collected data to ensure unwarranted analyses are not requested. Finally, when allocating and revising financial resources, all the associated costs need to be incorporated. Expenses that are frequently neglected include:

2. Trained field staff - Personnel who collect meteorologic, hydrologic and water quality data should be skilled in hydrography, field flow measurement techniques and the fundamentals of water chemistry. The increased use of electronic field data also requires field personnel to be skilled in the use of data 34

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TABLE 5.1: Key Planning Steps for Water Monitoring

The Key Planning Steps 1. Identify the potential receiving waters and their beneficial uses. 2. Outline the site resources (personnel, financial) which are available for the monitoring program. 3. Locate and review the presence of any existing data, environmental audits and reports. 4. Identify all Local, State and Commonwealth statutory requirements which must be met by the operation. 5. Select a reputable laboratory which can advise on sampling methodology, containers, preservation and storage, etc. 6. Using a site plan, identify the physical and chemical properties of all likely point and non-point sources of pollution, the network and the catchment partitioning. 7. Design and implement a “screening” monitoring program to identify all sources and types of contaminants (eg. suspended solids, zinc, phosphates, E. coli) from each location. The screening program should include all surface waters, groundwater, industrial and domestic discharges, receiving waters etc. Control or background sites should also be identified and sampled. This program should be undertaken during dry and wet weather periods and the results reviewed in detail to identify contaminants which should or should not be analysed for a specific location. 8. Identify all monitoring sites which require flow measuring facilities (if contaminant loadings are required for water balance data, for catchment yield characterisation and rainfall/runoff parameters). Ensure a proper program is in place for physical measurement of flows for calibration and for validation of all recorded data. 9. Design and implement a calibration, quality control and quality assurance program with appropriate control sites, blank and duplicate samples, etc., and ensure detection limits are appropriate. 10. Ensure rainfall guages (and climate stations as appropriate) are in place for catchment rainfall/runoff characterisation. 11. Implement a site-wide sampling program and review the data once they are available. Parameters that have been measured below the detection limit can be sampled less frequently. 12. Review all results against statutory requirements. 13. Design an appropriate computerised database management system so that results can be managed and retrieved with ease.

• consumable costs (sample bottles, acid rinsing of sample bottles, labels, coolers, field clothing);

5.3 Compliance Monitoring

• costs associated with calibrating streamflow data, which requires qualified personnel manually undertaking a program of streamflow gauging; and

In the past, licence and discharge criteria varied frequently between the States. Recently, a more uniform approach has been taken with a move towards the ANZECC Water Quality Guidelines (1992)1, which consider both discharge limits and receiving water quality. This document should be reviewed in order to understand the existing national approach to water quality management in Australia.

• database development, data analysis costs (eg. computer facilities and employees’ time) and implementation of an appropriate data management system.

1 Under revision 1997-98.

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5.3.1 AMBIENT, POINT SOURCE AND NON-POINT SOURCE POLLUTION

5.4 Data Collection - Quality

Ambient concentrations generally refer to natural or background levels of water quality parameters within a receiving water. It is important to determine if the background values reflect actual natural conditions or a natural system which may have been modified over the past two centuries.

The resources allocated to environmental data collection will depend on the phase of the mining operation (ie. exploration, construction, operating, closure). • Baseline studies and associated monitoring programs should be implemented at prospective sites prior to the commencement of any major earthworks or infrastructure development.

Discharge or point source criteria refer to the concentration of a contaminant or parameter at the point of discharge (eg. an outfall from a wastewater treatment plant). The criteria may specify a mean value and a higher level not to be exceeded at a given frequency.

• The resource evaluation and feasibility phases usually involve the collection of meteorological and hydrological data, if no long-term data exist for the local region. Long-term time series data will improve techniques for optimising tailings dam design, surface drainage works, water supply and flood mitigation.

Non-point source pollution refers to a diffuse source rather than a single discharge point, eg. unconfined stormwater runoff from a minesite, workshop and maintenance areas. Contaminants from diffuse sources may be measured as a concentration (eg. Mg/L), but usually contaminant loading data (eg. kg/ha/yr) are required and both quality and quantity data must be collected.

• The construction phase generally involves expanding the monitoring program as staff and financial resources increase. A target monitoring program during construction is often necessary to measure the impacts of the construction activities. It also allows fine tuning of initial “screening” programs prior to full-scale operation.

5.3.2 MIXING ZONES When assessing compliance with receiving water quality guidelines, the “mixing zone” of the waterbody must also be considered. This is a region of the receiving water at which elevated levels of a contaminant can be present due to a discharge source, before dilution to an acceptable level. ANZECC (1992) defines a “mixing zone” as an explicitly defined area around an effluent discharge where certain environmental values are not protected. All relevant mixing zones, both within and outside a lease area, should be clearly identified. Monitoring programs and interpretation of data need to consider that these areas exist.

• The decommissioning phase and the extent and duration of monitoring will depend on the nature of the operation and the requirements outlined in the mine decommissioning plan, agreements and licences.

Control strategies should ensure that the area of a mixing zone is limited in order that the value of the waterbody is not prejudiced.

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• The operational phase will normally involve frequent monitoring of all point source (eg. sewage effluent, potable water, process and tailings dam water), non-point source (eg. stormwater from the plant area, landfill leachate) and receiving water quality and quantities (waterbodies within and adjacent to the mine and mineral processing lease).

5.4.1 MONITORING DESIGN Initially both a statistical evaluation of the monitoring design and a review of the procedures T

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be compared with guideline values such as those published by ANZECC (1992) and NHMRC (1994). Locating best positioned flow monitoring stations, relative to the monitoring locations required, can also be assessed as part of the initial screening program.

and techniques to be adopted should be undertaken. Once a preliminary plan is prepared, the logistics (eg. staff and financial resources) need to be reviewed. Development of the statistical design and validation of the sampling program, analytical methods and final data set need to be undertaken by personnel with appropriate expertise. The use of blank samples, unidentified duplicate samples and inter-laboratory testing should be incorporated as key components of the monitoring program.

5.4.4 SAMPLING LOCATIONS The selection of suitable sampling sites within and surrounding a mining operation should be based on the potential for a specific area, process or activity to have an environmental impact.

Electronically collected hydrological data from streams and rivers should also be validated using appropriate statistical procedures and manual gauging methods during low, medium and high flow flood events. Electronically collected rainfall data should be validated similarly.

Selection criteria for sampling and control sites are shown in Table 5.2 It should be noted that the conditions required for an acceptable control site for biological monitoring programs are generally more stringent and complex than a control location for chemical monitoring programs.

5.4.2 IDENTIFICATION OF KEY MONITORING PARAMETERS

Sufficient samples should be collected to quantify accurately the concentrations and behaviour of a compound from the time it is discharged through to the point where it can no longer be detected above ambient concentrations.

The monitoring parameters selected (physical, chemical and biological) will depend on the ore being mined at the operation, the process technology and chemistry, the geographical location and the beneficial environmental uses which need to be protected. It is important to identify all the key monitoring parameters early in the program in order to avoid possible delays at some later stage of the development.

5.4.5 SAMPLING FREQUENCY The frequency interval selected for the collection of samples for a water monitoring program will depend on the following factors:

5.4.3 INITIAL SCREENING PROGRAM

• statutory and licence conditions (eg. weekly, monthly);

Prior to commencing a full-scale monitoring program, it is worthwhile undertaking an initial screening survey at all potential monitoring locations within the project area to determine which parameters are relevant, significant and measurable above analytical detection limits. This should be done in conjunction with the statutory authorities concerned and the analytical laboratory. Multi-­element screening of water samples for total and dissolved contaminants on a selected number of samples is a cost-effective technique to identify parameters which should be incorporated into the site monitoring program. Results from the initial screening program should 1

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• size and geographic location of the mining operation; • distance and ease of access to sample locations; •

variability of natural and seasonal conditions;

• availability of staff resources to collect samples and process data; and • type of analysis.

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TABLE 5.2: Selection Criteria for Establishing Sampling Sites

Sample Sites

Control Sites

The selection of sampling sites within and outside the project area should reflect the:

Control sampling sites are an essential component of any water monitoring program. The location and number of control sites selected will depend on:

• beneficial uses requiring protection; • geographic location and the area potentially impacted by the operation;

• the geographic and topographic location of the operation;

• the nature of the operation and the type of ore/minerals/metal produced;

• the spatial coverage of the proposed monitoring program; and

• conditions of the licence agreement;

• financial constraints.

• access to sampling sites (all weather if required); and

It is essential that control and routinely monitored sites:

• budget and analytical constraints.

•

An overview of the "typical" monitoring sites that should be sampled at an operation are:

are in similar locations, preferably in the same catchment;

• are not influenced by past or current mining operations or other human influences;

• within or adjacent to areas of beneficial use; • the discharge point for industrial or domestic waste streams prior to entering receiving waters; • monitoring of receiving waters upstream and downstream of the discharge point or property boundary, if a mixing zone is identified in licence conditions;

• have similar geochemical conditions, ie. either carbonate systems or organic systems; and • have similar meteorological and hydrological conditions.

• monitoring of all impounded water including tailings dams, retention pond water, seepage ponds; • monitoring of groundwater downstream from contaminated sites, eg. dirty water ponds, hazardous waste sites; and • below the confluence point of major tributaries within the region. 5.4.6 SAMPLING TECHNIQUES AND DESIGN

thoroughly documented, and all persons using them are adequately trained in their use.

There are numerous methods by which a representative sample can be collected, with the final technique selected primarily dependent on the type of waterbody or waste stream requiring assessment. It is particularly important that the procedures used, and any changes to these, be 38

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Surface Water Sampling Sample collection of surface waters (sewage effluent, stormwater, tailings dams, streams and estuaries) can range from simple grab sampling E

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To facilitate the collection of high quality samples and data interpretation, field log sheets need to be completed at the time of sample collection. Examples of field record data sheets are presented in Fact Sheet No. 1.

techniques through to sophisticated automatic samplers, which have the capacity to collect both discrete or composite samples over a specified period. When surface sampling techniques are to be used the following should be considered.

The reader is strongly recommended to review published guidelines and texts for the collection and preservation of samples prior to designing and implementing a monitoring program. Examples of such documents are provided in the references section of this handbook.

• The sample containers used must be appropriate for the chemical parameter being measured (eg. acid washed high density polyethylene for trace metals, organic solvent rinsed glass bottle with teflon lid for organic compounds). • Before filling, rinse the sample bottle out three times with the water being collected, unless the bottle contains a preservative. Ensure clean hands are used as dirty hands may contaminate the sample (eg. cigarette smoke or residual ash will contaminate low level nutrient and metal samples). For trace metal samples, prevention of contamination is paramount, and special techniques such as the use of non-powdered latex gloves are required.

5.4.7 SAMPLE TRANSPORTATION The remote location of most Australian mining operations means that samples may need to travel considerable distances to the laboratory at which the analysis will be performed. Water samples should be freighted in portable “coolers” containing ice, as many parameters require storage at 40C prior to analysis. Samples should be placed in designated “coolers” to allow the separation of low-level control samples from high level effluent samples.

• Avoid contamination of the sample and disturbance of the waterbody being sampled. • Exclude air from the sample containers.

Some parameters (for example alkalinity) require analysis within 3 to 24 hours of sample collection and so, it is recommended that these analyses be performed at the mining operation using properly calibrated instrumentation and clean conditions. Others, such as pH, EC and temperature should be measured in the field. The remaining samples should be rapidly transported to the allocated laboratory if possible by same-day or overnight transport.

• Appropriate sample preservation techniques must be implemented immediately after sample collection (eg. filtration and addition of AR grade HNO3 for dissolved trace metals, temporary storage at 40C for nutrients). Note that sample holding times vary between 3 hours and 28 days for different parameters being analysed. • Ensure the laboratory and the analytical techniques used are NATA (National Association of Testing Authorities) registered.

Appropriate chain-of-custody forms must also be dispatched with the samples, clearly identifying all sample details and the required analysis.

Variations in sampling and preservation techniques, storage times prior to analysis and the analytical methods chosen all contribute to incompatibility of data. Considerable time and effort should be allocated to ensure that the samples collected, and the results obtained, are of a consistent high quality.

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5.4.8 SAMPLE ANALYSIS The selection of a laboratory is an important decision in the design phase of the program. It is preferable that the laboratory and the methods

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used for a specific analysis are NATA registered. NATA registration means that the laboratory has been inspected by personnel from the governing authority; the analytical method has passed stringent quality control procedures and the method has been used in inter-laboratory quality control programs. The results of these inter­laboratory quality control programs should be requested prior to commissioning long-term work to a specific laboratory. The inclusion of duplicate and blank samples within all sample batches sent to a laboratory is recommended. Feedback should be provided to the laboratory to identify and remedy problem areas in the analysis.

Pilot plant and laboratory studies can often be more closely and easily monitored than fullscale field studies, as samples can be collected more frequently and the time, travel and cost of collecting samples is significantly less. Examples include the use of leach columns to test the acid generation potential and leachability of tailings, waste rock and other materials stored in bulk.

It is essential that all aspects of a QA/QC program are discussed with the selected laboratory once the site screening program is complete and prior to the implementation of a long-term site-wide monitoring program.

Where laboratory and pilot plant tests are conducted, it is important that findings and conclusions based on these studies are verified in the field under full-scale natural conditions.

5.4.9 DATA MANAGEMENT Data management is an important component of any environmental monitoring program, as vast amounts of data can be generated within short periods. Data management should be incorporated into the initial planning stages of the program in order that the database may be used to meet the initial objectives of the monitoring program.

5.5 Data Collection - Quantity

The use of spreadsheets for data storage and management is often insufficient for most long-term environmental monitoring programs. A relational database is more applicable due to its capacity to store and easily process vast quantities of data. It also has the advantage of rapidly retrieving information for a specific purpose, such as reporting to government authorities. In most cases, existing hydrologic, water quality and meteorological data which are stored in a spreadsheet or ASCII format can be imported easily to a central relational database. 1

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5.4.10 LABORATORY, PILOT PLANT AND LEACH TESTS In some circumstances, laboratory bench scale tests can increase the knowledge about the behaviour and removal of a pollutant within a treatment plant, sedimentation dam or tailings dam.

As a guide, the QAQC component of monitoring and analysis should account for at least 10-15% of the effort (and cost) of the monitoring program.

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A relational database linked to a geographic information system (GIS) provides a particularly powerful tool for the management and interpretation of data. For example, geographic trends, such as downstream dilution of groundwater contaminants, are easily identified and readily appreciated by management when presented visually.

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When considering the data measuring systems for the volumetric water parameters such as rainfall, evaporation, and stream flow, the specified use of the data is the primary consideration in selecting the appropriate recording system. The following is an overview of appropriate recording systems and controls for various climate and water-related parameters and the various circumstances when each may be utilised.

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rated controls in streams, pipe monitoring systems and manually flow rating the streams. Regardless of the type of hydraulic control structure it is imperative that the following basic rules be followed in establishing the flow recording system.

5.5.1 RAINFALL READING There are two methods of recording data. •

Manual recording of rainfall collectors, eg. a standard rain gauge, on a daily basis. These data are useful for general interpretation of rainfall trends and long-term water balance analyses. The data can also be used to verify automatic recording rain gauges.

•

1. Select the monitoring location that will maximise the reliability of data recovery for the range of flows that will occur. This may require construction of hydraulic control devices such as a flume or v-notch weir. Where natural controls are selected they must be robust.

Automatic recording rain gauges, which have a calibrated tipping bucket gauge with associated electronic data recording logger. The advantage with the automatic system is its ability to record the time sequencing of rainfall events. These data are valuable for characterising the storm intensities for an area and for the establishment of the rainfall runoff response at the site.

2. Select the appropriate flow depth recording hardware for the monitoring location. Typical flow depth recording sensors include pressure gauges, sonic systems, float gauges and capacitance probes. 3. It is essential that flow monitoring stations be rated for flow and height. This may be undertaken using a hydraulic structure that has a pre-determined rating relationship. Where natural controls are used, it is critical that the flows are rated by physically measuring the flows through the control and relating this directly to monitored flow heights at the station. It is not sufficient to rate a flow monitoring station using only theoretical and analytical hydraulic relationships that require subjective assessments of coefficients (eg. Mannings equation).

An automatic recording system is relatively inexpensive to install, with power from localised battery or solar panels. These systems can manually download data to a computer or can be connected to a telemetry system for data capture remote from the site of installation. 5.5.2 FLOW RECORDING Flow recording in existing streams and waterways and future waste streams or diversion works is essential for comprehensive characterisation of the site hydrology and water management plan.

4. Few chances occur to collect time related data, and therefore it is critical that both reliable and appropriate monitoring equipment be installed. As vital development and strategic decisions depend upon the values recorded at these stations, the hardware monitoring and recording equipment must be of a high calibre. The following questions help with the selection of suitable instrumentation:

The critical areas where flow recording instrumentation is either required or desirable for developing site specific characteristics are: • at licensed discharge locations from the site; • at stormwater discharge locations around the site; • on existing streams both upstream and downstream of the site; and • selected catchments where flow monitoring will provide useful design data.

• Will the equipment be intact and record throughout extreme events?

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• Is the site accessible during flow periods for manual flow recording (for rating relationship)? M

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• How often can loggers be downloaded and is a telemetry system required?

to the Australian Standard 3778 - “Measurement of water flow in open channels” and all its associated sub-sections. Care must be taken that specific requirements for the location of the system and measuring devices are followed, otherwise inaccurate monitoring data will result.

• What is the potential for vandalism or damage by animals or large trees? • Have rainguages been installed at appropriate locations for characterising the rainfall/runoff response?

5.6 Groundwater

• Do personnel responsible for collecting the data and maintaining the station have the required levels of expertise?

5.6.1 GROUNDWATER MAPPING Groundwater mapping involves the identification and location of groundwater resources. A typical groundwater map contains contour information representing piezometric levels. Groundwater contours should be shown relative to an absolute datum (eg. AHD or a suitable mine datum) rather than relative to ground level, as the ground contours may bear no relation to groundwater levels.

5. Measured and recorded data must be validated to ensure the data is correctly presenting the conditions being measured. The validation must take place as soon as possible after it is collected and should check: • that the data recorded are realistic; • any malfunctions in instrument recording; and • the calibration data.

Figure 5.1 shows a typical groundwater surface map.

Validation processes involve processing the raw data into physical outputs (height and flow), checking compliance against similarly recorded data, verifying where the data fall within the calibration limits and scanning the data for anomalies and unrealistic outputs.

Groundwater flow is always from a region of high water level or piezometric level to a region of low water level or piezometric level (see Figure 5.1). The following steps are required to construct a groundwater map. • Groundwater “borders” should be determined (eg. rivers, lakes, oceans and significant changes in types of soil and rock). Where practical,

For the installation and operation of flow monitoring systems, reference should be made

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then be used to derive groundwater parameters related to these events. These parameters allow calculation of quantities such as drawdown for various pumping rates, rates of recharge or speed and direction of contaminant flow.

mapping should include the entire groundwater resource as well as its borders.

• Observation bores or piezometers (see Section 5.6.2) should be installed in a relatively regular grid pattern over the area of interest. Piezometers should be located such that the difference in water levels between adjacent piezometers is less than the planned contour interval of the map.

Prior to establishing a groundwater testing program, hydrogeologists and analytical laboratories should be consulted to determine the appropriate testing, sampling and storage methods required for identification of individual compounds in the groundwater. Samples may need to be gathered and stored in non-reactive containers to ensure that they are not contaminated. Special care may be required for biologically active contaminants.

• Ambient groundwater levels should be measured at regular temporal intervals to identify seasonal fluctuations as well as responses to rainfall and periods of drought. Care should be taken to gather ambient data well before activities such as pumping are commenced.

Groundwater levels and quality may be monitored using piezometers. Piezometers extending into unconfined (water table) aquifers show water levels which represent the surrounding water table level. Piezometers extending into confined aquifers show water levels which represent the pressure existing within the aquifer. When there are strong flows within the aquifer, a component of the measured pressure may result from inertial forces as well as static groundwater levels.

• Interpolation packages available for computer simulation of contours may be used to generate maps from gathered data. Each map should be a snapshot of groundwater levels for the relevant period of monitoring. 5.6.2 TESTING AND MONITORING Groundwater testing and monitoring is carried out to establish water quality and changes in quality, and water levels and changes in levels. Testing and monitoring should be undertaken for ambient or pre-existing groundwater reserves to establish baseline groundwater characteristics. Testing and monitoring subsequent to events such as pumping, recharge and contaminant leakage can

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Figure 5.2 indicates the water levels given by piezometers in unconfined and confined aquifers.

