Overview of acid mine drainage
impacts in the West Rand Goldfield
Presentation to the Director General of DWAF: 2 February 2009
PRESENTED BY :
DATE:
TEL:
E-MAIL:
2.5 Ml
Courtesy of Phil Hobbs - CSIR
x 10
= 25 Ml/d
TDS load
~ 100 t/d
Treat. cost
R5 M/m
West Rand
Basin
WITWATERSRAND
BASIN
x 100
= 250 Ml/d
pH (Logarithmic)
14
9
8
7
Baking Soda
Distilled Water
6.5-8.4
6
Acid
5
6.0-9.0
Orange Juice
4
3
Vinegar
2
Lemon Juice
1
0
7.0-8.0
Battery Acid
Typical Wits Acid Mine Drainage
10
Aquatic Ecosystem
(Site Specific)
11
Ammonia
Irrigation
12
Bleach
Drinking Water
Alkaline
13
DWAF Guidelines
2.5-4
Pre mining situation
•
•
Before mining, the water table will have followed a natural piezometric surface
Springs may have developed in areas where this surface lies above ground level and where suitable
conduits exist by which water can flow to surface
Diagram modified after:
Hobbs, P. J. and Cobbing, J. E. (2007) A hydrogeological assessment of acid mine drainage impacts in the West Rand Basin,
Gauteng Province, CSIR/THRIP, CSIR/NRE/WR/ER/2007/0097/C, Pretoria: 59 p.
During active mining phase
Groundwater flow would have been drawn into the mine void
owing to dewatering and the creation of a cone of depresion
During Mining
•
•
Zone of AMD generation
AMD pumped from
workings
During mining, dewatering will have lowered the water table within the mine and surrounding areas
Springs will have dried up in the area affected by dewatering
Diagram modified after:
Hobbs, P. J. and Cobbing, J. E. (2007) A hydrogeological assessment of acid mine drainage impacts in the West Rand Basin,
Gauteng Province, CSIR/THRIP, CSIR/NRE/WR/ER/2007/0097/C, Pretoria: 59 p.
Post mining situation
Post Mining
AMD decanting
from low-lying
shaft(s)
Contaminated
groundwater flow
•
•
Water will tend to return to the pre-mining piezometric surface, although this is likely to be flat (or
flatter) due to the higher transmissivity of the mined out voids
Water will decant from mine openings or other conduits where these lie below the piezometric surface
Diagram modified after:
Hobbs, P. J. and Cobbing, J. E. (2007) A hydrogeological assessment of acid mine drainage impacts in the West Rand Basin,
Gauteng Province, CSIR/THRIP, CSIR/NRE/WR/ER/2007/0097/C, Pretoria: 59 p.
Downstream groundwater pollution
Treatment plant(s)
Post Mining
•
Recharge of karst aquifer with
contaminated surface water
Discharge of partially treated water
Polluted water discharged to the Tweelopie Spruit recharges the Zwartkrans Compartment, leading to
contamination of substantial karst groundwater resources and as yet poorly determined impacts on the
Cradle of Humankind World Heritage Site
Diagram modified after:
Hobbs, P. J. and Cobbing, J. E. (2007) A hydrogeological assessment of acid mine drainage impacts in the West Rand Basin,
Gauteng Province, CSIR/THRIP, CSIR/NRE/WR/ER/2007/0097/C, Pretoria: 59 p.
Water pollution management via
maintenance of water levels at the
ECL
Level which will prevent
decant of mine water to
surface via shafts and
springs
Level which will protect
local and regional
groundwater
•
•
•
This situation may be managed via maintenance of water levels at the Environmental Critical Level
(ECL), defined as that level where contaminated mine water will not impact on the environment.
The ECL for surface water has been identified for the West Rand only. Additional groundwater
investigations are required to determine the appropriate level for groundwater protection however it is
estimated to be 100-150m below the surface water ECL level.
ECLs have not been determined precisely for the other goldfields although there is a good conceptual
understanding of issues, particularly in the Central Rand.
