Posted on: March 16, 2011 Posted by: Diane Swarts Comments: 0

The December 2010 report on acid mine drainage in the Witwatersrand gold fields basin, outlines risks and treatment options to an AMD ministerial committee.

The report, collated by the Council for Geoscience, drew on a large team of mine water experts from Council for Geoscience, Department of Water Affairs, Department of Mineral Resources, Council for Scientific and Industrial Research, Mintek, Water Research Commission, and advisors including Prof TS McCarthy, Prof G Steyl, Prof J Maree, and Prof B Zhao.

The team was chaired by Dr T Ramontja, CEO of the Council for Geoscience. The report notes that there is no single or ‘one size fits all’ solution to AMD, and effective management depends on localised conditions and potential impacts.

Reef flooding and AMD risks

Witwatersrand gold fields Western, Central and Eastern Basins are identified as priority areas requiring immediate action because of the lack of current adequate measures to manage and control problems related to AMD, before current problems become more critical and approached densely populated areas.

These gold Reef flooding and AMD risks are identified;

• Contamination of shallow groundwater resources required for agricultural use and human consumption.

• Geotechnical impacts, such as the flooding of underground infrastructure in areas where water rises close to urban areas.

• Increased seismic activity which could have a moderate localised effect on property and infrastructure.

• Serious negative ecological impacts.

• Regional impacts on major river systems.

• Localised flooding in low-lying areas.

Mine flooding management

A generic approach to management of these risks is proposed for implementation in the three priority areas.

Decant prevention and management: Experience in the Western Basin has shown the severe impacts that can be expected if the mine void is allowed to flood completely and decant.

It is recommended that water levels in the basins be held at or below the relevant Environmental Critical Levels (ECLs) by pumping. In the Western Basin this will require pumping to lower the water level that is already at surface.

Ingress control would reduce the volume of water to be pumped and treated. Ingress control can be achieved by preventing recharge of shallow groundwater above the mine void by canalisation of surface streams and sealing of surface cracks and mine openings, of abstraction of clean groundwater from aquifers that feed the mine voids.

AMD water treatment

Various treatment options and technologies, including active, passive and in situ treatment technologies, have been identified and reviewed.

Given variability in water quality between the different basins and the possibility that the water quality in the mine voids will improve over time, it is likely that a suite of different technologies will be required.

Volume of decant has peaked at about 60 megalitres per day (Ml/d) in response to recharge during exceptionally wet summer rainfall seasons. More typical is a decant rate of 15 –20 Ml/d.

Basic treatment of this water currently permits the release of ~12 Ml/d into the Crocodile (West) and Marico drainage system. Existing pumping and treatment capacity is inadequate to effectively manage the impact of AMD, with the excess volume flowing untreated into the receiving aquatic environment.

Urgent treatment recommendations

it is recommended that AMD intervention and management measures are undertaken in the Western, Central and Eastern Basins as a matter of urgency.

In the Western Basin, this requires the establishment of a neutralisation plant with a capacity of 20 Ml/d. This is required to supplement the existing treatment capacity operated by mines in the area and the upgrade of mine water pumping facilities accordingly.

In the Central Basin, it is required that a pumping facility with a capacity of ~60 Ml/d be installed in one or more existing mine shafts, and a neutralisation plant or plants of matching capacity be established in close proximity.

In the Eastern Basin, the pumping capability in Number 3 Shaft of Grootvlei Mine must be secured.

It is also required that the existing treatment plant at this locality be returned to service as soon as possible. The volumes of water to be managed may be reduced by the timely implementation of ingress management measures, with a resultant reduction in operating costs. The design of the pump and treat systems will need to take this into account.

Neutralisation of mine water and discharge to the environment will produce conditions similar to the status quo during periods of active mining. In the medium to long term this may not be sustainable as it could result in excessive salt loads on the receiving water bodies, which will require the release of clean water for dilution, particularly in the Vaal River System.

Options of direct consumptive use of neutralised mine water or desalination and sale of the water to local users must be investigated. In the very long term, it is expected that water quality could improve significantly.

This will be confirmed by ongoing monitoring and will create a situation where the mines could be allowed to flood completely or to levels closer to the surface, reducing the costs of water management.

Immediate expansion of a programme to monitor mine water level, mine water quality, surface flow and quality, groundwater level and quality and seismic activity is required for the three priority basins.

Recommendations summary

• Pump water from the three priority basins to maintain water levels below Environmental Critical Levels, or, by agreement with stakeholders, the lowest level of underground mining or pumping activity in the basin.

• Reduce ingress of water into the underground workings.
• AMD treatment should at first raise low pH (high acidity), high iron and other metal content. In the medium to long term consideration should be given to steps to reduce mine water contribution to salinity of major river systems.

• Monitoring of mine water, groundwater, surface water, seismicity, subsidence and other geotechnical impacts of mine flooding should improve. A multi-institution monitoring committee should be established to implement monitoring and assessment of problems and solutions.
• Mine waste residues should be monitored and appropriately remediated to reduce AMD impacts.

• Environmental levy of operating mines to cover remediation costs, should be investigated.

Mpumalanga, KZN, West coast AMD

AMD also poses current or potential environmental and health problems in Mpumalanga and KwaZulu-Natal Coal Fields, and O’Kiep Copper District.

The situation in other mining regions of the country requires additional information, monitoring and assessments of risk, particularly in vulnerable areas such as the Mpumalanga Coal Fields, where the impact of mining on the freshwater sources in the upper reaches of the Vaal and Olifants River Systems is of serious concern.

Mine water abbreviations

AMD Acid Mine Drainage
ANZMEC Australian and New Zealand Mineral and Energy Council
ARD Acid rock drainage
CGS Council for Geoscience
CSIR Council for Scientific and Industrial Research
DEA Department of Environmental Affairs
DME Department of Minerals and Energy (former name for DMR)
DMR Department of Mineral Resources (formerly DME)
DST Department of Science and Technology
DWA Department of Water Affairs (formerly DWAF)
DWAF Department of Water Affairs and Forestry (former name for DWA)
ECL Environmental Critical Level
ERPM East Rand Proprietary Mines
GARD Global Acid Rock Drainage
HDI Human Development Index
HDS High Density Sludge
IMC Inter-Ministerial Committee
KOSH Klerksdorp, Orkney, Stilfontein and Hartebeesfontein
MEM Mine and Environmental Management
MEND Mine Environment Neutral Drainage
MPRDA Mineral and Petroleum Resources Development Act
OSMRE Office of Surface Mining Reclamation and Enforcement
RMCS Regional Mine Closure Strategies
SWM Strategic Water Management
USA United States of America
USEPA United States Environmental Protection Agency
WRC Water Research Commission

PHOTO; Geology academic Prof Terence McCarthy is one of the advisors on the large panel of mine water specialists that drafted the urgent AMD report to the government AMD team.

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