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becomes impractical, the well may be capped and a pressure transducer installed within it;

Piezometer Construction A piezometer is simply an open stilling well into which a probe may be inserted to measure water level or quality, or from which a sample of the groundwater can be collected. Piezometers are primarily made of either PVC (polyvinyl chloride) or ABS (acrylonitrile butadiene styrene).

• wells should be slotted or screened to facilitate a good connection to the aquifer. Open-bottomed, unslotted wells may be used effectively in granular soils; • slotted wells often form the cheapest alternative, as slots may be machined by the manufacturer or cut by hand on site. Slots should be cut liberally (either horizontally or vertically) but should be small enough to exclude significant intake of soil. Porous geotextile fabrics may be used to filter out soil particles if required;

The material chosen for piezometer construction should have strength, rigidity, low maintenance, resistance to galvanic and electrochemical corrosion, resistance to abrasion, high strength-to­ weight ratios, partial flexibility and low cost. Other considerations are:

• prevention of contamination is critical for the collection of water quality data; the installation of slotted or screened casing will be important. In these instances, a hydrogeologist should be consulted to provide appropriate well designs;

• piezometers may be installed using a variety of means from hand augers to drilling rigs. In all cases, the piezometer tube is installed after drilling a hole of sufficient diameter and depth; • the diameter of the piezometer used depends on the type of monitoring or sampling that needs to be carried out. The sizes of probes and sampling devices need to be considered. It is rare to find piezometers of less than 50 mm in diameter, and 100 mm diameter piezometers are common; • the length of the piezometer needs to be sufficient to measure the maximum possible drawdown;

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• a tether wire and concrete collar serve to anchor the piezometer and reduce the risk of slippage in unconsolidated material or accidental movement from outside impact; and • the lip of the piezometer should be surveyed into the mine datum or Australian Height Datum (AHD), as this is the most convenient point of reference for manual monitoring.

• when monitoring confined aquifers, the well may need to protrude significantly above ground, in order to measure the standing head of the water. However, if this protrusion

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• piezometers should be capped at the surface, preferably with a screw-in cap for ease of removal and re-application;

Figure 5.3 shows a typical piezometer installation.

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Remote monitoring carries a much higher risk of data contamination or error. A rigorous schedule of equipment maintenance, data verification using manual methods, and frequent calibration checks should be in place.

Monitoring Monitoring of piezometric levels may be performed manually or remotely. Manual devices include: • dip meters: these comprise an electrical sensor at the end of a graduated wire. Contact with water completes the electrical circuit between sensor and wire, causing a tone to be emitted (see Figure 5.4). The distance between the sensor and the reference point (eg. the lip of the piezometer) may be read off the graduated wire. Dip meters are popular because of the ease and speed of use; and

Groundwater Sampling Testing of groundwater quality may be carried out using in-situ methods or by the extraction of a representative sample. A range of field equipment exists for measuring such basic parameters as pH and conductivity, using probes which may be lowered into piezometers.

• graduated transparent piezometers or manometers (when the piezometric level is above ground).

Groundwater samples are normally collected from a piezometer or bore using one of two techniques: a bailer or submersible pump. Submersible pumps powered by a battery or generator are preferred due to the large volumes of water that need to be displaced from a bore prior to the collection of a representative groundwater sample.

Remote monitoring is carried out using a sensor installed within the piezometer. The sensor may be connected to a central monitoring system or to a data logger which reads, at regular intervals, the voltage output at the sensor. The data logger may be downloaded regularly using a portable computer, or may have removable memory banks which can be replaced and downloaded later. The recorded voltages are then translated into water levels via calibration relationships. Popular sensors include:

In addition to these two methods, groundwater samples may also be collected from sample valves located near aboveground pumps on water supply bores. When groundwater samples are to be collected, the following should be considered:

• pressure transducers;

• the piezometer or bore needs to be purged prior to sample collection. This technique must be used in order to obtain a representative

• capacitance probes; and • float levels.

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groundwater or aquifer sample. In the absence of extensive pumping, the sample collected will merely represent water held in the bore or piezometer which has been exposed to atmospheric conditions. Extensive pumping also reduces cross contamination of the sampling equipment between bores. Typically, three times the volume of water held in the piezometer or bore needs to be removed prior to sampling;

• if a bailer is used then extensive bailing of water held in the bore must be undertaken prior to sample collection. Most bailers only have about one litre capacity and consequently manual bailing of a bore can be a time consuming procedure. If sufficient funds are available, disposable bailers should be considered to eliminate the risk of sample contamination between bores; and

5.6.4 PREDICTION OF GROUNDWATER CHARACTERISTICS AND RESPONSES Prediction of aquifer responses to various scenarios allows “what if ... ?” questions to be answered. Predictive modelling may be carried out using analytical models (simplified equations) or, more recently, numerical models which use the technically rigorous and complex physics of groundwater flow. Numerical models have developed significantly in the last two decades and their popularity has increased. A brief discussion of the types of numerical models is presented in Fact Sheet No.12, and advantages and disadvantages of using numerical models are summarised in Table 5.3. Predictive modelling in groundwater now enjoys widespread use and offers significant benefits in assessing groundwater-related issues. An increasing environmental focus in the mining industry and the recognition of groundwater as a fragile natural resource has seen the expanding use of groundwater models. Models simulating contaminant transport in groundwater and rootzone behaviour are now widely available.

• appropriate sample containers, rinsing procedures and preservation techniques must be used, as for surface waters. 5.6.3 GROUNDWATER PARAMETERS Physical and chemical parameters are of interest when attempting to characterise and model aquifers in order to simulate various scenarios. Groundwater parameters are best obtained by stressing the aquifer and observing the response induced. These stresses are typically obtained by pumping water out of the aquifer or pumping water into the aquifer via bores.

Predictive modelling should always be used with a questioning attitude, and a rigorous process of calibration, verification and sensitivity analysis should be an integral part of any modelling program.

5.7 Review of Monitoring Data

A large range of pump tests and analytical methods exist for this purpose. Advice from qualified hydrogeologists should be sought to determine: • which parameters are of interest; • cost-effective methods of obtaining this data; and

For a vast majority of existing monitoring programs, insufficient time is spent actually reviewing and analysing the data. Regular screening of data can detect problems in sampling and analytical techniques as well as in hydrographic data recording systems.

• the applicability of these methods to site-specific conditions.

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TABLE 5.3: Advantages and Disadvantages of Using Numerical Models

Primary Advantages

Primary Disadvantages

• Ability to run complex and lengthy calculations in increasingly short times as computers evolve rapidly;

• Initially high level of labour intensity during setting up a numerical model; • development of a ‘black box’ mentality which results in the widespread use of models without understanding of concepts and limitations;

• a low level of labour intensity during simulations;

• a tendency among the public to perceive models as infallible and acceptance of results as the literal truth; and

• high capacity for testing the sensitivity to groundwater parameters; • the development of increasingly visual outputs, which allow the lay person to understand the answers proposed by the models; and

• high capacity for misunderstanding or misuse of models because of their complexity.

• flexibility in assessing a range of scenarios quickly and easily.

A review of the data set can establish seasonal trends and will detect analyses that are unwarranted (ie. those continually below the detection limit). Sites with data that do not fluctuate to any degree can be sampled less frequently to reduce costs.

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Regular review will also forewarn management of any impending changes which may effect the sites ability to obtain or discharge water or any breaches in compliance with statutory obligations. Presentation of data in a graphical format allows easy scanning of large numbers of results and identification of trends in the data.

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6. Water Supply

In a country as arid as Australia, mining and mineral processing operations will almost certainly require a regular supply of water. Therefore, identification, evaluation and maintenance of this supply will be critical to the continued operations. While this topic could demand a handbook of its own, some concepts will be introduced in this section.

6.1 Surface Water This section examines sources of surface water supply around typical minesites. 6.1.1 CATCHMENT YIELD

Rainfall runoff: the quantity and quality of rainfall runoff will be dependent on the catchment area soil type, topography and vegetation. A discussion on estimation of rainfall runoff is given in Fact Sheet No.2. Groundwater seepage: during periods of rain, a percentage of the water will seep into the ground as infiltration. Some of this water will percolate into groundwater stores. However, on sloping sites or areas underlain by shallow rock, most water will flow through the soil profile to the bedrock and percolate out into a watercourse or cutting. This water will continue to flow long after rain has ceased. Mine dewatering: surface and groundwater reserves that flow into mine workings are usually pumped out to a suitable storage. This aspect is covered in Sections 8 and 9.

When discussing the useful yield of surface water within a catchment it is important to realise that it can never be any greater than the facilities available for storing or continuously using water. This can include groundwater recharge, as discussed in the next section.

Outflow Outflows will result from any combination of the following.

The balance of processes contributing to the final yield at a given storage facility can be represented as

Releases: resulting from:

Yield = Inflow - Outflow.

• excess water overtopping storages and passing into the next catchment or off the lease;

Inflow The inflow into a storage may originate from any of the following sources. Imported water: reservoirs, irrigation schemes or major supply pipelines are often the major source of water for minesites in Australia. Recycled water: most minesites in areas of water scarcity are now recycling water from various stages of the mine process. This is discussed in the following section.

• water drained from darns to allow for maintenance, to make room for expected inflows or as regulated to provide water for downstream ecosystems or users; or • treated water which may be released after sufficient residence time to remove pollutants (eg. acidity, suspended solids, salinity).

Direct rainfall: within shallow storages covering large surface areas, the amount of direct rainfall may be appreciable. 48

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Evaporation: the loss of water from reservoirs through evaporation is appreciable in many regions of Australia. Where water supply is a critical issue, it can be worthwhile attempting to reduce evaporation by the use of a deeper storage or various cover techniques. Evaporation is also often used as a disposal method for highly saline or otherwise polluted waters.

It is usually a more environmentally sound practice to recycle lower quality water on a minesite rather than to discharge the water and use better quality water from clean supplies when it is not needed. Some examples of sources and uses of recycled water are given in Table 6.1.

6.2 Groundwater

Water use: this will depend on the location of the storage, the quality of the water and the scarcity of water on the site. Other potential users of the water must also be considered. A number of ideas for recycling water are presented in Section 6.1.2.

6.2.1 SOURCES OF SUPPLY There are two primary sources of groundwater supply; unconfined aquifers and confined aquifers. Perched water tables (see Fact Sheet No. 11) are a special form of unconfined aquifer. Unconfined aquifers may be used for water supply via the pumping of bores. Confined aquifers are generally under pressure and, in some cases, may not require pumping to extract water (eg. a flowing or artesian bore).

Seepage: although seepage through the ground has been identified as an inflow it is also an outflow mechanism. Any dam is likely to lose some water through seepage into the groundwater unless the groundwater level is higher than the base of the dam. In earth darns (as most minesite dams are) seepage may also occur through the dam wall.

Individual groundwater resources tend to be compartmentalised by geology, but are rarely truly isolated. Despite some connection to other aquifers, an individual groundwater resource should be viewed as a finite body of water. Replenishment of groundwater (or recharge) is a vital component in assessing the long-term viability of a source of supply. Recharge may occur through rainfall infiltration, or from rivers and streams, or from artificial recharge (such as pumping of surface water into aquifers).

If considering the yield of a specific catchment, it will be necessary to obtain specific information on all the above processes relevant to that catchment. Historical records of inflows and outflows will provide invaluable information for the calculations. The water balance method for identifying the inflows and outflows is a useful tool for understanding how the water supply for a minesite may be achieved by considering all the potentially contributing elements. The water balance allows the user to optimise parameter values for the most desirable outcome and to explore the probability boundaries when variations are introduced (refer also to Fact Sheet No.3 for probability information).

6.2.2 SECURITY OF SUPPLY Security of supply may be breached if the sustainable yield is compromised when a bore is overpumped or drawdown is quick but recovery slow. The quality is compromised when pumping stresses lead to dissolution of salts from the soil matrix and excessive salinisation of the pumped water or development of flow paths from neighbouring contaminated aquifers.

6.1.2 RECYCLING OF WATER Most minesites promote the use of recycled water. Recycling often occurs when water is scarce, or the discharge of polluted waters could be a hazard to the surrounding environment. Even where water is freely available, it may be more cost-effective to recycle water. 1

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TABLE 6.1: Sources and Uses of Recycled Water

Sources of Recyclable Water

Uses for Recycled Water

Dirty mine water: surface runoff from dirty areas, intercepted to remove suspended solids and/or other pollutants.

Dust suppression: dust control for haul roads, conveyor belts and transfer stations, loading facilities, dump hoppers, stockpiles (product and waste), construction sites and working faces does not require high quality water. Issues which may affect this are:

Clean mine water: there will be some limitations on the amount of water which can be intercepted from undisturbed areas. This is to ensure that downstream users and ecosystems are not disadvantaged.

• viral and bacterial micro-organisms which, if present in fine aerosol mists, are easily ingested by workers; and

Process water: most process plants or washeries will use large quantities of water which is often returned to a process water tank or dam, and then recycled back through the process.

• nutrient levels which can promote algal growth and block spray equipment.

Tailings liquor: tailings are deposited with varying percentages of water to allow pumping, and to ensure proper deposition and drying. Excess water remaining after solids have settled can be recycled directly or after passing through a filter dam.

Process water: processes which involve crushing, washing and screening are suited to using recycled water. Co-disposal tailings will utilise recycled water. Typical quality issues are:

Washdown water: vehicle and workshop washdown water should be passed through a settling pond and oil separator, after which it may be suitable for selected recycling.

• chemical make up of the water; and • suspended solids. Irrigation: rehabilitated areas, gardens and perhaps even neighbouring properties or stock may be a very efficient use of wastewater. Irrigation to rehabilitated areas may result in water dependant regrowth with shallow root systems which will struggle to survive if irrigation ceases. Water quality issues are:

“Grey” water: wastewater from showers, hand basins, laundries and kitchens should be treated to remove solids and can then be recycled. Chemical dosing (eg. chlorine) may be necessary if people will come into contact with the recycled water.

• chemical, salinity and pH extremes which may adversely affect plants and/or stock;

Treated effluent: package or site built treatment plants are used to treat sewage to acceptable levels after which it can be used for limited recycling applications. Treated industrial effluent from workshops may also be used for recycle water.

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• suspended solids, which may block pumping and spraying equipment;

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• suspended solids (as for dust suppression); • viral and bacterial micro-organisms. Wetlands maintenance: during rainy periods there will usually be enough dilution and flushing to keep wetland systems healthy. However during dry periods there may be a build up of pollutants from mine dewatering or simply a shortage of water. Quality issues are similar to those for irrigation.

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TABLE 6.1: Sources and Uses of Recycled Water (CONTINUED)

Sources of Recyclable Water

Uses for Recycled Water

Slurry transport water: at the end of a slurry pipeline, the slurry is dewatered, leaving large quantities of water. The location will often be environmentally sensitive, hence the water would require treatment to high standards before discharge; re-use may be a better option.

Washdown water: recycled “grey” water and treated wash down water can be used for washdown of mine equipment and workshop areas. Quality issues are: • build ups of oil or detergents; • viral and bacterial micro-organisms which if present in fine aerosol mists are easily ingested by workers. Potable water: in very arid and remote areas it may be viable to treat recycled water to very high levels and use it as a potable water source. Clean and dirty water runoff are obvious sources, but other sources can be used. All facets of water quality will obviously be vital if this is the intended use.

Constant monitoring of quantity and quality is an integral part of water supply evaluation and maintenance.

Sustainable yield is a significant parameter in water supply. It determines the maximum flow which may be extracted over the long term. This factor is determined by pump testing and analysis of drawdown. Borefields of two or more bores will incur some penalty in the sustainable yield of each bore because of interaction between the drawdown from each bore. More intensive analyses are required to identify the sustainable yields of borefields. The sustainable yield should be identified whenever bore water supply is considered. Expert advice should be sought before commissioning a bore drilling program.

• Quantities of pumped water should be noted throughout the life of a bore. Flow totalisers are a convenient and cheap method of monitoring quantity. These show the total volume of water pumped. When monitored regularly and used together with a record of pump down time, adequate information on pump rates may be gathered. • Aquifer drawdown should also be monitored on a regular basis. This may be done using adjacent observation bores and, where possible, within the pumping bores themselves.

The quality of water pumped out of a bore may depend on the rate of pumping exerted. The sustainable yield of a bore should be identified in conjunction with any deterioration in the quality of water being pumped. The likelihood of quality deterioration may increase with the rate of aquifer pumping. For example, in coastal locations seawater may migrate towards a bore which is pumped beyond its sustainable yield.

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• Water quality monitoring should be carried out regularly on representative samples pump from bores. Relevant water quality standards should be consulted, depending on the use of the supply. These may be for potable water, ablution water or process water. Site-specific process water requirements should be determined where the water is used for processing.

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If a licence is required for the bore or borefield, conditions such as these are generally included on the permit. The information gathered usually has to be provided to the licensing authority on renewal of the permit. The intensity of the monitoring program selected for water supply bores should reflect the importance placed upon the supply.

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7. Exploration

7.1 Surface Water

Water is an important component in exploration activities and therefore careful management is necessary as in any other aspect of mining and mineral processing.

Most exploration activities in Australia will be in areas where minimal knowledge of ground and surface water behaviour exists. Therefore the collection of all possible information that may be relevant is encouraged. At the same time, care of the existing environment is required.

A lack of water for process, potable and fire protection requirements or an excess of water (eg. high groundwater table, large aquifers, flood risks) can determine the subsequent economic viability of a mining project. Therefore serious consideration must be given to water constraints during the early exploration phases of a project. This should include data gathering of both surface water and groundwater resources as well as initial flood studies.

7.1.1 SURFACE WATER DATA COLLECTION The lack of water or the possibility of serious flooding may seriously impact the extent or timing of an exploration program. Information on rainfall, evaporation and stream flows in the project area is often inadequate, and important decisions are usually made using data extrapolated from many kilometres away. Exploration teams can provide important data to reduce the risk associated with these decisions.

The environmental significance and sensitivity of watercourses and other waterbodies (surface or ground) will determine the extent of exploration and subsequent mineral extraction allowed in any area. This will be dictated by the relevant legislative body (ie. Mining, Environmental and Water Resources departments) at both State and Commonwealth level.

Records should be kept of local surface water conditions. This can include evidence of previous flood heights through the location of debris and local knowledge, conditions of watercourses (ie. flowing regime, photographs), signs of erosion, and quality of water. Monitoring water quality will also provide valuable background information, which may form an important part of future license conditions.

Water will also play a role as a resource and/or hindrance to the actual exploration efforts. Rivers, streams, rainfall runoff and groundwater all need to be managed to avoid or minimise damage during exploration. Many exploration activities could be considered as miniature minesite operations; hence all sections of this handbook will be applicable, albeit at a modified level.

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If a new deposit has high potential and continuing exploration is likely, a remote weather station network as well as stream gauges in all major watercourses should be established. These installations should measure rainfall, temperature, wind speed and direction, evaporation and stream flows. A few years of local climatic data between

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the time of initial exploration and the stage of feasibility decisions will provide invaluable assistance in the design of water supply dams, tailings dams, evaporation ponds and any flood mitigation or mine drainage works required (refer to Sections 5.4 and 5.5, and also Fact Sheet Nos 3 and 10). 7.1.2 ACCESS TRACKS

disturbed areas will impact the undisturbed drainage line; and – keep tracks a reasonable distance away from watercourses to ensure a vegetation strip is maintained. • When constructing tracks: – avoid using heavy earth moving equipment to construct temporary tracks, as this will destroy root stock;

Exploration projects which cover a large area with many drill holes in different locations will often result in a “spider web” of access tracks linking the different sites. The clearing and constant traffic associated with such drill lines and access tracks can lead to serious erosion and sediment problems if precautions are not taken to minimise their impact. The construction and rehabilitation of access roads is dealt with in Section 6.8 of AMIC (1990), while the following points provide guidelines for reducing the impact of tracks on surface water.

– culverts are recommended for creeks and streams on more permanent tracks. These will reduce mud and keep tracks passable in most weather. For guidelines on the design of culverts, refer to Fact Sheet No.6; – runoff should not be allowed to concentrate on tracks. Flow should be shed off the road as quickly as possible by using reasonable crossfall (say 3%) side drains with regular take-offs and by allowing sheet runoff to flow uninterrupted across the track. Where road access cuts across steep hillsides, road stability may necessitate sloping the cross fall into the hill slope and into a side drain, which then discharges via a constructed drain built at a low point under the road or across an armoured road crossing;

• Minimise the area of disturbance by reducing the number of tracks and using the same routes (even if the journey takes slightly longer). It is also very important that four wheel drive vehicles remain on existing tracks whenever possible. • When locating tracks: – every effort should be made to minimise clearing and other disturbance to vegetation, especially in well vegetated areas with easily eroded soils (eg. wet tropical areas). Tracks should deviate around large trees; where this is impractical, use the timber to stabilise edges and low points;

– if it is necessary to cut roads greater than 2 m wide into the natural surface, then small v-type interception drains should be used to divert water from the batter slopes. Generally batter slopes should be no steeper than 2H:1V (0.75H:1V in rock); and

– avoid using gullies as convenient locations for tracks;

– any discharge points for culverts or table drains must be protected against erosion.

