Mine Water Management

Steve Perrens

Heaven or Hell for Hydrologic Modellers?

2

The Issues The Context Water supply

Excess water

Water quality

Community concerns

Environmental protection

Regulatory control

Mines constantly evolve

Runoff into pit

Groundwater inflow

Coal stockpiles

Haul roads

Vehicle maintenance

Fuel storage

Overburden runoff

Overburden leachate

Washery tailings disposal

3

Water QualityNumerous sources – Different pollutants

Coal dust, sediment;

Salinity, pH, iron;

Coal dust;

Sediment, coal dust;

Hydrocarbons;

Hydrocarbons;

Sediment;

Salinity, acid leachate;

Sediment, salinity, pH?

4

5

Comparison of sediment dam and drainage line water quality

0

50

100

150

200

250

300

350

400

450

500

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Total Suspe

nded

 Solid

s (mg/L)

Percentile

Dam 1 Dam 2 Dam 3 Dam 4 Dam 5

0

5,000

10,000

15,000

20,000

25,000

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Total Suspe

nded

 Solids (m

g/L)

Percentile

Rising stage  samplesMonthly 1 2 3 4 5

6

Mine Water Quantity & Quality

Highly variable over time;

Pit & overburden area variable throughout mine life;

Groundwater inflow to pit variable – generally related to depth of pit;

Groundwater inflow to U/G workings variable depending of mine layout;

Runoff highly variable depending on climate (typical annual variation: 20% - 300% of average)

Consequences of changes to mine plan

7

Management Issues Sufficient supply for mine operations:

Dust suppression Longwall operation Coal washing

Sufficient storage to meet operational requirements Reliability of supply Storage of excess – without compromising production Treatment and discharge in the event of excess?

Minimise discharge (zero discharge preferred) Divert external catchments Enhanced use/loss (irrigation, evaporation) Water quality (sediment, salinity) Treatment HRSTS.

8

Regulatory Issues

Water access licenses for incidental take Unavoidable groundwater make

Problem when linked to ‘cease to pump’ rules in the WSP

Aquifer interference policy

Return flows not counted

Water access licenses for supply; Availability of surface and groundwater licenses

Supply reliability

Discharge licenses/permits

Mine Water Management

Steve Perrens

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Issues

Complex interactions between Evolving mine landform

Rehabilitated and ‘natural’ catchments

Water management system

Storage requirements

Water balance highly dependent on climate

Increasing salinity: Deeper pits

Underground mining

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Issues

Alterations to mine plan and production

Short term variation in CHPP throughput

Operating rules for exchange of water between mines

Opportunities for discharge from sites

14

Predicted Annual ROM Supply and CHPP Water Requirement (Assumes slurry disposal)

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

5,000

5,500

6,000

6,500

0 5 10 15 20 25

ROM an

d Water Req

uiremen

t

Project Life (Years)

ROM (t x 1,000) CHPP Water (ML)

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Projected Tailings Storage Requirements

0

5

10

15

20

25

0 5 10 15 20 25

Cumulative Tailings V

olum

e (m

3x 1

06)

Project Life (Years)

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Mine Water Balance

Water sources (internal and external)

Water demands

Dust suppression,

Coal processing,

Underground operations

Losses / discharge

Storage

17

Water Demands

Dust suppression,

Mainly achieved by water spray (chemicals in special situations)

Differentiate between stockpiles and haul roads

Consistency of approaches by air quality modellers and hydrologists?

Underground operations

Typical longwall – 1 ML/day

Dust suppression

Dust Suppression

18

Dust suppression – haul roads

Mainly achieved by water spray (chemicals in special situations)

Dependent on haul area and weather (rainfall and wind)

Little benchmarked data in Australia

South African research – water required to maintain wet surface (effects of road albedo and wheel movement)

Dust suppression – coal stockpiles

Dependent on dump height and reclaim process

Consistency of approaches by air quality modellers and hydrologists?

19

Water Demands Coal processing – dependent on

Mining process and source characteristics (Open-cut ± 12% fines; Underground ± 8% fines)

Dewatering process

Underground operations Typical longwall – 1 ML/day Dust suppression

Treatment Relative Water Requirements

Slurry disposal 100% Secondary flocculation 90% Paste thickener 60% Belt press 40% Pressure filter 30% Solid bowl centrifuge 30%

20

Why do we use models?

Prediction of interaction between the mine, climate and the surrounding environment

Assessment (design) of the location and size of facilities necessary to manage water

Understanding the risks

Ongoing management of water at an operating site:

Are predictions valid/correct?

Does management need to change?

Are new/additional facilities necessary?

21

Modelling Considerations Adequacy of supply - enough water?

Adequate storage – Seasonal variation of rainfall and evaporation

Probability of extreme sequences of rainfall

Variation of groundwater make

Year-to-year carry-over of water

Discharge frequency, volume and quality

Relevant timescale to characterise runoff and mine operations

Data requirements and availability22

Mine Site Models

System models: Water gains and losses

Storages

Water conveyance (channels, pumps, pipelines)

Operating rules/triggers

Process models: Groundwater make

Runoff from different surfaces

Water uses (dust suppression, coal washing, etc)

Losses (evaporation, seepage)

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AWBM

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Automatic AWBM calibration for full data set with one year omitted

Use estimated parameters to model runoff for missing year

Assess adequacy of fit between modelled and observed (Total volume, R2, Nash-Sutcliffe coefficient of efficiency, flow duration, etc)

Repeat process taking out successive years of data

Assess statistics of parameters and goodness of fit to select parameters for adoption

Model Parameter EstimationLeave One Out Cross Validation

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AWBM

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0.01

0.10

1.00

10.00

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%Ru

noff (m

m/day)

Percent of Time Runoff is Equalled or Exceeded

Flow Duration

Actual

Calculated

0%

20%

40%

60%

80%

100%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Percen

t of T

otal Volum

e of Cum

ulative

 Run

off

Percent of Time over which Cumulative Runoff Volume Occurs

Cumulative Runoff 

Actual

Calculated

Derived from groundwater modelling

Relatively steady day to day

Significant variation over mine life –depends on mine plan

Groundwater Inflows to Pit and Underground Workings

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0

100

200

300

400

500

600

700

800

0 5 10 15 20 25

Ann

ual D

ewatering (M

L/year)

Mine Year

Storage Requirements – median rainfall Sequence: A

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Storage Requirements – median rainfall Sequence: B

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Storage requirements – risk profile (± 125 years rainfall data)

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Salt balance – conservation of mass

Final void salinity – water and salt balance:

Groundwater leaching through in-pit spoil

Surface runoff from void

Groundwater loss from ‘lake’ (or make)

Evaporation loss (accounting for depth of void)

Need for strong linkage between surface and groundwater models

Water quality modelling

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154

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168

170

172

174

0 100 200 300 400 500 600 700 800 900 1000

Void W

ater Le

vel (m AHD

)

Years Following Mine Closure

132

134

136

138

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142

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146

148

150

152

0 100 200 300 400 500 600 700 800 900 1000

Void W

ater Level (m

 AHD

)

Years Following Mine Closure

0

1,000

2,000

3,000

4,000

5,000

6,000

0 10 20 30 40 50 60 70 80 90 100

Salin

ity (m

g/L)

Years Following Mine Closure

Existing Climate Climate Change

0

2,000

4,000

6,000

8,000

10,000

12,000

0 10 20 30 40 50 60 70 80 90 100

Salin

ity (m

g/L)

Years Following Mine Closure

Existing Climate Climate Change

Dr Steve Perrens

Evans & Peck

(02) 9495 0500

sperrens@evanspeck.com