Diagram modified after:
Hobbs, P. J. and Cobbing, J. E. (2007) A hydrogeological assessment of acid mine drainage impacts in the West Rand Basin,
Gauteng Province, CSIR/THRIP, CSIR/NRE/WR/ER/2007/0097/C, Pretoria: 59 p.
pH
(Water discharged to the
Krugersdorp Game Reserve)
14.0
12.0
10.0
pH
8.0
pH
Directive - Low
Directive - High
6.0
4.0
2.0
0.0
5-Sep-05 24-Mar-06 10-Oct-06 28-Apr-07 14-Nov-07 1-Jun-08 18-Dec-08 6-Jul-09
Pre-treatment
Electrical Conductivity
(Water discharged to the
Krugersdorp Game Reserve)
600
500
EC (mS/m)
400
EC
300
Directive
Pre Treatment
200
100
0
5-Sep-05 24-Mar-06 10-Oct-06 28-Apr-07 14-Nov-07 1-Jun-08 18-Dec-08 6-Jul-09
Events: January-March 2008
14.0
600
12.0
500
10.0
pH
8.0
300
6.0
200
4.0
100
2.0
0.0
1-Dec-07
Low pH water released after period of rainfall
•No significant change in conductivity
•“High rainfall” period similar to normal years
0
20-Jan-08
10-Mar-08
29-Apr-08
EC (mS/m)
400
pH
Directive - Low
Conductivity
Downstream groundwater quality
(Krugersdorp Brickworks
Borehole)
200
3
180
2.5
160
2
120
100
1.5
80
60
1
40
0.5
20
0
0
02-Jul- 10-Oct- 18-Jan- 28-Apr- 06-Aug- 14-Nov- 22-Feb- 01-Jun- 09-Sep- 18-Dec- 28-Mar06
06
07
07
07
07
08
08
08
08
09
Nitrate (mg/l N)
EC (mS/m)
140
EC
Nitrate
Volumes and loads
•
Polluted water is discharged into a receiving environment
–
–
–
Volume = ~25Ml/d
Salt content = ~4g/l
Salt load = ~100 tons per day
20t
•
•
20t
20t
20t
20t
Dilution by around 5Ml/d from springs, much of it also contaminated, leading to little
dilution
This high salinity has had impacts on drinking water quality downstream and has been
proposed as the cause of animal deaths and health effects in the Krugersdorp Game
Reserve
Regional Closure Strategy
•
•
•
Regional Closure Strategies have been developed by the DME in partnership with CGS,
CSIR and Mintek.
Acknowledge the underground interconnection of mines, leading to cumulative and
integrated impacts on the environment and society, with multiple mines responsible for
impacts within regions
Key aspects
–
–
–
–
–
Addresses water management, dust, radioactivity, ground instability, land sterilisation and socioeconomic aspects
Water management by integration within the mining industry and other liable parties
Environmental protection can be ensured by the pumping of water to lower the mine void water
level to the ECL and treating of water to a standard suitable for discharge (Stock Watering
standard or better because of the proximity of the Krugersdorp Game Reserve) or sale to a third
party
Establishment of closure and monitoring committees to ensure meaningful stakeholder
interactions and to maintain technical oversight of the process. This has already partially been
achieved via the Technical Working Group and Monitoring Committee
Apportionment studies found that ALL mines in the Western Basin who had or currently have
rights to mine are jointly and severally liable (for example East Champ D’Or is listed as a asset
on DRDGold shareholder publications).
Conclusions
•
Pollution is as a result of current and historical mining activities which created and
exposed contamination sources connected to an artificial aquifer within a natural
groundwater system:
–
–
–
•
•
•
•
Ingress of water into the mine void
Flooding and decant of contaminated water is driven by natural processes
“Shutting down the mine” won’t work!
Enforce the “polluter-pays” principle
Ensure that water volume, quality and pollution load issues are adequately captured in all
EMPs and Water Use Licenses
Enforce legislation to promote the development and implementation of short, medium
and long-term solutions
Ensure meaningful involvement of stakeholders