– locate creek crossings in naturally rocky locations, or line sensitive or erodible crossings with rocks;

• Ensure all tracks to be used are located on field maps and that all personnel are instructed to use only those marked tracks. This will reduce people’s desire to create their own tracks and hence minimise disturbance.

– avoid permanently wet and boggy areas; – install silt fences or hay bales across watercourses where sediment from

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• the downstream or lower side of any cleared area should be arranged so as to intercept and contain sediment washed down by surface runoff or concentrated discharges. This is easily achieved by the use of interception drains and silt fences, hay bales, silt traps or filter dams, as described in Fact Sheet No.8; and

7.1.3 EXPLORATION SITES On any given project, the area physically disturbed will be reasonably small and control of erosion, runoff and discharges from these areas is relatively straight forward. Guidelines for minimising impacts on water include: • a buffer zone should be kept between the exploration activities and environmentally sensitive areas. The width of this zone will depend on the sensitivity of the area and may range from 10 m for a non-sensitive bank of a watercourse up to 3 000 m or greater for an environmental conservation zone;

• dams or diversions to watercourses should be thoroughly investigated to ensure any adverse effects are minimal. They should also be designed, constructed and maintained to ensure good water management (Fact Sheet No.5). It is important to advise the relevant Water Resources department in any State before undertaking such works. Dams which retain large volumes or which could risk life and property in the event of failure will often require licensing and much stricter design standards.

• as with access tracks, the area and degree of clearing should be kept to a minimum; • the discharge of wastes into watercourses must be avoided. Various waste can be handled as follows:

Exploration within a watercourse or riparian zone has the potential to severely damage the surrounding environment and hence will require more rigorous control than described above.

– fuel and oil storage tanks and dispensing areas must be bunded and sealed. Oil absorbent booms should be used across storm water drainage points away from these areas; – sewage should be treated to recognised levels using septic systems or commercially available package treatment plants or contained and removed from site; – toxic and saline wastewater must be stored in ponds either permanently or until treated or degraded to safe levels; and – sludges and silt resulting from drilling or processing operations must pass through sumps to settle or filter out fines before the water is discharged;

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8. Open Cut Mines

8.1.1 FLOOD MITIGATION

Management of surface and groundwater flows around open cut mines is critical to safety and the operation of the mine. This is a specialist topic and detailed design and engineering should be undertaken by relevant experts.

There are numerous factors which dictate the type and extent of flood mitigation works best suited to a particular site. Every mine will have a different set of conditions; hence only the major issues will be covered in this handbook.

However, the environmental officer may play an important role in tasks such as:

Type of Flooding

• providing the base data to determine the likelihood of an event; • routine monitoring to evaluate the performance of the control structures; and

Before considering any mitigation works, the extent of flooding likely to occur naturally should be estimated. This should include conservative estimates of the following:

• advice on the best means of disposal of excess waters.

• total volume of surface runoff entering the pit (Fact Sheet No.2);

Consequently, it is important that there is close consultation between the expert and the officer charged with site management responsibilities.

• the peak rate of flow into this pit (Fact Sheet No.2); and • the major drainage paths by which water enters the pit.

The following section provides some basic information to assist the environmental officer in understanding some of the specialist hydrological engineering issues.

Safety This is the highest priority in mining and the possibility of injury or death due directly or indirectly to pit flooding will be the primary determinant of flood mitigation measures.

8.1 Surface Water Runoff

Economics

Flooding of open cut mines can be a very real problem if a mine is located in a valley or in the path of a stream or a river with a significant upstream catchment. Depending on how quickly it occurs and how severe it is, flooding can cause a variety of problems such as loss of life or injury, damage to machinery and infrastructure and, far more likely, loss of access to the pit due to water and silt and subsequent loss of production. All of these scenarios are highly undesirable to mine operators.

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If safety is not a deciding factor, a cost/benefit study should be carried out. For the proposed schemes, the capital and annual maintenance costs should be added to the residual costs due to annual flood damage (eg. the costs incurred when the scheme fails). The scheme which gives the lowest total cost will then be the most effective solution. This approach is illustrated in Figure 8.1. It is rarely practical to eliminate totally the risk of flooding and hence protection of the flood R

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mitigation works against overtopping damage should also be considered.

habitat, it may be better to build an upstream flood control reservoir than widen the channel.

Pit Location

Maximising Waterway Capacity

The location of the pit in relation to the catchment will determine whether a particular scheme is feasible, ie. a pit at the bottom of a steep valley will have fewer alternatives than a pit located in a wide gently sloped flood plain.

The intent of this method is to optimise the ability of existing rivers, streams or drainage channels to carry flood waters away from the pit. This can be done by: • altering cross section - increasing the cross section size will give a greater flow capacity Note that, if the existing waterway is prone to erosion, the channel should be made wider only. If the existing waterway is prone to silting the channel should be made deeper only (Take care that the existing system does not incorporate both erosion at high flows and silting at low flows.) Impacts on downstream unaltered sections must also be assessed;

Appropriate Risk The level of risk (of failure) associated with a given flood mitigation scheme is linked to both the safety and economic issues. When deciding at what level of risk to design a scheme, an important consideration is that a very low level of risk (ie. failures are very rare) may lead to a lack of contingency planning such that when a very large flood occurs the results may be disastrous.

• upstream erosion protection - a reduced sediment load can prevent clogging problems in the lower reaches of a waterway. This can be achieved by protecting steep sections (usually the upper reaches) of a stream against erosion, using methods such as drop structures, check dams, bottom sills, vegetation and channel armouring (Fact Sheet No.8); and

8.1.2 METHODS OF FLOOD MITIGATION There are many flood mitigation methods available to the mining engineer. Each method has different environmental impacts and these should be addressed as part of the design criteria. For example, if the waterway is a valuable riverine

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• coarse sediment traps - another method of reducing sediment load in flows is to create a coarse sediment trap upstream of the area to be protected against flooding. This can consist of a wide shallow pond or flood plain area which will allow the same flow to pass at a much lower velocity, hence allowing sediment to settle. It is important to note that sediment traps require regular cleaning to maintain their performance. Dykes Constructed embankments either side of a natural waterway can give a large increase in flow capacity. The final capacity is determined by the height of the embankments and their distance apart. Where space is available it is better to have low embankments spaced far apart. This configuration will be cheaper, safer and result in less erosion. For a meandering stream the dyke system should form a band which envelopes the stream (Figure 8.2). Upstream and downstream impacts of these structures must be assessed.

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Flood Control Reservoirs If the catchment upstream of the pit is steep and subject to short heavy storms, it is likely that flooding will be short in duration and have a high peak flow (refer Fact Sheet No.2). In this situation a useful method of flood control is to attenuate this peak flow (eg. temporarily hold back some of the flood water until the peak flow downstream has passed, and then release it at an acceptable rate). The simplest method of achieving this is to build a dam or basin with an open outlet at the base to gradually release the intercepted water. Flood Diversion If it is feasible, the most effective way of flood mitigation is to divert water away from the mine. Diversion channels can direct water to a number of different points, such as: •

same waterway downstream;

• adjacent flood plains; or • nearby lakes or streams.

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sized culverts should be installed (refer to Fact Sheet No.6). Inexpensive and re-useable corrugated steel pipes (Armco culverts) are suitable; however attention must be paid to installation and cover requirements.

If a diversion method which returns flow to the same waterway further downstream is used, it is important to assess any backwater effects. For example, a sudden increase in flow downstream may cause the waterway to back up and flood the pit from the downstream end.

Sumps

8.1.3 IN-PIT DRAINAGE

The size and configuration of sumps will vary to suit individual conditions. However the following guidelines should be followed:

All open cut mines are likely to have water entering the pit and ponding at the lowest point. In most cases this water will need to be removed from the pit to avoid disruption to mining activity. The amount of water to be dealt with will depend on the area of the pit and access ramps (which will determine the amount of direct rainfall), the effectiveness of flood mitigation and pit interception drainage schemes (refer to Sections 8.1.1 and 8.1.4) and management of groundwater inflow (Section 8.2).

• for safety and convenience locate sumps away from trafficked areas; • incorporate the sump location at the mine planning stage to ensure floor slopes and seam slopes are accounted for; • if pumping out is used, locate the sump to give a suitable route for the pipeline to the required discharge point (Section 8.3.1);

The quality of water will, in part, depend on the residence time in the pit. Water may be exposed to mineralised or acidic material and become contaminated, or may contact spilled hydrocarbons. In both cases treatment may be necessary prior to release. Rapid disposal of in-pit water will limit the problem.

• locate the sump to give maximum life before pit development dictates a new location; • due to typically high sediment loads in in-pit runoff water, the sump should ideally have at least two cells. The first cell will allow the silt to settle or be filtered out of suspension and should be easily cleaned by in-pit equipment; and

Drains

• the size of the sump does not necessarily need to cater for the total flow into the pit but rather should be located such that all water eventually drains into it (ie. once the dewatering system catches up with the inflow).

Design criteria will need to consider: • the main access ramp into the pit must be kept trafficable. Hence ramp side drains should cater for high peak flows; • drains on the pit floor must be kept away from main traffic routes. This saves the drains from damage by large vehicles, keeps the pit accessible by small service vehicles (eg. surveyors) and avoids mud on vehicles;

Dewatering Options Three commonly used methods to dewater mine pits are pumping, shaft and tunnel, and slot drainage. If water discharging from the pit is not retained, the impact of variable flows and water quality on the downstream surface water or groundwater bodies will need to be considered. Water disposed of in these ways may need to be monitored continuously as its quality will be affected by the length of contact with mineralised zones in the pit.

• where possible, drains should be maintained at a slope between 1% and 3% to avoid silting and erosion problems; and • drains which cross major traffic routes should be hard lined “swayles” (wide shallow ‘v’ drains). If large flows are expected then correctly 1

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drain around the pit. If the pit is at the bottom of a natural bowl this technique is ideal. It is usual to design these drains for a 20 Year ARI or an ARI to suit the acceptable risk for the open cut (refer to Fact Sheet No.3). For the design of open channel drains refer to Fact Sheet No.4.

8.1.4 INTERCEPTION DRAINAGE AROUND PIT Where open cut mines do not have flooding problems there will usually be some runoff towards the pit from the immediate surrounding areas (Figure 8.3). If this water enters the pit it may be exposed to acid generating rock not present on the surface and will also necessitate larger pit pumps, generally causing inconvenience and delays to in-pit operations. Therefore interception drainage should be installed around the pit.

Gully dams: simple contour drains will not be effective if a number of gullies run towards the pit. In this situation it is necessary to cut off the gullies using dams (refer to Fact Sheet No.5). These dams should be sized such that the overflow spillway is high enough to direct flow into an adjacent gully which is not flowing into the pit, or into a high level contour drain which can avoid the pit.

Interception drains should be installed as close to the top of the pit as practicable. It is also good practice to use these drains to separate clean water (ie. runoff from undisturbed catchment) from dirty water (ie. runoff from disturbed catchment). This may require parallel drainage systems but will result in much smaller sediment loads and in some cases a reduction in treatment facilities (Figure 8.3).

Flow detention basins: small scale versions of the flood control reservoirs discussed in Section 8.1.1 can be used to detain and regulate flows as part of an interception scheme. Where a number of small catchments feed into a single collector drain, detention basins can be used to delay flows from some of the areas and hence reduce the peak flow in that drain. If pumping is necessary as part of the interception scheme, detention basins can be effectively used to regulate flow to the pump. This will reduce the required pump size. As with the flood control reservoirs, it is important that these

Providing interception drainage can be difficult if the mine is in rough terrain or located in a valley. There are many techniques that can be used to develop an interception scheme. Runoff Interception Techniques Contour drains: the simplest method is to use the natural topography and run an open channel

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sediment from being eroded and transported. If this is not feasible, it is then necessary to contain the sediment in controlled locations where it can not cause these problems.

dams should self drain to ensure they are empty when a storm occurs. The size and design of detention basins is dependant on the area, steepness and ground cover of the catchment as well as the design storm (Fact Sheet No.3) and degree of detention required.

Avoiding Erosion and Transport of Sediment Clearing control: the most effective way to prevent soil erosion is to not disturb the natural (stable) ground. In open cut mining, clearing of vegetation and stripping of topsoil and overburden is necessary and must be carried out in advance of pit operations. Care must be taken not to strip this area too early, and to minimise the area actually cleared.

In-pit systems: if the terrain is extremely difficult it may be too expensive to create an effective interception scheme. In such cases it may be possible to use the benches of the pit as a drainage path. In strip mines, where the pit is continually moving forward, this is especially effective. If possible, the back bench of the pit should be sloped towards a deep gully where the water can be discharged away from the pit. In some cases, however, the only feasible direction to drain water is into the pit. If this is necessary, careful thought should still be given to doing it in a controlled manner so that drainage paths remain stable and pit pumps can cope with the inflows (Section 8.1.3).

Effective rehabilitation: rapid rehabilitation of disturbed mine areas will stabilise soil, and so prevent erosion. It is advisable to direct runoff from rehabilitated areas into the dirty water system for some time after completion of the area, to ensure that any sediment that is eroded can be contained before flow is discharged offsite. Open channel erosion control: controlling erosion in open channels is very important for effective flood control and interception drainage. Prevention of scour in drains is achieved through good design and adequate protection (refer to Fact Sheet Nos 4 and 8).

8.1.5 SEDIMENT CONTAINMENT The containment and control of sediment in and around open pits is important for efficient mine operations and is vital for the protection of the environment surrounding the mine (Note: for containment of sediment on and around waste dumps refer to Section 11.2.4). Some of the adverse effects from uncontrolled sediment transport and deposition are:

Increasing infiltration: erosion and transport of sediment is caused by water flowing at high velocities entraining soil particles. To prevent this it is necessary to reduce the amount of runoff and to slow it down. On large disturbed slopes, such as stripped or recently rehabilitated areas, this can be achieved by ripping along contour lines using grader or dozer tines. This will increase infiltration and inhibit overland flow. Important considerations for ripping are covered in AMIC (1990).

• upstream erosion; • clogging of pump inlets and sumps; • blockage of culverts; • reduction of drain capacities; • access problems for light vehicles;

In-pit sediment: an open pit is naturally a highly disturbed area. Therefore as a sediment management technique, it is best to have as much sediment as possible in the pit where it does not have to be controlled. Large and shallow sediment traps upstream of pit pump-out sumps are an effective way of achieving this (Section 8.1.3).

• damage to vegetation; • loss of habitat; and • off-lease discharges exceeding license limits for suspended solids and/or turbidity: The best way to avoid these problems is to prevent 1

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problems. They must be designed and constructed with care to ensure the correct level of water and balance of vegetation are maintained.

Removal of Suspended Sediment from Flowing Water The two techniques for removing suspended sediment from flowing water are filtration and settlement (refer to Fact Sheet No.8).

8.2 Groundwater

Filtration: by passing water through a fine media or by causing it to percolate slowly through an obstruction, silt will be removed. For overland flow this can best be achieved using synthetic or hay bale silt fences for small to medium sized cleared areas, or strips of heavy vegetation where these have not been cleared. When planning for clearing of an area, vegetation should be left undisturbed wherever possible. For channel flow it is best to use rock filter dams. Settlement: allowing water to flow into a large wide body of water will significantly reduce the flow velocity and will allow sediment to settle out of suspension. These sediment ponds can be designed to allow even the smallest sediment particles to settle. Shallow heavily vegetated wetlands are extremely efficient sediment traps as they both settle and filter suspended solids. They can also be effective in the treatment of acid drainage and heavy metal

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8.2.1 GROUNDWATER INFLOW Groundwater inflow to open cut mine pits is controlled by three primary factors: • hydraulic gradient (the slope of the water table in an unconfined aquifer, or the piezometric pressure in a confined aquifer); • hydraulic conductivity (often referred to as permeability) of the soil or rock; and • the area through which flow occurs. An idealised example of pit inflow in a homogeneous unconfined aquifer is shown in Figure 8.4. Visual evidence of the flow through area is given by the existence of a “seepage face” on a pit wall. This is characterised by a slick or wet appearance of the soil or rock surface, and close examination of this region may reveal trickling flow.

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Where a permeable fracture or a similar preferred flow path exists (in a non-homogeneous aquifer), the seepage face is often a discrete feature and may only show up as a long, thin line rather than a plane, as shown in Figure 8.4.

• Grout curtains are formed by injecting grout (which may be in a liquid, slurry or emulsion form) under pressure via grids of staggered wells. Solidification of the grout then provides a barrier to groundwater flow; and

8.2.2 MANAGING GROUNDWATER INFLOW

• Sheet piling is applied by driving sheets of steel into the ground until contact is made with bedrock. Improved hydraulic retardation is obtained by using interlocking sheets to form a more continuous barrier.

Groundwater inflow may be accommodated in the mine plan by restricting and/or containing the flow, and routing it elsewhere (dewatering). Flow Restriction

Figure 8.5 shows, schematically, the effect of placing a barrier to groundwater flow

Groundwater flow may be restricted by reducing the hydraulic conductivity and/or reducing the area through which flow occurs. These may be achieved by any of the following methods (Bedient et al, 1994):

Containment and Re-routing of Flow • Dewatering is commonly carried out to lower the watertable by pumping water out of the aquifer and away from the mine. A series of bores or spear points may be positioned in areas of good hydraulic connectivity to allow pumping at a sufficient rate to draw down the aquifer. Drawdown of the watertable reduces the flow

• Slurry walls may be constructed perpendicular to the direction of groundwater flow. These are generally installed at sufficient depth to intersect bedrock so that the aquifer is “barricaded”;

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through area of groundwater near the mine pits. Ideally, the watertable should be drawn down below the floor of the pit so that groundwater inflows are eliminated altogether. Figure 8.6 indicates the effect produced by dewatering.

A channel may be constructed to lower the water table and drain the water to downstream catchments. However, lowering of the watertable in this manner is generally less effective because of the reliance on steady gravity drainage. Figure 8.7 shows the method of channel dewatering.

• Channel dewatering: groundwater may also be intercepted outside the pit if the topography, groundwater regime and mine plan allow this.

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• co-disposal with tailings water; and

When groundwater flows are not highly significant, the water is often intercepted in the pit, collected in a sump and pumped to a retention dam for treatment or storage as required.

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treatment followed by disposal to receiving waters.

The option decided upon will depend on the quantity and quality of the water needed to be disposed.

Each method of managing groundwater inflows will have different environmental impacts. These will need to be evaluated prior to implementing a control technology. Issues such as volume of flows, water quality, effect on other users of the groundwater, surface drainage systems and receiving water bodies should be addressed.

8.3.2 ACID DRAINAGE Acid drainage can occur within an open pit when sulphide bearing minerals are exposed to air and water. The resulting low pH water can readily dissolve heavy metals that are contained in the orebody, overburden and waste rock. Additional detail outlining the chemistry and conditions favourable to the formation of acid drainage are provided in Fact Sheet No.7.

8.3 Water Quality 8.3.1 PIT WATER DISPOSAL

Acid water within an open pit is a problem if the water within the pit migrates to groundwater via rock pores or fissures or if the water from the pit is pumped to a storage area which may leach or overflow to receiving waters. It may also be an operational problem; for example, corroding structures and pumps.

Water held at the base of an open mine pit may be derived from direct rainfall, surface runoff from outside the pit and groundwater seepage. The contaminants which can be present include: • oils and greases from light and heavy machinery; • dissolved and particulate metals resulting from the dissolution of metalliferous minerals;

Hutchinson and Ellison (1992) identified three generally accepted approaches to the prevention or abatement of acid generation and leachate migration. These measures are applicable to acid drainage from open pits, waste rock dumps and stockpiles and include, in order of preference:

• nutrients from explosive residues; • acid drainage; • suspended sediments; and • salts. If acid drainage is present from the oxidation of sulphide minerals contained in the rock within the pit, then specific treatment and management strategies need to be considered. Options for the prevention and alleviation of acid drainage problems are provided in Section 8.3.2.

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control of the acid generation process;

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control of the migration of the leachate; and

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collection and treatment of acid drainage.

Options available for the disposal of pit water include:

While considerable research is being undertaken on this topic, options for the prevention of acid drainage at new mining operations and the control and elimination of problems within existing. open cut mines are generally limited to:

A combination of these three measures can often be the most applicable solution.

• disposal to evaporation ponds; • direct or indirect use as process plant water; • irrigation of rehabilitated areas within the minesite (eg. waste dumps); 1

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• avoiding or restricting the exposure of sulphide bearing rocks to the atmosphere. This may be achieved by selective mining of the orebody or modifying the overall mine plan;

In arid regions, evaporation ponds are the most common method for the disposal of saline or contaminated pit water. However care must be taken to avoid discharge of the water, and disposal of potentially contaminated bottom sludge must also be considered. Some mines dispose of hypersaline water to natural salt lakes, but this technique is not favoured by regulatory authorities. Depending on the quality of the pit water, other techniques such as irrigation within the release area may also be considered. Potential impacts on vegetation would need to be reviewed if irrigation is considered as an option. The potential for deep well disposal may also exist.

• ensuring long-term slope stability within the open cut as deterioration can result in the long-term exposure of fresh rock to conditions which lead to acid generation; • removal of the water as quickly as possible; and • incorporating acid neutralising rock (eg. limestone) in flow channels within the mine pit. A number of standard laboratory tests may be undertaken to determine the capacity of waste rock or ore to generate acid and mobilise heavy metals. Laboratory tests available include:

In temperate and tropical regions, where rainfall can equal or exceed evaporation, alternate methods of disposal must be developed. Site specific techniques and management practices usually need to be implemented within these areas.

• acid neutralising capacity (ANC) - the ability of a sample to neutralise acid generated from sulphide oxidation; • net acid producing potential (NAPP) the difference between the maximum potential acidity (MPA) and ANC; and • net acid generation (NAG) - a direct evaluation without measuring the MPA and ANC separately. Where these static tests indicate the potential for acid drainage, it may be useful to perform kinetic (or leach) testing. The data from both types of testing can then be used to derive appropriate management strategies to reduce the incidence or treat the outcome of acid drainage. Expert advice at the testing and planning stage can reduce the need for costly and long-term chemical treatment of polluted discharges. 8.3.3 SALINITY Mine pits which contain highly saline waters require specific management strategies which allow dewatering of the pit with minimal environmental impact. The strategies implemented will be dependent on the geographical location of the mine and local climatic conditions.

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High flow conditions in surrounding rivers and streams may also provide opportunities for discharge. For example, in the Hunter Valley of New South Wales saline mine waters are discharged to the Hunter River during times of high or flood river flows when the assimilative capacity of the river is high and the saline water can be quickly flushed to the ocean. This practice is now regulated by the NSW Government through the Hunter Salinity Trading Scheme.

8.4 Pit Closure Pit closure strategies are formulated to ensure that protection of the water environment, both within the site and downstream from the operation, is continued following pit closure. Final pit geometry is dictated by the balance of borrow and fill of earth, from the mining operations to the rehabilitation operations. However, water management concerns should be addressed interactively during pit closure design.

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In some cases, flooding of the open pit may be desirable, especially if sulphide rock is exposed to the atmosphere. In order to accelerate flooding, adjacent streams may be diverted into the pit. Such pits can also provide reliable sources of water for stock or irrigation. However, monitoring of the water quality will be necessary to ensure that it does not degrade due to, for example, acid generation from exposed sulphide rock.

Open cut mine closure leaves voids which may extend hundreds of metres below the water table. Consequently groundwater is often a primary issue in pit closure. An open void (see Figure 8.8) will tend to fill with water from the adjacent groundwater until a level of long-term equilibrium is attained. This will impact on the surrounding equilibrium groundwater levels. Recharge areas such as streams or rivers may be affected by these equilibrating processes. Surface water drainage into the open void and evaporative losses will form part of these processes. Pit closure strategies should be viewed as a water balance exercise, assessing the regional significance as well as the local significance of the presence of the void. Hydrological, surface water and groundwater issues should be addressed to quantify and minimise environmental effects of the final void on the hydrological cycle and vegetation of the region.

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9. Underground Mines

A sudden and large flow of water into an underground mine can have disastrous results and minor quantities will also cause inconvenience to personnel and machinery accessing the shaft.

Unplanned interception of adjacent flooded workings, especially in coal mines, can have disastrous consequences on workers and machinery. Blasting and drilling operations which tap into sources of water may result in a quick and widespread impact of the inflow in connected working areas.

The potential for this type of problem and hence the level of preventative works is dependant on the mine locality.

9.2.1 MANAGING GROUNDWATER INFLOW Managing groundwater inflow in underground mines can take many forms. Some techniques are:

9.1 Surface Drainage Away from Head Works

• preventative, using flow restriction, containment and re-routing of flow (Section 8.2.2). Bore dewatering, in particular, provides an effective way of reducing the effects of groundwater inflow to the underground mine by removing a proportion of the groundwater resource;

The most cost-effective method to avoid water entering a shaft or decline is to locate the shaft away from any watercourses or flood plains. If the general topography or the geological formation of the ore body makes this impossible, it will be necessary to undertake more pro-active flood protection civil works. For a discussion of flood mitigation and interception drainage techniques refer to Sections 8.1.1, 8.1.2 and 8.1.4. Due to the importance of a mine’s access shaft, flood protection and mitigation works must be designed to give a very low risk of failure. Where flooding is possible the level of risk must be very carefully analysed. If flooding may be life threatening, it is advisable to cater for the probable maximum flood (PMF) (refer to Fact Sheet No.2).

• contingent, allowing for the inflow of ground­ water. The confined nature of underground mines makes the design of adequate drainage into an adit or shaft used exclusively for collection of groundwater (ie. a sump) essential. Drainage to an adit which passively discharges to the environment may prove to be a long-term problem if acid drainage is present. Control and treatment of such drainage streams after mine closure is difficult and expensive; • depressurisation at the interior surface of the underground working, which involves progressively tapping into water bearing strata to “bleed” water and hydrostatic pressure at several points; and

9.2 Groundwater Inflow Groundwater inflows may originate from lateral connections to local and regional groundwater resources at working faces, vertical seepage from roofs of underground pits and local seepage from water bearing strata or “pockets” of groundwater. 68

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• pumping to the surface from sumps or pumping to abandoned shafts from temporary sumps may also be used to move volumes of water from areas in which they are not wanted.

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If acid drainage is present within the underground workings, then treatment of this water will be required, as outlined in Sections 8.3.2 and 11.4.1. In addition to the water extracted for treatment, consideration should be given to water that may potentially escape through mine shafts, adits and bedrock cracks and fissures.

In wet areas, the plugging of old shafts and surface exploration drill holes can reduce water inflows quite significantly

9.3 Water Quality Water present within underground mines is normally derived from direct infiltration of rainfall and seepage of groundwater into the excavation. Water extracted from underground mine workings may be contaminated with:

If at all possible, clean water flowing into a mine should be kept separate from dirty streams and removed as quickly as possible. This will prevent contamination of the water and reduce the quantity which then has to be treated.

• increased dissolved and particulate metals resulting from the abrasion and dissolution of metalliferous minerals (eg. acid drainage); • nutrients from explosive residues; • high concentrations of suspended sediments; and • oils and greases from underground machinery 9.3.1 TREATMENT AND DISPOSAL OF UNDERGROUND MINE WATER Water extracted from underground mine pits should be pumped to a central holding facility where suspended sediments can settle. If possible, the settling facilities should be underground, so that the sediment does not become a problem on the surface. Appropriate treatment technologies can then be implemented for the removal of any hydrocarbons, heavy metals or acid drainage.

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10. Heap Leach Processes

10.1 Introduction Management of a heap leaching operation is effectively the management of flows: the flow of barren leach solution to the leach pad and through the heap; containment of the pregnant solution; removal of the dissolved metals and recycling of the barren solution. In order to maximise the recovery of metal values and avoid environmental damage, catchments must be clearly separated and all drainage systems sized to contain the normal and abnormal flows. Inadequate design means both a loss of the resource and contamination of stream flows by process solutions. The design and management of a heap leach operation is a specialist skill. However, some traditional operations may decide to treat low-grade material using the principles of heap leaching. In these cases the design of the water and solution management systems may fall to the site engineer. The following sections are provided to assist site personnel in obtaining useful site specific information for the design, operating management, decommissioning and rehabilitation of a heap leach facility.

by process water, a proper understanding of the underground flow conditions and water chemistry will predict the extent and environmental significance of any process water seepage and enable the rapid implementation of remedial actions. Chemical parameters to be measured should include both the natural groundwater constituents, process chemicals and any chemicals which might be formed or liberated as a result of the process chemicals interacting with the soils or rock. 10.2.2 RAINFALL EVENTS, ACCEPTABLE RISK, CONTINGENCY PLANNING The collection system must be designed to accommodate the solution from both the leaching process and storm runoff without overflow or erosion occurring. The facility will need a water balance to properly manage the flows and containment ponds. Climatic factors to be considered include high intensity rainfall and long-term wet or dry periods. Local climatic data normally provide the most reliable data for predicting hydrological events. Suggested minimum design event frequencies are presented in Table 10.1.

10.2 Planning for Heap Leaching

Design event frequencies should be determined in conjunction with a risk analysis.

10.2.1 BASELINE EVALUATION

Storm design parameters must consider the critical duration of the design event, whereas seasonal variability is important for the design of water supply and containment ponds sizes. The ponds will need to contain:

It is essential to define and isolate catchments, and size drainage lines and ponds to ensure that clean and contaminated flows are separated and that the drains and containment ponds are not over-topped. The groundwater system beneath the pads and process ponds should be defined with regard to its hydrology and chemistry. In the event of contamination

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• minimum operating volumes to enable the pumps to operate;

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TABLE 10.1: Suggested Minimum Design Event Criteria for Heap Leach Operation

Facility

Type of Design Event

Access road, culverts and drainage ditches

10 year ARI to 50 year ARI flood peak

Drainage courses and ditches outside of leach pad perimeter berm, pregnant pond and barren ponds

100 year ARI flood peak

Internal freeboard within leach pad, pregnant and barren solution ponds

Maximum of: • average hydrological conditions plus a shortterm, 100 year ARI storm event; and • a longer term equivalent 100 year ARI event over a period of several months or years.

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The geochemistry of the process solution should be fully evaluated to determine the best indicators of contamination. For example, with regard to a copper heap leach operation, elevated sulphates in the groundwater may be identified in perimeter bore hole samples long before elevated copper concentrations.

heap drain down volume;

• rinsing cycles; • normal seasonal fluctuations in water volume (based on average climatic conditions); • flood surge (based on the critical design event); and

10.2.4 CLOSURE PLANNING

• extreme event discharge outlet or spillway.

The chemical characteristics of the spent leach pile and the long-term leachate stream should be determined during the design phase. The characteristics will depend on the nature of the ore, the process solutions used and the degree of rinsing and/or chemical treatment of the heap once active leaching has finished. It is important that the process ponds are sized to contain the volume of solution generated during the rinsing process.

Contingency plans should be developed (and preferably tested) prior to an event resulting in the release of process solution. Useful equipment to have on site or in daily operation may include: • a continuous flow monitor on the receiving creek to enable estimates of dilution; and • emergency chemicals and dosing equipment to neutralise overflows. 10.2.3 BASELINE GROUNDWATER MONITORING Many materials are available to seal the heaps from the underlying soil and for use as pond liners. These include PVC, asphalt and clay. It should be assumed that all ponds and heaps will potentially leak, so a groundwater monitoring program should be implemented to determine if there is any loss of process solution and contamination of the groundwater. Routine field monitoring should evaluate changes in the water table and the water chemistry. 1

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Where heaps are constructed sequentially, experience gained during the operation should provide the information needed to establish closure criteria for water quality and heap stability. Revegetation of the heaps may be problematic due to slope angles, chemistry and water retention of the spent ore. Ongoing treatment of the heap leachate may be required for some time after the last heap has ceased active leaching and it is important that adequate provisions are made to ensure containment of any contaminated water during the closure phase. A

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10.3 Solution Control During Operations

10.4 Water Management on closure 10.4.1 CRITERIA FOR LONG-TERM LEACHATE QUALITY

10.3.1 MAINTENANCE OF DRAIN AND POND CAPACITY Heaps should be designed to avoid slope failure and/ or erosion of the ore and subsequent blockage of the drains. Erosion and sedimentation within the drains reduces the capacity for containment and may result in overtopping of containment structures. Testing should be undertaken on both saturated and unsaturated heaps as the shear strength of the material will vary with different pore water pressures. The effects of earthquakes should not be overlooked. 10.3.2 INTEGRITY OF THE PAD OR LINER The pad should be protected from flood flows in the natural drainage systems by appropriately sized berms. These should also extend around the process solution ponds. It is recommended that the 100 year ARI storm event be the minimum design standard.

10.4.2 RESIDUES AND LONG-TERM CONTAMINATED SITE MANAGEMENT All heaps should be contained as safe, stable structures which will erode at an acceptable rate. This rate will need to be determined through project specific field trials as the slope angles, particle size and length of slope will influence the rate and extent of erosion. The use of vegetation to control erosion may be subject to both geochemical and physical limitations and the early establishment of field trials should provide the data needed to evaluate this option.

During construction of the pad care is required to ensure the integrity of the liner. The strength of the liner should be commensurate with the hydraulic pressure to be applied and the chemicals to be used. Multiple use of a pad increases the risk of tears in the liner and subsequent seepage. Careful inspection is required to ensure integrity of the liner prior to the construction of the heap. A leachate collection system should be constructed to collect seepage.

Leachate and surface runoff from the heaps should not cause degradation of watercourses downstream from the site through either siltation or long- or short-term toxicity. The operation will need to implement a monitoring program to evaluate the success of its rehabilitation and leachate management strategies. This will include both surface and groundwater quality monitoring and should include contingency plans for the implementation of alternative control strategies should they be required. Relinquishment of the lease can be expected once the operation has attained an acceptable discharge quality and stable surfaces.

10.3.3 INTEGRITY OF PIPING AND VALVES All pipes containing process solution should be located within bunds which are sized to contain the amount of solution which would be released should the pipe or valve fail plus any additional flows due to rainfall within the bund catchment. Routine inspections and leak detection equipment should be used to identify leakages and these should be repaired immediately to avoid contamination of the groundwater. Preventative maintenance rather than the repair of leaks should be the underlying operating philosophy. 72

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The leachate discharge criteria should be developed on the basis of the downstream beneficial uses of the surface and groundwater flows. State regulations (and/or catchment or river specific environmental protection policies) and the ANZECC (1992) guidelines for receiving water quality will provide a basis for determining the appropriate long-term leachate quality. These, in conjunction with the flows and chemistry of the leachate stream and of the receiving waterbody, will determine the final discharge quality, and where applicable, the size of a mixing zone.

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11. Waste Dumps

11.1 Waste Dump Construction for Water Management

• new waste dumps should be located within catchments serviced by dirty water interception and treatment facilities;

Attention to waste dump construction with a view to the final rehabilitation plan will minimise erosion potential and facilitate a drainage system that reflects the final drainage network. Accordingly, waste dump planning and construction should attend to the following critical matters.

• where possible, natural drainage paths should be maintained, and room should be left around the base of the waste dump for interception drainage; • waste dumps should not be constructed immediately adjacent to natural or uncontaminated watercourses. Provision must be made for intercepting runoff, leachate and seepage before it enters such watercourses;

11.2 Surface Water

• room should also be left for construction of retention ponds, or it must be possible to direct interception drains into existing ponds for the removal of suspended materials and the treatment of chemical contaminants; and

The information provided in this section should be read in conjunction with Section 6.1 of AMIC (1990). The type of material to be stored in the waste dump will determine its design and ongoing construction. The presence of acid or other undesirable leachateproducing waste may necessitate a capped waste dump which will generate high volumes of surface runoff. Alternatively, if the material is inert it may be desirable to encourage infiltration. The types of contaminants to be expected are discussed in Section 11.4. To ensure this contamination is minimised and contained there are many critical design issues for waste dumps. These are discussed below.

• avoid locating road culverts immediately downstream of waste dumps. The high sediment load in waste dump runoff can easily cause blockages. Where this is not possible, ensure that sediment retention dams are located upstream of the culverts. Culvert inlets should be carefully designed to maximise velocities into the culvert and outlets designed to ensure that sediment is removed from the outfall.

11.2.1 LOCATION OF WASTE DUMPS

11.2.2 EROSION ON WASTE DUMPS

The location of waste dumps should be planned well in advance to cater for the expected waste volumes, the final and intermediate design profiles, visual and noise screening of mine operations and the interaction with groundwater. The following surface water issues should also be considered in the plan:

Severe rilling on waste dump batters and the problems associated with high sediment loads in waste dump runoff can be reduced by proper design and construction of the waste dump. This should include close attention to batter slopes, benching, armouring and drains. Apart from these

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• introducing storm water retention basins into the final profile to reduce the magnitude of peak flows.

‘geometric’ design guidelines, the following points should be considered. Capped Waste Dumps

11.2.3 INTERCEPTION DRAINAGE AROUND WASTE DUMPS

Where acid drainage and other leachate formation is to be minimised by capping the waste dump with impervious clay or rock, there will be very high volumes of runoff. It is important to incorporate erosion control when constructing the capping layer. This will include properly designed drains, spillways, drop structures, armoured batters and immediate topsoil and grassing. It is also very important to ensure the impervious material selected is not excessively dispersive (clays) or soluble (weak limestone).

Contaminated runoff or leachate derived from waste dumps must be intercepted and directed towards ‘dirty water’ treatment ponds. The degree of treatment required to match the quality of natural watercourses in the area can vary from none at all, to removal of nearly all suspended solids and treatment for acid, salinity, and heavy metals. Typical techniques for runoff interception are discussed in Section 8.1.3 which, along with the following guidelines, will ensure that the interception system works effectively.

Encouraging Infiltration

Separation of Water Streams

If seepage of water into the waste dump will not cause structural instability or contaminated leachate and groundwater seepage, it can be very beneficial to encourage infiltration. This will greatly reduce runoff volumes and hence reduce erosion. Increased infiltration can be achieved by contour ripping of the surface, “moonscaping” (refer to AMIC, 1990), creation of small detention ponds or sink holes on top of the stockpile.

To avoid excessive volumes of water entering the dirty water treatment systems, runoff from undisturbed catchments around the waste dump should be kept separate from dump runoff and associated disturbed areas. If large quantities of dust from the waste dump settles on nearby areas, then these areas should be included in the dirty water system. Vegetation Filters

Erosion Control

The retention of natural vegetation between the waste dump and the interception drains can be highly effective for removing sediment from runoff and reducing contaminants in the leachate.

Erosion control can be achieved through: • effective and early revegetation of completed waste dumps or even of completed sections of active waste dumps. This will require thorough advance planning of final dump profiles, but in so doing may prevent double handling of waste;

Drainage Design If sediment cannot be retained on the waste dump then it must be kept in suspension until it reaches a designated location for sediment removal (ie. a sediment pond). Drainage velocities must be sufficient to keep sediment suspended but not too fast so as to cause scour.

• armouring or effective slope reduction which will reduce scour. Planning of open channels to achieve stable profiles and slopes (ie. 0.5% - 1.0%) is also important (refer to Fact Sheet No.4); • reduction of slope lengths by construction of contour banks and/or drainage benches; and

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11.2.4 SEDIMENT CONTAINMENT AROUND WASTE DUMPS

or percolates through this zone. Similarly, the migration of contaminants is strongly vertical.

Containment of sediment on the stockpile is the ideal solution and can be maximised using silt fences, hay bales, silt traps, filter dams, retention basins and any other method which will temporarily reduce runoff water velocities to allow suspended solids to settle. A description of these techniques is given in Fact Sheet No.8.

Flow in the capillary zone is complicated by the strong and variable presence of air in the soil matrix. This results in a variable hydraulic conductivity of the soil, which, in turn, results in variable groundwater infiltration characteristics between ground level and the top of the aquifer. The main factors influencing groundwater contamination are:

When de-silting ponds, sediment should be dumped in a location where it will be exposed to minimal surface runoff. Methods of containing the sediment either on the waste dump or in a dirty water system are dealt with in detail in Fact Sheet No.8. If wetlands are used, they should only be used to remove very fine sediment particles and a pre-settling pond should be constructed upstream.

• travel time of contaminated water from the ground surface to the water table; • the fraction of contaminant that reaches the water table; and • the rate at which the contaminant enters the aquifer from the capillary zone. Characteristic behaviour of contaminants include:

11.3 Groundwater

• soluble contaminants collect near the water table in a floating lens and are then transported across the water table where horizontal dispersion occurs;

11.3.1 INFILTRATION TO GROUNDWATER Between ground level and the top of the aquifer, the level of saturation in the soil may vary from zero (dry) to fully saturated (Figure 11.1). This zone, referred to as the capillary zone, contains water which is held under negative (suction) pressures within the soil matrix.

• solvents which are denser than water migrate downwards to the bottom of the aquifer and are then transported by a process of advection and diffusion; and • residual (free phase) chemical contamination in the soil matrix above the water table has the potential to generate long-term problems.

Flow in the capillary zone is strongly vertical and only weakly horizontal. Therefore water infiltrates

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Control of infiltration may be achieved through:

and the piezometers should extend into the subsurface groundwater regime. Monitoring and sampling should be carried out both upstream and downstream of the prevailing groundwater flow direction near the waste dump (Figure 11.2).

• liners or impervious layers placed between the waste dump and the soil matrix (eg. polyethylene, PVC, non-reactive clays or soil-bentonite mixtures);

Monitoring and sampling should include:

• surface capping to insulate against the infiltration, percolation and contaminant migration via rainfall through the waste dump. Surface capping materials may be impermeable materials such as clay, concrete or liners; and • adequate waste dump drainage to confine runoff to the surface, where it may be more easily contained and treated if required.

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groundwater levels or piezometric heads;

•

pH and salinity; and

•

chemical and/or biological analyses as appropriate.

When sampling for chemical or biological analysis, standard sampling procedures should be used (Section 5.4).

Attenuation of groundwater contamination may be achieved by isolating the groundwater near waste dumps using:

Contaminants may react within the soil matrix, so that groundwater monitored at the periphery of waste dumps may not directly reflect some characteristics of the primary contaminant infiltrating from waste dumps.

• slurry walls (Section 8.2.2); • grout curtains (Section 8.2.2); and • sheet piling (Section 8.2.2).

11.4 Water Quality

In addition, groundwater control methods such as dewatering bores and capture trenches (Section 8.2.2) may be used to collect water for pumping to treatment facilities. However, these methods should only be employed after source control methods have failed.

Waste rock dumps may be a source of contaminants to local streams and receiving waters. The range of problems that occur from these structures include:

11.3.2 MONITORING Groundwater should be monitored as close as practical to the perimeter of the waste dump

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acid drainage;

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saline runoff;

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suspended solids runoff; and

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heavy metals in runoff and leachate.

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• gas sampling within the partially saturated zone; and

11.4.1 ACID DRAINAGE Acid drainage from waste rock dumps is normally a more significant problem than that from within open cut or underground mines. This is primarily a result of increased surface area of exposed reactive sulphides, higher porosity and infiltration within waste rock dumps and the difficulty in containing and/or treating leachate. The extent of the acid drainage and subsequent metal solubility problems within a waste rock pile will depend on the following physical, chemical and biological conditions:

• sampling and analysis of soluble acid drainage products in the waste rock and underlying geologic formation. Specialised sampling techniques are required when monitoring for acid drainage and the reader is referred to Hutchinson and Ellison (1992) for further information. A wide range of prevention and remedial strategies are available for acid drainage problems from waste rock dumps. These are shown in Table 11.1

• physical size and geological characteristics of the waste rock;

11.4.2 SALINITY

• the presence and type of sulphide bearing minerals;

Saline runoff from waste dumps can be a common problem at mines located within arid regions and regions with specific high salinity geological formations, for example, much of the Hunter and Bowen Basin coalfields. Overburden and waste rock that originated from within saline parent material can have high concentrations of dissolved and precipitated salts. Once this material is removed and placed on waste rock dumps, rainfall infiltration can result in highly saline runoff and leachate.

• the extent of rainfall infiltration; • the permeability of the waste rock dump to air and water; • the presence of acid neutralising rocks within the waste rock dump; and • the level of microbiological activity, including the presence of bacteria. Monitoring techniques that can be used to identify acid generation within a waste dump include:

Runoff and leachate from saline waste rock dumps should be intercepted and directed to storage ponds for:

• the presence of “hot spots” on the waste surface that are warm to the touch;

• evaporation;

• the appearance of steam from sections of the dump, particularly after rain events;

• recycling if suitable; • dilution with low saline water if available and subsequent use;

• red and brown coloured water around the base of the dump, red or brown colouring on stream bottoms and banks, or the presence of colloidal yellow precipitate in the water;

• treatment if feasible; or • controlled discharge, for example under flood flows where natural dilution occurs.

• the use of remote imaging techniques, such as thermal infra-red, to identify higher than ambient temperatures in the dump;

The chosen option will depend largely on the water’s suitability for use on site and the characteristics of the receiving waterbody.

• in-situ temperature sensing;

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TABLE 11.1: Prevention and Remedial Strategies for Acid Drainage

Control of Acid Generation • pre-treatment to remove or exclude sulphide minerals • use of an impermeable cover to exclude rainfall infiltration and oxygen • waste segregation and blending to control pH • use of bactericides to control bacterial oxidation of sulphide minerals • avoid exposing reactive minerals to atmospheric conditions by modifying the mine plan or avoid mining sections of the deposit

Control of Acid Migration • use of covers and seals to exclude infiltration • controlled placement of waste to minimise infiltration • interception and diversion of surface and groundwater

Collection and Treatment of Acid Drainage • use of a physical and/or chemical treatment system • use of biological treatment systems such as wetlands Modified from Hutchinson and Ellison (1992)

• heavy metals which may be dissolved by acid forming processes; and

11.4.3 SUSPENDED SOLIDS Common techniques used to control sediment runoff from waste dumps have been outlined in Sections 11.2.2 and 11.2.4. Further techniques applicable to erosion control and the rehabilitation of waste rock dumps are provided in Fact Sheet No.8 and AMIC (1990).

• acid and alkaline waste streams from naturally forming inorganic acids and natural carbonates or alkaline silicates. Specific treatment of these waste streams may be required, and special disposal techniques may be needed for sediment derived from these materials and deposited in sedimentation dams.

11.4.4 LEACHATE AND OTHER CONSTITUENTS Additional contaminants that may emanate from waste rock dumps include: • asbestos fibres from naturally occurring minerals; • soluble cations and anions such as chlorides, sulphates and carbonates;

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12. Tailings Water Management

• co-disposal of tailings which is the combined disposal of coarse rejects material and fine tailings usually by combined slurry pumping. The mixture produces a stable landform at the point of disposal with major advantages for rehabilitation. Significantly larger volumes of water are required than for conventional tailings disposal. The advantages of co-disposal are the stable ongoing and end landform, the reduction in area for waste disposal, the potential for recycling most of the discharge water and fewer environmental impacts. The technique does require large volumes of water, and there are greater potential seepage losses and large recycling pumps are required to return the water for the ongoing co-disposal process. Co-disposal techniques are being used at coal mines but are also applicable to metalliferous mines where there is a rejects component that, when combined with tailings will produce a well graded stable in-situ landform.

All tailings disposal systems require management of the water component in the tailings. Management strategies are closely linked with the method of disposal, design of containment facilities and the potential for impacts both on and off the site.

12.1 Disposal Methods Tailings disposal methods can be separated into four major categories: •

saturated tailings management, where the tailings are transported and discharged as a slurry. The saturated tailings are held in a dedicated containment area where gravity separation isolates a percentage of the water from the tailings solids. As deposition of the tailings is in a wet slurry, tailings beach slopes are flat and, consequently, large containment areas required. To minimise storage requirements, the separated water should always be recycled as much as possible;

In all these processes, the effectiveness of the dewatering processes is a function of local conditions, the type of waste solids, size distribution, statutory requirements and economics.

• semi dry or thickened tailings management, which involves discharging the tailings to a containment area at higher solids content than the saturated tailings management. Depending upon the stacking characteristics of the particles in the tailings, higher beaching slopes are possible, with resulting smaller containment areas for tailings and decant water;

It is critical for the rehabilitation of tailings facilities that the disposal and decommissioning methods are compatible and decided upon in the planning stage. For example, if a tailings storage facility is planned to be decommissioned by drying out the surface and covering it with waste rock or other material to encourage revegetation, disposal of the tailings under water (sub-aqueous disposal) could lead to poor settlement and ineffective drying of the surface. Conversely, a facility which will be decommissioned using a wet cover, typically used

• dry stacking, which permits the extraction of most of the water before deposition. This allows the solids to be transported into a solids rejects dump from where they can be taken to waste dump areas for contouring, topsoiling and revegetating; and 1

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to inhibit acid drainage, should not be operated with dry beaches where oxidation of the sulphides can take place.

• diversion of drainage from surrounding catchment areas in order to reduce inflow as much as possible; • the need for separate reservoirs for water to be recycled eg. in co-disposal;

12.2 Characteristics and Management of Tailings Water

• recycling of tailings decant water should be encouraged as much as possible;

12.2.1 NATURE OF THE WATER

• tailings pipelines should be bunded and have collection sumps to contain spills from leaking or ruptured pipes;

The water used to transport tailings and co-disposal tailings or extracted during thickening of the waste becomes contaminated during the process. In some cases, such as in the goldfields of Western Australia, the water itself is a risk to the environment because of its hypersaline nature. Tailings water can be acid or alkaline, have elevated concentrations of heavy metals or contain concentrations of cyanide which can have considerable environmental impacts if it is released to the environment. It is important to characterise the tailings water through a monitoring program and manage the water accordingly.

• infiltration monitoring systems are required around the containment site to detect contaminants escaping from the impoundment; and • discharge monitoring for disposal systems with continuous discharge of tailings liquor and/or solids.

12.3 Seepage Management

In some cases, it may be necessary to treat the water before disposal to the tailings storage facility. Denaturing or recovery of cyanide from gold process liquors is frequently practiced in order to reduce costs and also to reduce the potential environmental impacts.

Seepage can occur through the walls and through the floor of a tailings storage facility (Figure 12.1). Infiltration through the floor of the tailings storage facility usually decreases with time as tailings are deposited in successive layers and form a retardant to vertical flow. In the long-term, the majority of tailings water seepage occurs through the dam wall and via infiltration through the ground surface on which the wall is built.

12.2.2 MANAGEMENT The following are the key elements that need to be considered in tailings water management: • the sensitivity of the containment area to infiltration and hence the requirements for lining the storage area need to be evaluated;

12.3.1 SEEPAGE CONTROL Seepage may be controlled to some extent by constructing the tailings facility using permeable (for filter dam segments) and impermeable soils where applicable. In addition, geofabric liners may be used to increase the insulation against seepage flow.

• the ability of the storage area to contain stormwater inflows should be assessed. The potential impact of discharges from the tailings storage during storm events must be assessed. This will necessitate a risk assessment (see Fact Sheet Nos 2 and 3) with a resulting design storm event for containment;

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Under-drains may be installed in the floor of the facility before deposition of tailings in order to collect and channel water to a collection system. Similarly, interception drains and trenches may be

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as well as evaporation rates, are critical to forming a water balance management scheme.

installed around the facility to collect seepage before it can escape into the environment. In extreme cases, impervious slurry walls and interception systems have been installed in the preferred seepage paths to prevent escape of potentially contaminated water into sensitive environments downstream of the facility.

Tailings water control may be implemented using containment measures such as:

12.3.2 MONITORING

• sizing the TSF sufficiently to hold large volumes;

Monitoring of seepage flow through the wall of a tailings storage facility (TSF) is readily accomplished using piezometers to determine the geometry of the phreatic surface (Figure 12.1). This may be translated to seepage flow rates using standard groundwater flow theory.

• constructing filter dams to allow selective seepage of water into retention ponds or evaporation ponds. Water extracted in this way may be more acceptable for recycling in processing plants;

It is also common practice to install piezometers around the base of the impoundment wall in order to detect seepage escape into shallow aquifers under the facility. Such piezometers should be installed in appropriate locations so as to be able to detect a contamination front moving from the impoundment early enough to take remedial action. Indicator elements should be determined from a knowledge of the chemical composition of the tailings water.

• sizing and locating outlet structures to hydraulically control discharges from the storage; and

12.3.3 WATER CONTROL

• staging of containment wall construction to facilitate drainage from the co-disposal area;

• sizing evaporation ponds to reduce water levels at sufficiently high rates. The re-use of tailings water is often limited because of specific water quality requirements of the process. In general, the characteristics of tailings water is process-specific, as is the acceptability of tailings water for re-use.

Water balance monitoring of TSFs enhances the overall understanding of the site water circuit. Monitoring should be carried out within the tailings pond, in the dam wall and in any downstream evaporation ponds. Adequate knowledge of tailings settlement and water retention in voids,

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13. Mine Infrastructure

Water is essential for many aspects of a mining operation. As well as the core function of extracting the ore, virtually every other part of the mining infrastructure uses water in some way. After coming in contact with the operations, this water can pick up contamination. It is important to be aware that this contamination can exist, and of ways to minimise it. Water used in these support functions needs to be managed in the same way as other water on the site. This section examines three main areas of an operation where good water management is essential. It is important to ensure that all operators are aware of the potential environmental impacts from failure to follow procedures, and that they are adequately trained in the operation of all pollution control systems.

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• process reagents; • increased dissolved and particulate metals resulting from the dissolution of metalliferous minerals;

Remedial measures and technologies available for the containment and treatment of contaminants from the process plant include: • improved housekeeping strategies to identify the locations of spillage (eg. conveyor transfer points) and the implementation of appropriate remedial measures; • bunding of all process chemical storage areas and the interception and treatment of all stormwater from within these areas; • drainage of all process plant runoff to a central treatment facility (eg. sedimentation or evaporation pond);

A risk analysis and the associated contingency plans should be undertaken at the planning stage. Engineering solutions should be commensurate with the level of acceptable risk, safety hazards and environmental harm which could result from an event. 9

• oils and greases;

13.1.2 CONTAINMENT AND TREATMENT TECHNOLOGIES

The quality of surface runoff from the process plant is dependent on the type of ore being processed and the metallurgical process adopted, eg. flotation, beneficiation, cyanide leaching.

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• suspended sediment;

• nutrients from residual nitrates from blasting.

Water used in a process plant is normally confined within its designated piping and storage facilities. It is only through washdown, pipe ruptures, spillages and overflows from process water tanks and dams that significant volumes of process water can enter receiving waters.

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The process plant and associated ore stockpile area can be a source of the following contaminants:

• strong mineral acids and bases such as sulphuric acid and lime; and

13.1 Process Plant

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13.1.1 CHARACTERISTICS

• provision of quiescent conditions in retention ponds to enable settlement of fine grained sediment. More rapid settling can be achieved using a flocculent such as alum;

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• construction of the facilities to collect and contain minor spillages outside the bunded area during refuelling operations; and

• pH correction using lime dosing or other suitable material may be necessary if the retention pond water is acidic or incompatible with receiving water quality; and

• diversion of oil contaminated bund water collected during rain events through oil interception or separation facilities.

• interception and treatment of stormwater runoff containing hydrocarbons through a oil-water separation facility or alternatively, materials contaminated with hydrocarbons may well be suited to treatment using Bioremediation Technology (Fact Sheet No.9).

Workshop and Truck Washdown Areas General principles of design and operation of these areas include: • better control of hydrocarbons, eg. central bulk storage and reticulation throughout the workshop rather than the use of 20 or 200 L drums;

The treatment of soluble contaminants is dependent on the volume and quality of the waste stream. Wastewater or contaminated runoff can be diverted to a retention pond, tailings storage facility or evaporation pond. Some waste streams may require more advanced forms of treatment such as activated carbon or ion exchange.

• design of dispensing facilities to prevent drips and spillage; • covering of the working area to prevent storm water picking up contaminants;

13.2 Industrial and Workshop Areas

• installing a drainage system to separate clean and contaminated water streams from within and surrounding all workshop areas;

The industrial area and its associated workshops can be a frequent source of contaminants such as lubrication oils, greases, solvents, surfactants (water and solvent based products), suspended solids from vehicles, atmospheric sources, spillage, and metal shavings from lathes. Stormwater runoff is the major transport route of these pollutants to local watercourses and receiving waters.

• diversion of oil contaminated water to a separation system, which can range from simple concrete sumps through to more sophisticated mechanical systems such as coalescing plate separators, skimmers and centrifugal separators; • use of dry cleaning methods such as industrial vacuum cleaners and absorbents rather than water to clean floors and other surfaces;

13.2.1 CONTAINMENT AND TREATMENT TECHNOLOGIES

• phasing out of solvents for cleaning applications in favour of new generation water-based detergents, suitable for the cleaning of hydrocarbons soiled equipment (solvents are more difficult to treat and remove in wash water than heavy lubricating oils); and

Fuel Storage Areas General principles for the design and operation of storage areas include: • bunding to the appropriate Australian Standards in order to contain spillages;

• more effective dispensing, mixing and use of detergents by operators, which can also reduce consumption.

• frequent inspection of storage tanks and piping for corrosion and any above ground and underground leaks;

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13.3 Haul Roads

course layer. This will minimise the potential for saturation of this layer (Figure 13.1).

Controlled drainage from haul roads is essential for the maintenance of the road integrity for haul truck usage. The drainage systems have environmental impacts in terms of both the structures adopted and the quality of the drainage waters collected for disposal. Both surface and groundwater drainage issues should be addressed.



• if the grade of the road exceeds 2-3%, erosion protection along side drains may be required to prevent undercutting of the pavement layers. The erosion protection may be in the form of lining (rocks, concrete, synthetic materials) or barriers for inducing flatter slopes; and

13.3.1 ENVIRONMENTAL ISSUES Haul roads are potentially a source of contamination in water, notably from suspended particulate matter. Any spillage of mined material onto the road surface is a source of these particulates and, depending on its nature, also a source of chemical contamination. Any pyrite present in the ore or waste could oxidise, leading to acid drainage and mobilisation of heavy metals.

• haul road drainage crossings should be through culverts, with attention given to upstream and downstream erosion protection. Appropriate slopes and surface level designs are necessary to facilitate sediment movement without deposition and consequent culvert blockages.

It is important to ensure that, wherever possible, haul roads are constructed of material which will not lead to further environmental impacts.

13.3.3 GROUNDWATER DRAINAGE Groundwater investigations will reveal the necessity for any groundwater drainage systems. The primary purpose of groundwater drainage systems associated with haul roads is to minimise the potential for saturation of the haul road sections and possible failure. The environmental consequences of such failures can extend to washouts of the road with excessive sediment loads and destruction of the integrity of the surface water drainage systems.

There are recorded instances where materials used in the construction of haul roads have led to environmental contamination along the entire length of a road. 13.3.2 SURFACE WATER DRAINAGE The important elements in surface water drainage on haul roads include:

Typical groundwater protection mechanisms include:

• water must be cleared from the pavement or wearing surface quickly to avoid excessive soaking of the surface base course layer and without creating deeply incised scour paths. Generally; maximum cross fall slopes of 3% will facilitate both these criteria (Figure 13.1);

• slotted pipes in gravel beds; • rock fill “pipes”; • rock fill blankets to facilitate both the construction and haul road operation; • synthetic geotextile materials to separate layers and provide strength; and

• side drains are required to catch surface water from the pavement and runoff from cut bank slopes. The side drains should be sized such that the design flow depth is no higher than the underside of the pavement top course or base

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It is preferable to direct drains off the haul road at cut and fill interfaces or otherwise down batter slopes at designated locations via erosion protected chutes;

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• dewatering by mechanical means (pumps) in extreme cases.

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References

AMEEF (1996). Environmental Management in the Australian Minerals and Energy Industries. Ed. David Mulligan.

DME (nd). Groundwater Quality and Water Well Maintenance. Information Sheet No. 10, Department of Mines & Energy, South Australia.

AMIC (1990) Mine Rehabilitation Handbook. Minerals Council of Australia, Canberra (under revision 1997).

EPA (1995). Environmental Monitoring and Performance. One Module in a series on Best Practice Environmental Management in Mining. Environment Australia, Canberra.

Anderson, M.P. & Woessner, W.W. (1992). Applied Groundwater Modelling; Simulation of Flow and Advective Transport. 381pp, Academic Press, New York. ANZECC (1992). Australian Water Quality Guidelines for Fresh and Marine Waters. Australian & New Zealand Environment & Conservation Council (under revision 1997-98). APHA (1994). Standard Methods for the Examination of Water and Wastewater. 18th Edition. Washington, USA. AWRC (1992). Draft Guidelines for Groundwater Protection. Australian Water Resources Council.

EPA (1997) Managing Sulphidic Mine Wastes and Acid Drainage. One module in a series of Best Practice Environmental Management in Mining. Environment Australia, Canberra. Faust, S.D. & Aly, O.M. (1983). Chemistry of Water Treatment. 723 pp, Butterworths, Boston USA. Fetter, C.W. (1994). Applied Hydrology. 3rd. Ed. MacMillan College Publishing Co., New York. Haan, C.T. (1994). Design Hydrology and Sedimentology for Small Catchments. Academic Press, USA.

Bedient, P.B., Rifai, H.S. & Newell C.J. (1994). Groundwater Contamination; Transport and Remediation. 541pp, Prentice Hall, New Jersey. Bureau of Meteorology (1994). The Estimation of Probable Maximum Precipitation in Australia: Generalised Short Duration Method. Bulletin 53, December 1994. Australian Government Publishing Service, Canberra. Chow, VT. (1973). Open Channel Hydraulics. Intl. Student Ed. McGraw-Hill, Tokyo, Japan.

Hart, B.T. (1974). A Compilation of Australian Water Quality Criteria. AWRC Technical Paper No.7, Australian Government Publishing Service, Canberra. Hart, B.T. (1982). Australian Water Criteria for Heavy Metals. AWRC Technical Paper No. 77, Australian Government Publishing Service, Canberra. Hutchinson, I. & Ellison, R. (1992). Mine Waste Management: A Resource for Mining Industry Professionals, Regulators and Consulting Engineers. Lewis Publishers, USA.

DEH (1995). Water Quality Sampling Manual- For Use in Testing Compliance with the Environmental Protection Act 1994. Department of Environment & Heritage, Queensland. DME (1995). Technical Guidelines for the Environmental Management of Exploration and Mining in Queensland. Department of Minerals & Energy, Queensland.

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references

Shaus, E.M. (1994). Hydrology in Practice. 3rd Edition. Chapman Hall.

International Organisation for Standardisation (1994). ISO Standards Compendium (Environment - Water Quality): Volume 1 - General; Volume 2Chemical Methods; Volume 3 - Physical, biological and microbiological methods. First Edition. Switzerland.

Smith, A. & Mudder, T. (1991). The Chemistry and Treatment of Cyanidation Wastes. Mining Journal Books Limited, London.

Kinori, B.Z. & Mevorach, J. (1984) Manual of Surface Drainage Engineering, Vol II. Stream Flow Engineering and Flood Protection. Elsevier, Amsterdam The Netherlands.

Vick, S.G. (1983). Planning, Design and Analysis of Tailings Dams. Wiley Williams, R.E., Winter, G.V., Bloomsburg, G.L. & Ralston, D.R. (1986). Mine Hydrology. 169pp, Society of Mining Engineers, Colorado.

Nelson, K. D. (1991). Design and Construction of Small Earth Dams. Inkata Press, Melbourne. NH&MRC (1994). Draft - Australian Drinking Water Guidelines. National Health & Medical Research Council, Canberra. Pilgrim & Cordery (eds) (1987). Australian Rainfall and Runoff. Institution of Engineers, Australia. (This document is revised regularly)

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Glossary

Advection

The process by which solutes are transported by the motion of flowing groundwater.

Anisotropy

The condition under which one or more of the hydraulic properties of an aquifer vary according to the direction of flow.

Antecedent conditions

The moisture conditions existing in a catchment at the onset of a storm.

Aquifer

Rock or sediment in a formation, group of formations, or part of a formation that is saturated and sufficiently permeable to transmit economic quantities of water to wells and springs.

Aquifer, confined

An aquifer that is overlain by a confining bed. The confining bed has a significantly lower hydraulic conductivity than the aquifer.

Aquifer, perched

A region in the unsaturated zone where the soil may be locally saturated because it overlies a low-permeability unit.

Aquifer, unconfined

An aquifer in which there are no confining beds between the zone of saturation and the surface. There will be a water table in an unconfined aquifer. Watertable aquifer is a synonym.

ARI - (Average Recurrence Interval)

The average or expected value of the period between exceedances of a given event (eg. rainfall, discharge etc.).



This period is a randomly distributed variable.

Bailer

A device used to withdraw a water sample from a small diameter well or piezometer. A bailer typically is a piece of pipe attached to a wire and having a check valve in the bottom.

Basecourse

A layer of granular fill material constituting the uppermost structural element of a road pavement immediately below the wearing course.

Capillary zone

The zone immediately above the water table, where water is drawn upward by capillary attraction.

Capture trench

A trench which extends below the water table and into which the groundwater drains.

Catchment

The area which drains into a given stream or dam by way of natural ground slopes or constructed drainage systems.

Clean water

Surface runoff which has not picked up any solid or dissolved pollutants through contact with disturbed or contaminated surfaces.

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glossary

Co-disposal

The combined disposal of tailings and coarse reject material.

d/s

Down stream (eg. d/s of a dam).

Dewatering

The process of removing water from a given source (eg. pit dewatering, aquifer dewatering).

Diffusion

The process by which both ionic and molecular species dissolved in water move from areas of higher concentration to areas of lower concentration.

Dirty water

Surface runoff which has picked up solid or dissolved pollutants through contact with disturbed or polluted surfaces.

Drawdown

A lowering of the water table of an unconfined aquifer or the potentiometric surface of a confined aquifer caused by pumping of groundwater from wells.

Finite-difference model A digital computer model based upon a rectangular grid that sets the boundaries of the model and the nodes where the model will be solved. Finite-element model

A digital ground-water-flow model where the aquifer is divided into a mesh formed of a number of polygonal cells.

Gabion

A flexible wire basket filled with stones and used to retain earth and sediment or to control scour. (Typical size: 1m wide x 1m high x 2m long)

Geotextile, geofabric, geosynthetic material

Any permeable synthetic textile material, fabric or net used with earth, soil, rock or foundations as an integral part of an engineering structure. Mainly used to improve structural and/or hydraulic properties of soil, to reinforce or stabilise embankments, as a filter layer in drainage applications or for erosion control.

Groundwater

The water contained in interconnected pores located below the water table in an unconfined aquifer or located in a confined aquifer.

Groundwater, confined The water contained in a confined aquifer. Pore water pressure is greater than atmospheric at the top of the confined aquifer. Groundwater, perched

The water in an isolated, saturated zone located in the zone of aeration. It is the result of the presence of a layer of material of low hydraulic conductivity, called a perching bed. Perched groundwater will have a perched water table.

Groundwater, unconfined

The water in an aquifer where there is a water table.

Grout curtain

An underground wall designed to stop ground waterflow; can be created by injecting grout into the ground, which subsequently hardens to become impermeable.

Heterogeneous

Pertaining to a substance having different characteristics in different locations. A synonym is non-uniform.

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glossary

Homogeneous

Pertaining to a substance having identical characteristics everywhere. A synonym is uniform.

Hydraulic conductivity A coefficient of proportionality describing the rate at which water can move through a permeable medium. The density and kinematic viscosity of the water must be considered in determining hydraulic conductivity. Hydraulic gradient

The change in total head with a change in distance in a given direction. The direction is that which yields a maximum rate of decrease in head.

Hydraulic radius

A measure of waterway geometry used in hydraulic calculations. The cross sectional area of flow in a drain or pipe divided by the wetted perimeter (ie. length of wetted surface) perpendicular to the direction of flow.

Hydrogeology

The study of the interrelationships of geologic materials and processes with water, especially groundwater.

Hydrologic cycle

The circulation of water from the oceans and other waterbodies through the atmosphere to the land and ultimately back to the ocean.

Hydrology

The study of the occurrence, distribution and chemistry of all waters of the earth.

Infiltration

The flow of water downward from the land surface into and through the upper soil layers.

Isotropy

The condition in which hydraulic properties of the aquifer are equal in all directions.

Laminar flow

That type of flow in which the fluid particles follow paths that are smooth, straight, and parallel to the channel walls. In laminar flow, the viscosity of the fluid damps out turbulent motion. Contrast with turbulent flow.

Manning's coefficient (n) A dimensionless value defining the roughness of a surface (eg. pipe wall or sides of a drain) with regards to water running across that surface. Used in hydraulic calculations such as Mannings equation. Manning’s equation

A formula used for calculating the flow in a given waterway (eg. pipe or open channel drain).

Model calibration

The process by which the independent variables of a digital computer model are varied in order to calibrate a dependent variable (eg. head) against a known value (eg. water table).

Model verification

The process by which a digital computer model that has been calibrated against a steady-state condition is tested to see if it can generate a transient response, such as the decline in the water table with pumping, that matches the known history of the aquifer.

Numerical model

A model of groundwater flow in which the aquifer is described by numerical equations with specified values for boundary conditions that are solved on a digital computer.

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glossary

Phreatic surface

“Free” surface of groundwater; pressures are equal to atmospheric along this surface.

Piezometer

A non pumping well, generally of small diameter, that is used to measure the elevation of the water table or potentiometric surface. A piezometer generally has a short well screen through which water can enter.

Piezometric head

Pressure head experienced by a given body of water, comprising both static levels and inertial forces.

Piping failure

Failure of an earth dam wall caused by excessive seepage of water through the embankment.

PMF - (Probable maximum flood)

The flood caused by runoff water from the probable maximum precipitation.

PMP - (Probable The greatest depth of precipitation for a given duration meteorologically possible for maximum precipitation) a given size storm area at a particular location at a particular time of year. Porosity

The ratio of the volume of void spaces in a rock or sediment to the total volume of the rock or sediment.

Recharge

The process of replenishment of a water resource (recharging of aquifer, recharge of dam).

Rational method

A procedure for calculating the peak discharge from a small to medium sized catchment, resulting from a storm of a given ARI and duration.

Reno mattress

A low profile flexible wire basket filled with stones and used to control scour. (Typical size: 2 m wide x 6 m long x 0.3 m deep)

Revetment mattress

A hard surface armouring formed by using pocketed pervious fabric filled with concrete. Used to control scour.

Rip Rap

Irregular rocks of medium to large size, used for the lining of embankments, drainage channels, dam spillways etc. for prevention of erosion.

Runoff

The total amount of water flowing in a stream. It includes overland flow, return flow, interflow and baseflow.

Sediment barriers

Structures placed in a drainage channel to promote settling out of sediment until a stable flow slope is achieved between each barrier. Used for erosion prevention.

Sediment fence / silt fence

A low fence of woven geotextile designed to filter suspended solids from overland flow, (sheetflow). Used for containment of sediment in disturbed areas.

Seepage

Common term for groundwater flow, encompassing the characteristic “slow flow” processes (see laminar flow).

Sheet piling

Physical barrier applied by driving solid sheets of impermeable material into the ground.

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glossary

Slurry wall

An underground wall designed to stop groundwater flow; constructed by digging a trench and backfilling it with a slurry rich in bentonite clay.

Soil matrix

Skeletal structure of soil, within which “honeycombs” of pores exist.

... % Standard compaction

An earthworks term used to specify the amount of compaction effort required (or compaction achieved) in engineered earthworks.

Surface water

Water found in ponds, lakes, inland seas, streams and rivers.

Time of concentration

The time required for rain falling at the farthest point of the catchment to flow to the point at which the discharge is being calculated. Used in hydrology calculations such as the Rational Method.

u/s

Up stream (eg. u/s of a dam).

Water table

The surface in an unconfined aquifer or confining bed at which the pore water pressure is atmospheric. It can be measured by installing shallow wells extending a few feet into the zone of saturation and then measuring the water level in those wells.

Wetlands

Areas where water is over or near the ground surface for long enough each year to maintain saturated soil conditions along with related vegetation (eg. marshes, bogs, swamps etc.).

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FA C T S H E E T N O . 1

Field Record Data Sheets Example of Sampling Report Form for Marine Waters Site

Site Code

Date

Time

Latitude

Longitude

Site Description

HYDROGRAPHIC CONDITIONS Tidal Currents: Direction

Approx. velocity

Time of high water

Time of low water

WEATHER CONDITIONS Wind Direction

Force

Cloud cover

State of sea

Depth (m)

Temperature (ºC)

Salinity

Dissolved Oxygen (% sat.)

Sample Number

Time

Sampling method Analysis profiles Remarks

Sampler

Signature

Date MODIFIED FROM IS0 STD 5667-9:1992 (E)

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FA C T S H E E T N O . 1

Field Record Data Sheets Example of Sampling Report Form for Groundwaters Site

Site Code

Date

Time

PUMPING DETAILS Height of riser/bore pipe above ground level

(m)

Water level within aquifer (before pumping)

(m)

Water level within aquifer (after pumping)

(m)

Pumping Time Volume Extracted (estimated) SAMPLING DETAILS Time: Start

End

of sampling

Depth of sampling Sampling method Sample appearance

Details of preservation techniques employed

Details of sample storage method employed/required

Remarks

Sampler

Signature

Date MODIFIED FROM IS0 STD 5667-11:1993 (E)

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FA C T S H E E T N O . 1

Field Record Data Sheets Example of Sampling Report Form for Surface Waters (LAKES, STREAMS, WATER STORAGES AND TAILINGS DAMS)

Site

Site Code

Date

Time

Site Description Water Depth

Volume

Time: Start

End

of sampling

Sampling method DEPTH-INTEGRATED SAMPLE Withdrawal between

and

m

OBSERVATIONS AT THE SAMPLING POINT Turbidity, caused by sediment particles

/plankton

Colour

Odour

Water plants Estimation of the discharge of the streams/river: (high/medium/low) LOCAL WEATHER CONDITIONS Air temperature Wind force Direction of wind Cloudiness (%) Remarks

Sampler

Signature

Date

MODIFIED FROM IS0 STD 5667-4:1987 (E)

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FA C T S H E E T N O . 1

Field Record Data Sheets Example of Sampling Report Form for Domestic and Industrial Wastewater Site

Site Code

Date

Time

Sample method: Grab

Composite-time dependent



Equipment Used

Interval of flow between samples

min or m3

Volume of grab samples

mL

Sampling started

Sampling ended

Preservation method

FIELD MEASUREMENTS

Test

Result

Unit

Time

Remarks

Sampler

Signature

Date MODIFIED FROM IS0 STD 5667-10:1992 (E)

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FA C T S H E E T N O . 2

Estimation of Surface Runoff overlying an impervious or rock layer, or where the groundwater level is very near the surface (eg. at valley bottoms or near streams) it will not take long to saturate the surface soil. Once this occurs, infiltration ceases and water will flow over the surface as saturated overland flow. Alternatively in sandy areas, or areas of deep permeable soil overlying impervious layers, water can rapidly flow downslope through the soil and percolate out of the soil when it intercepts a saturated zone. This is known as interflow and is differentiated from groundwater flow by the speed with which it reports to watercourses. The efficiency of these runoff processes is again dependant on soil types, as well as rainfall intensity, the geology of the area, catchment slopes and groundwater levels.

This fact sheet examines surface runoff processes and techniques used to estimate total catchment runoff and peak flows generated by runoff for small to medium sized ‘non urbanised’ catchments (< 250 km2). Accurate estimation of these quantities depends on a large number of site characteristics. Hence it is not within the scope of this handbook to give precise techniques for every region in Australia. Instead, the general principles will be discussed and references provided to locate the information specific to a given region.

Runoff Processes Losses: When rain falls on a catchment surface, a portion of it will be held back as ‘losses’ before the remaining ‘excess rainfall’ reports to streams or drainage channels as surface runoff. The losses combine a number of rainfall and interception mechanisms. In the early stages of a storm, much of the rain is intercepted by trees, grass and other plants and stored on leaves and branches etc. as interception storage. When these stores are full, water will reach the ground surface and commence filling small depressions. As these fill and overflow, large depressions begin to fill until this depression storage is full and overland flow commences. There are continuing losses through infiltration into the soil which starts at a high rate if the soil is initially dry and then rapidly decreases until approaching a steady rate known as the infiltration capacity of the soil. Evaporation from the vegetation and ground surfaces will also contribute to the losses. From this discussion it can be seen that losses (and hence rainfall excess) are affected by vegetation type and density, soil type and degree of disturbance, catchment slope and the number and efficiency of watercourses in the catchment.

Design losses: When estimating total or peak runoff values it is necessary to estimate the losses, as it is only the rainfall excess which contributes to the runoff. With losses depending on so many site specific variables it is almost impossible to realistically model the processes. Even within a Single small catchment there will be a large number of sub areas responding differently due to varying physical characteristics. To simplify matters, a number of methods have been developed for applying general losses across a whole catchment. A full discussion of these methods, along with typical loss values for regions throughout Australia can be found in Chapter 6 of Australian Rainfall and Runoff 1987 (AR&R). The simplest and most popular of these methods are (refer to Figure FS 2.1): (i) Constant fraction (proportional losses/ runoff coefficients): Loss is assumed to be a constant fraction of the rainfall. This can be viewed in two ways: a) A runoff coefficient (ie. 0.7) is applied to the rainfall. If a catchment large distinct areas (ie. undisturbed, stockpiles, sealed areas etc.) then a different coefficient can be applied to sub areas; and

Runoff types: Once losses have been absorbed there are two major runoff routes by which water reaches watercourses. In areas where soil is thinly 1

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FA C T S H E E T N O . 2

Estimation of Surface Runoff b) If a predictable proportion of the catchment is known to become saturated during rain then this area can be viewed as the proportion of the catchment contributing runoff. (ii) Constant loss rate: If a catchment has minimal interception or depression storage and the infiltration into the soil is fairly constant (ie. if the catchment is already wet from previous antecedent rain) then a constant loss rate matching the infiltration capacity of the soil is a valid approach. (iii) Initial loss - constant loss rate: In line with the above discussion of interception losses through vegetation and depression storage, followed by ongoing losses due to soil infiltration and evaporation, is the concept of having no runoff until an initial loss is satisfied and then having a constant loss rate for the remaining duration of the rain. As well as AR&R there are many other sources of information for loss values applicable to an area: • Consulting engineers/hydrologists; • State government water resources departments; • State government mining departments; • State government agriculture/primary industries/forestry etc. departments; • Local Landcare groups; and • Local government engineers. To obtain accurate estimates of losses it is important to note that there is no substitute for site measured data. A historical record of rainfall and streamflow (or dam levels, releases and overflows) will enable a hydrologist or engineer to develop much more accurate versions of the above loss models.

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FA C T S H E E T N O . 2

Estimation of Surface Runoff Estimating Total Runoff

Detailed discussions of estimation procedures can be found in AR&R. For typical mine catchments, the best method to obtain a quick estimate is the rational method which is of the form:

The total volume of runoff (saturated overland flow and interflow) from a catchment is important when examining overall site water balances or storage capacities required for water supply dams etc. The general procedure is to simply apply rainfall from the period of interest (eg. a single storm, a typical year or a long sequence of wet or dry years) to the catchment, subtract the appropriate losses as discussed previously and assume the excess rainfall reports as runoff to a stream, dam or pond. (The long-term processes of evaporation and seepage losses from a storage area must also be taken into account for long-period water balances.) The rainfall data required is discussed in Fact Sheet No. 10: Hydrological Data for Design Purposes. Computer programs are available for applying long-term daily rainfall records to a catchment, varying the loss values to suit historical stream flows or dam levels. These can be used for projecting catchment yields into the future to examine water storage and recycling opportunities. One such model gaining popularity in Australia is the AWBM model.

QY = 0.278. CY. Itc, Y . A (Eqn 5.1 AR&R) where • QY = Peak flow rate (m3/s) of average recurrence interval (ARl) of Y years • CY = Runoff coefficient (dimensionless) for ARI of Y years •

• Itc, Y = Average rainfall intensity (mm/h) for the design duration of tc hours and ARI of Y years. The way to use the rational method is as follows: • first decide on the appropriate risk level, hence selecting the average recurrence interval of storm to be used (refer to Fact Sheet No.3); • the duration of storm to give the worst flood is then selected. The principle here is that the shorter the storm the higher the intensity will be for a given ARI. However, if too short a time is used then runoff from far reaches of the catchment will not have had a chance to contribute to the flow. Hence the critical duration, known as the time of concentration tc, is selected as the time required for the most remote part of the catchment to begin contributing to runoff at the point of interest. Different methods for calculating tc are presented in AR&R for various regions in Australia. Most of these depend on stream lengths and typical catchment slopes;

Estimating Peak Flows As discussed throughout this handbook, interception drainage, erosion protection, settling ponds and essential drainage infrastructure (eg. culverts, spillways etc.) must all be carefully designed to suit the expected peak flow they are expected to experience. A confident estimate of this flow is essential to: a) prevent under designing drainage infrastructure, which may result in damage and hence disruptions to mine operations and ongoing repair and upgrade works; and

• determine the average rainfall intensity (mm/hour) associated with the selected ARI and tc. Intensity; duration, and frequency rainfall curves for the specific minesite will be required. These can be developed using guidelines in AR&R or can be obtained through the Bureau of Meteorology.

b) avoid over designing, which is of course uneconomical. 1

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FA C T S H E E T N O . 2

Estimation of Surface Runoff

They will simply need the longitude and latitude of the minesite (refer to Fact Sheet No. 10);

• calculate the runoff coefficient for the site using the methods defined in AR&R for each region within Australia, or if available using values developed for your specific area and type of land use. (Neighbouring mines, land care groups, soil conservation departments or universities involved in runoff management in your area may have previously developed such coefficients); and

Once the PMP is determined, small losses are applied to determine the rainfall excess. The losses will be small due to the high likelihood of antecedent rainfall. It is suggested that values of zero or slightly below the lowest specified loss values for the area can be used. Having determined the rainfall excess, it is then a matter of using methods as described above, or more complex flood routing techniques (depending on catchment size and complexity) to determine the probable maximum flow (PMF). Section 13.4 of AR&R gives basic descriptions of the techniques used in such calculations.

• measure the plan area (km2) of the catchment feeding into the point of interest, taking into account pits, diversion drains, ridges etc. Having obtained all the above information, it can be used in the previous equation to give the peak flow.

Probable Maximum Flows (PMF) When designing spillways on large dams or examining major flood mitigation works where lives may be at risk, it is usually wise to use the maximum possible flow rate. This will ensure that the given element is unlikely to ever fail. Due to the importance of such calculations, experienced engineers or hydrologists should be consulted before using these flows for design purposes.

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Before it is possible to calculate peak flows, it is necessary to determine the probable maximum precipitation for the given area. For small areas and short-duration storms the Bureau of Meteorology has published an upgraded method of calculating PMP in Bulletin 53 (December 1994) The Estimation of Probable Maximum Precipitation in Australia: Generalised Short Duration Method. For larger areas or long storms, the Bureau will provide estimates of PMP for a set charge.

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FA C T S H E E T N O . 3

Understanding Event Probability Water management is not an exact science as rainfall is an integral part of the hydrological, and therefore the water management, cycle. Just as it is impossible to accurately predict quantities of rainfall, it is impossible to provide definitive answers to most water management questions. However, it is possible to define probabilities and risks of occurrence of particular events.

The AEP is often used for the probability expressions associated with large and extreme events and some flood estimation methods. The AEP is generally expressed as a fraction or percentage.

Probability (P) of Exceedance in L Years Probability of exceedance in L years is a descriptive risk term that relates the event exceedance probability to the design or useful life of the resource or structure. In probability terms it can be expressed as:

Care should be taken when communicating and interpreting probabilities and risks, and rigorous terminology should always be used. Probabilities and risks which are based on historical data carry an implicit assumption that history will repeat itself.

P = l-exp(–L/T where T is the ARI.

The following are more common risk terminologies used in water management practices. More detailed descriptions and understandings can be found in Australian Rainfall and Runoff 1987 (AR&R).

Probable Maximum Precipitation (PMP) The probable maximum precipitation refers to the greatest depth of precipitation for a given duration that is meteorologically possible for a given size storm area at a particular location at a particular time of year. The Probable Maximum Flood (PMF) has a similar definition and is related directly to the PMP (Also refer to Fact Sheet No.2.)

Average Recurrence Interval (ARI) The average recurrence interval is the average interval between exceedances of that value or event when viewed in the long (ideally infinite) term. All data above an arbitrary base value are used when ranking event values for determining the ARI. The ARI is usually expressed in years. It should be noted that, a rainfall (or flood) ARI of 100 years does not imply the event will only occur every 100 years; it is also feasible that the event will occur five times in five successive years and not occur for another 495 years. The terms “100 year return interval” and “the one-in-hundred-year­ storm” falsely advocate the former interpretation.

Due to the variable nature of the hydrological cycle, the use of risk analyses and probabilities should be encouraged in water management strategies. Where historical data are used to determine these risks, care must be taken to include as much relevant historical data as are possible. This reduces the element of skewing in risk analyses. In this way, although absolute answers are rarely available, water management strategies may be assessed a logical and justifiable manner.

Annual Exceedance Probability (AEP)

Because water management involves expressions of risk, the impacts of failure must always be assessed. Where appropriate, contingency failure strategies should be established and regularly audited and monitored.

The annual exceedance probability is the probability of exceedance of a given event within a period of one year. It is based on data that uses only the highest event in each year of record.

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FA C T S H E E T N O . 3

Understanding Event Probability Where hydrological analyses are used in a water management study, it should be clearly understood that a large proportion of the quantitative analyses is probabilistic only. The broad assumptions and the extent to which historical data play a part are documented in the industry standard AR&R. This document should be referred to when a more detailed understanding of event probability is required.

Sensitivity analyses provide means of assigning boundaries or limits to water management scenarios by asking “what if...?” type questions. Sensitivity analyses should be carried out on parameters which are thought to be important or on those which are not very well understood, such as hydraulic conductivity of soil, process plant water use etc.

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FA C T S H E E T N O . 4

Open Channel Drains Manning’s Equation:

The basic principles behind locating and sizing an open channel drain for normal depth flows are:

Q = A.R2/3S1/2 n

• determine the size of the catchment feeding into the base of the proposed section of drain. On reasonable size catchments, it is often worthwhile to separate the proposed drain into sections. By doing this it may be possible to have a smaller cross section in the upper section of the drain which only services the upper reaches of the catchment; • for a suitable ARI (commonly 5 to 20 yrs) calculate the peak flow in each section of the drain as described in Fact Sheet No.2; • calculate the slope of the drain. If it is not possible to achieve a uniform slope along the length of the drain it should again be separated into sections of similar average slope. (Note: Wherever possible the slope of the drain should be in the range of 0.5% to 1.0% or to suit local soil conditions. This will drastically reduce the cost of erosion control measures. It is preferable to ‘snake’ drains down steep slopes rather than taking the shortest possible route;) and



where:



Q = Flow (m3/s)



A = Cross sectional area of flow (m2) R = Hydraulic Radius (= A/WP)



WP = Wetted perimeter; length in m of wetted contact between water and the channel measured at right angles to the direction of flow



S = Slope of channel section (m/m)



n = Manning’s roughness coefficient.



Typical values of Manning’s n are:



Smooth concrete lining

0.014 - 0.018



Smooth graded earth

0.025 - 0.03



Grass cover

0.04 - 0.06



Rock lining

0.04 - 0.06

In uniform section open channels, regard for flow and hydraulic radius should be considered for Manning’s n (refer Chow, 1973).

• having established the flows and slopes for the proposed section of drain, a cross section size can be calculated using the Manning’s equation (shown below) with suitable roughness coefficients. (Note: Roughness coefficients are determined by the type of lining there is in the drain, ie. a smooth bare earth channel will have a low roughness coefficient while a channel lined with large unevenly placed rocks or dense vegetation will have a high roughness coefficient.) A freeboard of between 100 and 300 mm is added to the flow depth to give the design drain depth.

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FA C T S H E E T N O . 4

Open Channel Drains The best method for using this equation is to trial different drain cross sections and flow depths until sufficient flow capacity is achieved. • As an iterative procedure with the previous step, the type of erosion protection to be used in the drain should be decided at this stage. As described in Fact Sheet No.8, a different level of protection is required as the flow velocities increase; however the erosion protection method will also affect the flow velocity (Q/A) hence the need for iteration. • The following tips should be followed for selecting a drain cross section: – steep side slopes should be avoided (2-3 H to 1 V recommended); – the cross fall of the natural ground will affect the actual slopes used; – v-shape drains are recommended for minor drains while trapezoidal shapes should be used for large drains. The base width of a trapezoidal drain should be sized to suit earth moving equipment to be used; – a contour drain should be cut into the cross slope sufficiently to provide a balance of cut to embankment fill; and – embankments should be compacted to a minimum 90% Standard Compaction. Note: where large channels are required, expert advice should be sought due to the potential for backwater and downstream effects.

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FA C T S H E E T N O . 5

Construction of Small Earth Embankment Dams The intent of this fact sheet is to allow the mine operator to build small earth dams (“farm dams”) for minor or temporary water supply or to form part of a diversion drainage scheme. If the dam is an important water supply or flood mitigation tool then input from civil engineers and hydrologists is vital. The calculation of expected catchment yields and flood flows are covered elsewhere in this handbook; hence this fact sheet will cover the selection of a dam site, dam design and dam construction. The information in this fact sheet is collated from the text Nelson (1991).

valley must remain stable when saturated to avoid land slips into the dam; • the foundations for the dam must be sufficiently strong to support the embankment without excessive settlement and must be impervious to seepage. Stiff inorganic clay is ideal while sedimentary rock can be acceptable. Fractured igneous rock or deep layers of sand and gravel should be avoided; • the availability of suitable material nearby is vital. Available quantities will determine the type of embankment used as illustrated in the attached table (Figure FS 5.1). Impervious material for embankment construction should contain 20%-30% clay with sand, silt and some gravel. No rocks greater than 75 mm size should be present. As a safety factor, two to three times the expected quantities should be available; and

Selecting a Dam Site The easiest and most efficient dams involve constructing an earth embankment across a small valley. These are commonly known as gully dams and will be the focus here. Other types of small dams, including hillside dams, turkeys nest ponds and excavated tanks, are feasible alternatives if a suitable gully is not available, and involve many of the same principles to be discussed. The important points to consider when selecting a dam site are as follows:

• a subsurface geotechnical investigation should be carried out on favoured sites to assess the above factors as well as groundwater levels, cutoff trench depths and borrow pit boundaries. The investigation should include excavated pits along the dam centreline, spillway and in borrow areas followed by geotechnical testing of samples.

• minesite licence conditions should be checked or local water resources authorities contacted to ensure a dam is allowable under environmental, water use and dam safety restrictions;

Dam Design

• the storage volume should be selected to suit the expected catchment runoff volumes. This will prevent excessive earthworks or an eroded spillway; • unless the dam is for sediment capture purposes, the upstream (u/s) catchment should not be excessively disturbed. If this is unavoidable, an u/s silt trap will have to be installed and constantly maintained (ie. emptied);

Good design of the dam and spillway is vital to ensure a stable embankment and to prevent failure due to erosion or excessive seepage leading to piping failure. Piping failure results from seepage water transporting material out of the embankment causing a ‘pipe’ which rapidly expands leading to massive failure. The basic geometric design principles for a stable dam are illustrated in Figure FS 5.1. The following points should also be accounted for in the dam design.

• an ideal site is on a flat gradient watercourse in a wide flat-bottomed valley immediately upstream of a narrow gorge. Sides of the

• Cutoff excavations are used to prevent seepage under the embankment by providing a impervious barrier linking the embankment to

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FA C T S H E E T N O . 5

Construction of Small Earth Embankment Dams impervious foundation material. It must be connected directly to the impervious embankment material and must be keyed into suitable foundation material as shown in the table below. If a cutoff trench is impractical due to excessive depths, an effective alternative where foundations are moderately pervious is to use a clay blanket 0.6 m thick (approx.) extended 35 m (approx.) u/s from the embankment.

Good control of construction methods and material condition is vital to achieve a water tight dam. The following construction phases and guidelines should be adopted: • prior to commencing construction of the dam a surveyor should identify the extent of

Suitable foundation

Required penetration

Width of cutoff

Batter slopes for

material (SFM)

depth into SFM

trench at base

excavated trench

Clay

0.6 m

2.5 m minimum

1 :1

Rock

0.3 m

0.3 m

vertical

• Spillway flows must be diverted away from the downstream (d/s) toe of the embankment to avoid erosion. A small return wall at the spillway may be required, as shown in the figure. If continuous small flows are expected over the spillway it is advisable to install a trickle pipe or a small flow channel just below the main spillway level. This will prevent scour erosion. • Outlet pipes are sometimes necessary to create a gravity supply, supply a pump, drain water for dam maintenance, satisfy legal requirements or to allow the dam to be used as a flood flow detention storage. If these requirements are not applicable it is best to avoid outlet pipes.

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inundation, the embankment centre line and batter toe lines, the spillway and borrow pits; • ensure the proper equipment is available. This should include scrapers and dozers for small embankments while larger projects will also require graders, rollers and water carts; • if the dam is located in a gully or stream which flows regularly it will be necessary to dewater the site. This is best achieved using an upstream weir and a gravity drain which bypasses the dam. Groundwater in trenches will need to be pumped out; • a reliable water supply is important if the material used in the embankment needs conditioning (ie. addition of water to allow proper compaction);

• Freeboard is required on dams to allow for uncertainties in flood flow estimation, inaccuracies in construction and wave action. The heights shown on the figure assume a maximum 500 mm flow depth over the spillway. If an alternative spillway arrangement is used, the freeboard must be altered to suit.

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Dam Construction

• the area to be inundated by the dam water must generally be cleared and grubbed. This includes removing all trees, shrubs, rocks and any debris. This can be modified if aquatic habitat is to be an ancillary function of the storage. This should be burnt or pushed downstream of the embankment. At the same time the area under the embankment should be cleared and have all topsoil stripped (100 mm minimum) and stockpiled; R

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FA C T S H E E T N O . 5

Construction of Small Earth Embankment Dams • outlet pipes, if required: – must only be placed in a trench cut into natural ground or compacted embankment. The trench should be at least 100 mm deeper than the pipe diameter;

• the cutoff excavation should then be carried out and impervious material placed and compacted to bring the level back up to that of the stripped foundation. The whole foundation area is then lightly scarified (50 mm deep) in preparation for construction of the embankment;

– between three and six cutoff collars (1.2m x 1.2m) shall be evenly spaced along the pipe to prevent seepage of water along the pipe;

• borrow pits should ideally be within the area covered by the stored water. They should have side slopes of 3H:1V and should be positioned a minimum of 6 m away from the upstream toe of the dam embankment;

– do not place pipes at the very base of the dam if sediment is likely to be a problem; – it is advisable to include a trash rack at the inlet to the pipe;

• embankment construction requires control of: – the moisture content of the embankment material when placed must generally be within the range 3% dry to 2% wet of optimum moisture content. This is the moisture content which allows the maximum density to be achieved by the compaction equipment used;

– valves should be placed at the discharge end or in a pit on the d/s slope of the embankment; and • topsoil to a depth of 100 to 150 mm minimum and good holding grass such as kikuyu or couch should be placed over the entire embankment (u/s and d/s) and spillway. This should be fertilised and irrigated if necessary to ensure rapid growth and hence immediate erosion prevention.

– the loose thickness of layers placed should not exceed 100 mm if dozers and scrapers are used for compaction or 200 mm for sheepsfoot rollers; – the degree of compaction achieved should be 95% Standard Compaction or 90% Modified Compaction. This will usually require between four and eight passes with a sheepsfoot roller; – batter slopes should be controlled using a template (timber triangle with the required horizontal and vertical length ratios ie. 3H:1V) and spirit level; • the spillway must be constructed absolutely level to ensure there are no preferential flow paths which will erode. When cutting is complete the surface should be topsoiled, grassed and compacted;

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FA C T S H E E T N O . 5

Construction of Small Earth Embankment Dams FIGURE FS 5.1

GEOMETRIC DESIGN CRITERIA Homogenous

Dam Element

Zoned Dam

Diaphragm Dam

HEIGHT OF DAM (m)

0-3

3-6

6-9

0-3

3-6

6-9

0-3

3-6

6-9

CREST WIDTH (m)

2.8

3.5

4

2.8

3.5

4

2.8

3.5

4

UPSTREAM BATTER SLOPE (H : V)

3:1

3:1

3.5:1

2:1

2.5:1 3:1

3:1

3:1 3.5:1

2.5:1

3:1

3:1

2:1

2.5:1 3:1

2.5:1

3:1

3:1

0.6

0.85

1.1

DOWNSTREAM BATTER SLOPE (H : V) DIAPHRAGM THICKNESS ‘D’ (m) (Perpendicular to dam face) FREEBOARD (m) :

FETCH < 1000 m

1.0 m - Assuming 0.5 m maximum spillover depth

FETCH > 1000 m

1.5 m - Assuming 0.5 m maximum spillover depth

SETTLEMENT ALLOWANCE (mm) (Construction level above required crest level)

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FA C T S H E E T N O . 5

Construction of Small Earth Embankment Dams

SPILLWAY DESIGN FLOOD FLOW

MINIMUM INLET WIDTH

(m3/s)

(m)

<5%

5-10%

10-15%

15-20%

20-25%

3

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6.5

10

15

18

20

6

11

13

21

30

35

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19

31

44

53

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26

41

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70

80

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33

52

74

87

100

MINIMUM OUTLET WIDTH (m) (Various Return Slopes.)

CONSTRUCTION MATERIAL (in order of preference)

CODE

DESCRIPTION

GC

Clayey gravels

SC

Clayey sands

CL

Inorganic clays (Low liquid limit)

CH

Inorganic clays (High liquid limit.)

GW

Well graded gravels.

GP

Poorly graded gravels

SW

Well graded sands

SP

Poorly graded sands

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FA C T S H E E T N O . 6

Culvert Crossings • headwater: the greater the level of water at the inlet to a culvert compared to the outlet, the greater flow it will pass. It is generally acceptable to design culverts to flow with water up to a level just below overtopping of the road (ie. 300 mm to 1.0 m), for the design peak flow;

Culverts are commonly used to provide road crossings over drains or small creeks, and there is a wide variety of culvert shapes and materials that can be selected to best suit a particular application. The correct design and installation of these culvert crossings will prevent blocked or eroded drainage channels as well as costly road repairs. There are a number of areas that need to be addressed.

• downstream depth: in contrast to the upstream depth, the normal depth of flow immediately downstream from the culvert should be kept as low as possible to maximise the efficiency of the culvert. To achieve this a deep or wide channel is advisable downstream of the culvert; and

Flow Capacity The first and perhaps most obvious concern is to construct a culvert which is large enough to pass the design flow without overtopping the road or embankment. It is not practical to design culverts to take all possible flows; hence the designer must decide what risk level is acceptable for overtopping of the road and calculate a design flow of a suitable ARI (refer to Fact Sheet Nos 2 and 3). A culvert installation must then be sized to pass this flow. The hydraulics of culverts are surprisingly complex and rely greatly on the site conditions (ie. downstream flow depths, culvert sizes, shapes, lengths and slopes). It is not feasible to cover all possibilities in this handbook; however suppliers of culverts, State government roads departments, and many open channel hydraulics text books provide charts for determining the flow through various culverts. The basic controlling factors are as follows:

• inlet design: the design of the inlet can greatly affect the flow capacity of a culvert flowing under inlet control. Greater flow can be achieved be shaping the approach to the culvert to funnel flow into the culvert. If the water entering the culvert has a high suspended solids load, it is important to keep this water moving through the culvert. Any ponding at the inlet will inevitably result in the culvert becoming blocked. To avoid this, drops or chutes can be utilised to accelerate flow into the culvert.

Inlet/Outlet Protection Flows forced through culverts with a high head water will accelerate into the pipe and can discharge at a high velocity. High levels of turbulence will also result from water spreading out into basic channel flow again. To ensure that this high energy flow does not cause massive erosion at the inlet and outlet and under scour of the pipe it is important to provide erosion protection. This is usually achieved with headwalls and aprons of reinforced concrete, a concrete revetment mattress or grouted rock. At the downstream end, rock Rip Rap is also advisable for a further distance downstream from the apron. The level of protection required will depend on the outlet velocity, as described in Fact Sheet N0. 8.

• inlet/outlet control: a culvert which is able to pass water at a greater rate than is being supplied is said to be flowing with inlet control. If the culvert inlet geometry, flow resistance or depth of water in the downstream channel result in water being supplied at a greater rate than it can flow through the culvert, it is said to be under outlet control. When using design charts it is important to examine both control cases and adopt the worst case value (ie. the highest headwater or least flow);

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FA C T S H E E T N O . 6

Culvert Crossings rely on the strength of the fill around them and hence require very good compaction in the side zones. Numerous Australian Standards, as well as material supplied by manufacturers, give excellent advice on correct installation. One important factor to note is that many mine vehicle axle loadings will exceed standard highway values and hence special care must be taken when selecting the class of culvert (ie. wall thickness) required.

This will normally form part of the above hydraulic calculations.

Installation Correct selection of culverts and supervised installation is vital to ensure that heavy vehicles passing over will not damage the culvert. Depending on the culvert material and shape selected, there will be varying requirements for cover (fill depth) over the culvert and compaction requirements around the culvert. Concrete culverts rely on their own strength and require good foundations and substantial cover, while corrugated steel culverts

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FA C T S H E E T N O . 7

Acid Drainage • treatment of acid drainage: – lime or alkali treatment of the drainage; blend solids with alkaline material eg. limestone;

Acid drainage occurs when sulphides (usually iron sulphides) are oxidised according to the following, highly simplified, equation:

– use bacteria for sulphide precipitation;

FeS2 + xO2 + yH2O –> Fe(OH)3 + 2H2SO4.

– use plants to uptake and store metals, eg. wetlands; and

The process is bacterially mediated and temperature and moisture all affect the rate and expression of the problem. However, the neutralising capacity of the gangue is probably the most significant factor in reducing or preventing the formation of acid drainage. The geochemical reactions and indicators of sulphide oxidation and acid generation are shown in Figure FS 7.1.

– use concentration/recovery process, eg. cementation. Considerable work has been undertaken around the world and the status and outlook for key control technologies are summarised in Table FS 7.1. In high rainfall environments, the volumes of contaminated water that are generated can be extremely difficult and costly to contain and/or treat. This potentially ongoing, long-term cost should be factored in to any development decision.

In addition to the generation of acid, the low pH of these waters can mobilise trace and heavy metals, resulting in the potential for widespread contamination. There are many techniques available to foresee if acid drainage is likely to be a problem, including: • chemical prediction/materials characterisation (NAPP, ANC, NAG, solution indicators); • models (eg. for location of acid generating material in a model of the orebody and waste, rates of acid generation, timing of appearance in mining schedule, predictions and schedule of cost of treatment); and • predictions of ecological impacts. Once acid drainage is present, opportunities to manage it are limited to: • prevention of the generation of acid: – separate the acid producers (for sale or entombing); – cut off oxygen (wet or dry covers); – pacify the mineral surface; – solidify the waste rock or waste rock mass; and – minimise water movement (generation and transport of acid); and/or

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FA C T S H E E T N O . 7

Acid Drainage

Note 1: Non ferrous metal sulphides such as CuS, PbS, NiS, ZnS are acid neutral. Sulphides such as Cu2S are acid consuming. Note 2: Siderite (FeCO3) is not included since it has nil net neutralising capacity in an oxidising environment. Note 3: pH of site drainage may initially increase in response to sulphide oxidation and acid neutralization reactions. Note 4: Other precipitates such as CuCO3, MnO2, CuSO4 can also be observed over a wide pH range. Note 5: Jarosite iron oxide/hydroxide equilibria is a strong pH buffer and can maintain the pH as 3 even after all pyrite has been oxidised. Jarosite and iron oxides coat soil mineral surfaces and dominate the mineral solution chemistry. ENVIRONMENTAL GEOCHEMISTRY INTERNATIONAL PTY LTD

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FA C T S H E E T N O . 7

Acid Drainage TABLE FS 7.1: Status and Outlook for Key Control Technologies Technology

Applicable

Current status

Research outlook

Major limits

Chemical prediction

All

Inexact

Good

Costly

Prediction models

All

Incomplete

Good

Complex

Pre-treatment

Some

Beginning

Good

Site specific

Dry covers

Many

Field demonstration

Very good

Cost

Wet covers

Many

Laboratory/Field

Very good

Site specific

Selected

Laboratory

Fair

Cost

Lime neutralisation

All

In practice

Excellent

Perpetual

Sludge disposal

All

Emerging

Good

Volume/ Containment

Bio-treatment

Partial

Laboratory/Pilot

Fair

Capability/ Efficiency

Metal recovery

Selected

Laboratory/Pilot

Poor

Economics

Fixation

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FA C T S H E E T N O . 8

Erosion Control and Sediment Containment Erosion Control Methods

Minimising erosion and capturing sediment contained in surface runoff is a major environmental issue on minesites. Site discharge licences will normally specify a suspended solids limit for discharge offsite from a storm of a given risk level (eg. a 5 year Average Recurrence Interval).

The prevention of erosion is achieved by protecting soils from the erosive forces of water and/or by controlling the flow of water to reduce erosive forces. Large areas subject to sheet runoff should be protected as follows.

There are four main control options and an effective site program will need to incorporate all of these.

• Contour ripping: Bare or newly revegetated areas should be cross contoured to slow down flows. This will also prevent concentrated flow paths from forming. An added benefit of this technique is the retention of water stored in the furrows which will aid the growth of new vegetation and will reduce total quantities of runoff.

Minimising disturbance and rapid revegetation of disturbed areas: Mining by its very nature involves disruption of natural vegetation and soils. This results in a huge increase in erosion potential and sediment transport. The impact of such areas can be minimised by better planning of clearing and rehabilitation to ensure that the minimum possible area of soil is left unprotected at any time.

•

• Grassing as described above is the most effective large-scale method. If moderate slopes and suitable topsoil are provided such that good growth occurs, this will effectively protect soils against sheet runoff from very heavy and intense storms.

• Drainage control: Water erosion is increased when concentrated flows pass over unprotected or steep sloping ground. A properly designed and maintained drainage system will avoid this occurring. The most important principles are to divert uncontaminated drainage away from erosion prone areas, and to control flows by using properly constructed drains at gentle grades as discussed in the fact sheet concerning drainage design.

• Surface covers: Steep slopes such as creek banks or cut and fill batters are hard to revegetate due to the difficulties in keeping topsoil and seeds etc. in place. Layers of jute, geosynthetics or mulch are very effective in protecting these layers until the root system of the grass has developed. These layers must be securely fastened with pegs and the upslope layer must overlap the top edge of the downslope layer.

• Erosion control: The best method for controlling erosion is to prevent its occurrence. Methods for preventing erosion are discussed below.

Drains or gullies subject to concentrated flows can be protected using the following techniques.

• Sediment containment: In areas where erosion prevention is not feasible it is necessary to trap the suspended sediment before the water passes offsite. In-stream sediment traps can be used along the drainage path to remove the bulk of the solids, however, constructed sediment retention ponds may be necessary to ‘polish’ the water immediately prior to discharge. Containment methods are discussed below. 1

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• Grassing: (reference Chow 1973) Having grass lining in a channel will significantly retard the flow and hence reduce the velocity and erosion potential. Grass will also stablise the channel consolidate the soil and check the movement of sediment along the channel bed. The selection

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FA C T S H E E T N O . 8

Erosion Control and Sediment Containment – batter slopes steeper than 2.5 H to 1.0 V will not reliably support rip rap;

of grass should be “fine and uniformly distributed sod-forming grasses” where the main flow occurs. The use of bunch grasses should be avoided in drains because they will channelise the flow creating scour lines. Grassing can also be used successfully in combination with rock fill to provide a very stable and well interlocked matrix. In establishing a grass cover in drains it is important to:

– a layer of medium-weight geofabric should be placed under all rip rap to prevent scour of the soil. (Due to the rough nature of rip rap which retards the flow, there will be much turbulence around the rocks which can easily result in under scour beneath the rocks making them unstable)

– ensure there is a mixture of fast and slow germinating varieties to ensure immediate and long-term protection;

– a uniform grading of rock size (ie. a good range from small to big rocks) is vital to create a good interlocking mattress;

– irrigation should be provided as necessary to ensure good germination if the seed is planted outside the wet season (usually the ideal time to build drains);

– if rip rap is used on steep drops it must be carried a short distance into the flatter sections preceding and following the drop.

– provide adequate protection for the seed if flow is likely in the drain immediately after construction. (This can be achieved using degradable natural fibre type covers which stabilise the surface and allow the grass to grow up through the fabric); and

• Reno mattresses/gabions: Reno mattress or gabion lining is a form of rock lining where a low-profile wire cage is used to hold the rock in place. This enables the use of smaller diameter rocks but requires more careful placement. Mattresses are available in thicknesses of approximately 170 mm, 250 mm, 300 mm and 500 mm. This type of protection can be used where very high velocities or extremely turbulent conditions are expected. This may occur on very steep slopes (when very large rip rap is not available or not preferred), at culvert outlets, or at the base of drops. Reno mattresses are also aesthetically pleasing and may provide a good alternative to rip rap in highly visible areas.

– when laying topsoil on drain batters prior to seeding, tyne the batters parallel to the direction of flow in the drain. This will result in long furrows along the drain which will both retain water and help to prevent scour paths down the batter slopes. • Rip rap lining: Rip rap simply refers to a lining of large rock placed in the drain to armour the natural ground against erosion. The rock is sized to ensure its stability during the peak flow conditions. Size of the rock is based on the flow depth and velocity. Rip rap should be carefully machine placed to ensure that a uniform ‘mattress’ of interlocking rock is achieved. This is very important to ensure that the rock does not get displaced during early flows before silt and grass fill the spaces between rocks thereby locking them in position. The following points should be noted when installing rip rap: 116

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• Concrete filled ‘revetment’ mattress: Revetment is also a form of hard armouring, utilising a pocketed pervious fabric with concrete pumped through it. This creates a solid layer moulded to the shape of the natural ground below. Small penetrations between the pockets allow for drainage of subsurface water preventing any lifting pressures. As with Reno mattresses this type of protection can be used where very high velocities or extremely turbulent conditions are expected. R

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FA C T S H E E T N O . 8

Erosion Control and Sediment Containment This may occur on very steep slopes (when very large rip rap is not available or not preferred), at retention dam inlets and spillways, at culvert outlets, or at the base of drops. With good preparation of the base, revetment will provide a very neat and durable protective layer.

steel chutes will eventually rust out and are therefore suitable only for medium term projects.

In-stream Sediment Containment Fast flowing surface runoff with a heavy load of suspended solids can cause major problems downstream by clogging culverts, blocking inlets or causing short-circuiting through sediment retention ponds etc. The solution is to have a number of in-stream sediment traps along the drainage path.

• Bottom sills: In small steep drains where continuous minor erosion is likely, bottom sills can effectively prevent the propagation of deep scour gullies. Concrete or gabion barriers are set into the base of the drain such that scouring will only occur until a stable slope is formed between sills (see Figure FS 8.l).

• Sediment barriers/filter dams placed across the drainage channel with rock protection downstream will trap heavy suspended solids as well as providing effective scour protection. The important feature of these barriers is that they should be semi-impermeable to water. This will cause water to pond behind them and hence silt will settle out of suspension and build up behind the barriers such that steps are formed in the channel floor. These barriers are positioned so that the final slope between the toe of one step and the top of the next is approximately 0.5%–1%. The rock downstream protects the channel from scour at the base of

• Corrugated steel chutes: In situations where an intercept drain or a gully at the top of a cutting must drop down a very steep slope into a drain below running in a perpendicular direction it is advisable to create a lined chute down the slope. This will prevent large scour gullies forming. A simple method of lining such chutes is to use half round corrugated steel pipes. These should be lapped at the ends of each pipe section with the upstream section on top. The pipe sections can be held in place by either using ‘tent peg’ style posts or by providing a small concrete beam down each edge. It should be noted that

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FA C T S H E E T N O . 8

Erosion Control and Sediment Containment • Silt fences: In areas where flow is not channelised but carries a high sediment load it is possible to filter out the suspended solids using a silt or sediment fence. This may be desirable during the construction of roads, at the base of stockpiles, or along the length of natural watercourses which receive sheet flow off disturbed areas. There are many proprietary brand sediment fences available today which only require posts to be supplied and have their own ties and support bands (usually marketed by suppliers of civil products or geotextiles) (see Figure FS 8.3).

the drops while the flow velocity between the drops is reduced enough to prevent erosion. These structures are effective and economical at drain slopes up to 3%-4% and can be formed from either timber, gabions, or graded rock (see Figure FS 8.2). Points to keep in mind are: – as well as being required downstream, rock protection is required upstream of the barriers for a short distance. This is to prevent scour around the edge of the barriers which may occur from the highly turbulent water spilling over the drop; – rock protection is also required up the batter slopes in the vicinity of the barrier. This will prevent side scour as the water spills over; fabric must be placed under the downstream rock to ensure the underlying soil is not washed out;

• Vegetation strips: An alternative to silt fences for capturing silt in sheet flow is to pass the water through heavily grassed strips. These can ideally be placed adjacent to catch drains or road edge drains. An advantage of a vegetation strip is that as the sediment builds up the grass grows up through it. Detailed information on the design and effectiveness of vegetative filter strips can be found in Haan (1994).

– the downstream rock must be cut in, such that the top of the rock is level with the natural drain surface, to ensure that another step is not induced at the end of the rock apron; and the lowest section of the barrier crest should be over the drain centre line such that low flows are preferentially directed away from the drain edges.

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FA C T S H E E T N O . 8

Erosion Control and Sediment Containment

Sediment Removal Ponds

the bulk of the coarse sediment. The second cell is then a polishing and treatment pond for sediment and other quality parameters. Ideally the second pond should also be drained in a controlled manner after each runoff event, however it can be left full as a water storage facility. The draining should preferentially take water from the surface of the pond near the outlet end or should slowly discharge water through slotted riser pipes or rock/sand filters; and

Surface runoff with levels of suspended solids higher than licence levels will need to be intercepted and treated prior to discharge offsite. • Sediment settling ponds: These are the most common method for settling out solids. Usually positioned immediately upstream of a monitored discharge point they also provide a useful location for controlling other water pollution problems such as pH, BOD etc. and may also be used as a storage for recycling water. For optimum removal of sediment these pond systems should address the following design issues: – the length to width ratio should be approximately 3:1;

– a volume over and above that required for efficient pond operation must be incorporated for storage of sediment. A mechanism for completely draining the pond, and access into and around the pond must be provided for periodic removal of captured sediment. There are many methods for designing sediment ponds. A good rule of thumb is the CALM method as developed by the NSW Department of Conservation and Land Management.

– the inlet and discharge point shall be positioned to ensure the maximum flow length between them. Baffles should be used if necessary to prevent short circuiting. It is beneficial to have two successive cells. The first cell can then be free draining and hence provide flood detention as well as capturing

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FA C T S H E E T N O . 8

Erosion Control and Sediment Containment – supply of water to plants, especially when young. Wetland plants rely on a saturated base but must not be drowned (short periods of total inundation are tolerable); and

• Wetlands for sediment removal: The use of artificial wetlands to improve storm water quality is becoming increasingly popular. The sediment removal efficiency of wetlands is known to be high; however in a mining environment care must be taken that excessive sediment loads are not imposed on the wetland plants and that water is always available. The design of artificial wetlands requires much care and consideration in the following areas:

– spread of plants. The plants most effective for use in wetlands are typically invasive species that will take over existing wetland areas if given the opportunity. Deep water should be used to keep open ponds clear of the plants, and great care must be taken to prevent spreading if fragile wetland ecosystems exist in the area.

– the hydraulics of flow through dense vegetation; – the selection of plants. The common approach is to use emergent macrophytes such as reeds or bulrushes that are common to the area. These plants are fast growing and tolerant of high pollution loads and some fluctuation of water levels;

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FA C T S H E E T N O . 9

Bioremediation Technology Bioremediation is a process which relies on micro-organisms to break down and detoxify organic chemicals such as hydrocarbons, and some organo-chlorines. Carbon dioxide and water are the final degradation products for hydrocarbon wastes using this process.

This procedure involves the use of soil micro­ organisms, water or effluent application, nutrients (usually fertiliser) and oxygen (air). This technique is highly suited to minesites in arid regions, due to the higher degradation rates that can be achieved with high air and soil temperatures.

Bioremediation has a number of applications within the mining industry; including the treatment of the following types of wastes:

Prior to commitment to this technology, the soil and effluent stream need to be assessed by a suitably qualified laboratory for the following:

• oily sludges;

• soil type (particle size analysis, organic content, etc.);

• hydrocarbon contaminated effluent (eg. heavy equipment washdown pads); • hydrocarbon contaminated soils;

• the level of activity of hydrocarbon degrading microbes (ie. C17: pristane ratio);

• specific low volume oil spillages; and

• the nutrient status of the material to be degraded;

• workshop and power station liquid wastes.

• the moisture content; and

Within Australia, bioremediation is being used as a cost-effective alternative for the treatment of wastewater effluent and hydrocarbon contaminated soils. Most applications involve the construction of a ‘bioremediation pad’, and implementing the process known as landfarming. Landfarming involves the spreading of wastes (usually about 30 cm thick) over the ground to enhance the natural degradation process.

• concentration of specific hydrocarbon fractions.

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In the event that insufficient numbers or incorrect species are present, then the waste stream can be inoculated with microbes that are grown within an on-site bioreactor.

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FA C T S H E E T N O . 1 0

Hydrological Data for Design Purposes There are various formats for hydrological data to suit both design and reporting outputs. The reporting formats may be tabulated or graphed with time frames to suit the receiver of the report. Design formats will depend upon the design process for which the data is to be utilised. The following table presents the various data formats and the principal design processes for which they may be utilised.

Design Process

Peak Flows

Hydrological Data Format Rainfall • Intensity Frequency Duration curves (IFD Curves) (see Figure FS 10.1)

Hydrograph Analysis

Water Balance

Water Pollutant Tailings Storage Dispersion Storage

✓

• Monthly and Seasonal Rainfall

✓

✓

• Annual Rainfall

✓

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• Continuous Rainfall (minutes) ✓

✓

✓

✓

✓

✓

✓

✓

✓

✓

✓

✓

• Annual Flow

✓

✓

Evaporation • Daily

✓

• Monthly

✓

✓

• Annual

✓

✓

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• Monthly and Seasonal Flow

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✓

✓

• Daily Flow

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✓

Streamflow • Continuous Flow

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• Daily Rainfall

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Wastewater Disposal

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• Rainfall patterns (Hyetographs)

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FA C T S H E E T N O . 1 0

Hydrological Data for Design Purposes

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FA C T S H E E T N O . 1 1

Groundwater Groundwater is the generic term identifying water resources which are resident in soil or rock pores and matrices. By far the major proportion of groundwater resides under positive pore pressures within aquifers, but some water lies in the interstices between ground surface and the aquifer within the capillary zone. Aquifers are generally fully saturated, whereas the capillary zone contains a significant proportion of air as well as water. Aquifers may be confined (pressurised between layers of relatively impermeable ground or aquicludes), or unconfined (a water table aquifer with a phreatic or ‘free’ surface). In both cases, the flow dynamics are similar in that flow is generated by differences in pressure from one point to another. A perched water table is a special type of unconfined aquifer which may exist within another unconfined aquifer, and is ‘perched’ on a thin impermeable lens such as clay. Flow in aquifers is generally laminar, or seepage flow. In some cases where preferential flow paths may exist (eg. permeable faults and fractures in rock), turbulent flow may be generated. Flow in aquifers is always from a region of higher pressure or higher potential energy to a region of lower potential energy.

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Most aquifers are interconnected, and it is very rare that a single aquifer will exist in isolation. Connections between aquifers may be weak or strong depending on the porous media and the geological stratification. The single intrinsic soil or rock parameter that determines the characteristics of groundwater flow is the hydraulic conductivity. This is often (and strictly incorrectly) referred to as the permeability. The hydraulic conductivity is a quantitative measure of the velocity of seepage flow of water reached whilst being generated by a unit pressure gradient. Hydraulic conductivity may vary in space (heterogeneous porous media) as well as in the direction of flow (anisotropic porous media). A homogeneous and isotropic groundwater regime is an ideal saturation that rarely occurs in nature. Groundwater, while recognised as a separate entity in the hydrologic cycle, is nevertheless strongly interactive with other components of the hydrologic cycle such as rain, rivers, lakes and oceans. Although the time scale of processes in groundwater is long because of the laminar nature of flow, its interaction with surface water components of the hydrologic cycle should always be considered.

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FA C T S H E E T N O . 1 2

Numerical Modelling The process of ‘calibration’ and verification is an integral part of numerical modelling. Because a numerical model may operate using several parameters describing the physical processes (eg. frictional stresses, soil-water conductivity) a historical event for which cause-and-effect data exists should always be simulated. This allows the modeller to ‘tune’ the parameters against an observed event.

Numerical modelling is the process of solving the equations describing a physical process using a step-wise approximation. Solutions are obtained by performing iterations (successively improved approximations) at each step until the numerical answer satisfies all the equations being used. The approximation is improved by decreasing the size of the steps, much like drawing a curve using a series of short, straight lines. Decreasing step size, however, increases the amount of labour. With the rapid advances in computer processing speed, this is becoming less of a concern.

The complexity of the model chosen should realistically reflect the extent to which the relevant parameters may be measured or inferred with accuracy, as well as required accuracy of modelled answers in a particular project. The sensitivity of the model to prime parameters should always be investigated and quantified. The use of models as decision making tools often have greater value in sensitivity analysis than in absolute predictions.

The advantage of numerical modelling is that, once the model is set up and established, a range of scenarios may be investigated with relatively little effort, and complex problems may be solved using numerical models. Nevertheless, numerical models should be viewed with caution as their complexity and their ‘black box’ appearance may promote errors of judgement in their application.

The applicability of simpler (one dimensional) models should be investigated first before adopting complex (eg. three dimensional) models under the philosophy that complicated models have a greater opportunity for errors, both judgemental and numerical. Finally, the limitations of the model should always be clearly understood.

Numerical models were developed in the early 1960s and are now well established tools. Finite difference (FD) and finite element (FE) models are currently popular. These subdivide the physical area of interest into small fragments which are each treated in a simplified manner. FE models are more adaptable to complicated boundaries, but the methods of solution are slightly more complex than FD models. Other models which have limited use are boundary integral and method-ofcharacteristics formulations, but these presently lack the practical applicability of FD and FE methods. Numerical models may be applied to a wide range of problems in hydrology, flood flow and groundwater flow. In recent times, advances in the understanding of contaminant transport, sediment transport and complex boundary conditions have resulted in a generation of problem-specific models. Before choosing a model, its applicability to a specific problem must be questioned in depth.

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