Acid Mine Drainage - can it reach the RNC?
Published on Thursday, 22 April 2010 01:43
Garfiled Krige of "African Environmental Development" summarises the situation, and provides relevant facts. Decide for yourself if you should be concerned, involved or vocal on the issue.
Issues relating to Decanting of Mine Water from the Flooded Gold Mines of the Witwatersrand
During the month of August 2002, an event took place that would have a far-reaching impact on the Cradle of Humankind World Heritage Site (Cradle) and the dolomitic aquifer underlying it, the Zwartkrans Compartment. This event was, of course, the first decanting of water from the flooded gold mines of Krugersdorp and Randfontein, collectively referred to as the Western Basin Mine Void. The water, decanting from a disused mine shaft, flowed through the Krugersdorp Game Reserve and entered the dolomitic aquifers of the Cradle. Although predicted by specialists as early as 1996, the magnitude of the event still seemingly caught many people, mining houses and authorities by surprise.
The decanting of polluted mine water was the final stage in a sequence of events which took approximately 115 years to complete. The process started back in 1887 when mining started on the West Rand (a year after gold was discovered on the Witwatersrand). During this mining process, relatively “clean” water was pumped from the mine workings into the streams feeding the dolomitic aquifers of the Cradle, via the Tweelopiespruit, to enable deeper mining to take place. This pumping continued until around 1998, when a final decision was made to stop the last pumping operations at Harmony’s Central Ventilation Shaft and allow the mine void, created during the previous 110 years, to flood.
For the next 4 years the mine void gradually flooded, mines’ ownership changed and the predictions of 1996 were all but forgotten, until 2002, that is, when the water in the mine void finally reached the surface and started to decant into the Tweelopiespruit, upstream from the Krugersdorp Game Reserve, less than a month earlier than what was predicted way back in 1996.
Initially the water decanting from a dolomitic borehole, drilled into a dolomitic inlier above the Witwatersrand rocks in which the mine void was made, was of a relatively good quality. However, some two weeks after the initial decant started from a borehole, the increased water pressure opened a forgotten Black Reef incline shaft and the volume decanting from the mine void increased progressively while the quality of the decanting water decreased as the dolomitic water was gradually replaced by mine void water from the man-made aquifer below the dolomitic aquifer.
Several attempts have been made to stop the water from flowing into the dolomite, all of which have been unsuccessful so far.
The following statistics are relevant to the water in the Western Basin Mine Void:
Should residents of the Rhenosterspruit Nature Conservancy be concerned about the acid mine drainage orginating on the West Rand? Is there a chance that our groundwater will be contaminated?
- The Western Basin Mine void has a volume of approximately 45 Million m3 currently filled with water and overflowing into the Tweelopiespruit.
- The water in the Tweelopiespruit initially flows on surface over Witwatersrand rocks, but promptly disappears into the ground shortly after reaching the dolomite, when it enters the Zwartkrans dolomitic compartment.
- All water entering the mine void is “made” locally, i.e. it has its origin from rainwater and seepage from river/streambed/mine-pits losses. No water enters from areas outside the general area.
- Currently, approximately 18 to 20 Ml flows into the dolomitic groundwater compartment daily.
- This water is of a poor quality with sulphate concentrations ranging between 2 500 to 4 500 mg/l. With sulphate concentrations in this range, it can be assumed that the water will also be contaminated with many other metals associated with gold mining ores. This includes radioactive substances.
- It has been calculated that the sulphuric acid in each 1 Ml of mine void water will dissolve a volume of dolomite (carbonate rock) of approximately 317 litres. In April 2009 the mine void water, containing sulphuric acid, will have been flowing into the Zwartkrans compartment for 8 years. The corresponding void in the dolomite that has been formed so far, amounts to a staggering 16.7 Ml (i.e. 16 700 m3). Although this void space will be spread over a large area, most of it will be concentrated in the area where the water in the Tweelopiespruit first comes into contact with the dolomite. This is also the area where the N14 road crosses the Tweelopiespruit. This road is in serious danger of collapse due to sinkhole formation! A gravity survey and drilling/grouting (if needed) operation should be implemented immediately to safeguard the lives of people using this section of the N14 road.
- Over the last 8 years the sulphate concentration in many of the boreholes in the Cradle of Humankind World Heritage Site has also increased significantly, indicating that the plume of pollution is spreading rapidly through the groundwater aquifer. Sulphate concentrations in excess of 1 500 mg/l have been recorded in boreholes used for domestic purposes. These water users are presently unaware of the potential poisoning of their water supply and should be provided with a clean source of water before people/animals using this water become ill. In 2006 the pollution plume was already recorded in boreholes well beyond the Sterkfontein caves along the Blaauwbankspruit valley (also referred to as the “Valley of the Ancestors”).
- Although the mine void water flowing in the Tweelopiespruit is the only source of contamination related to gold mining activities entering the Zwartkrans compartment, there are other sources of pollution also entering the compartment, including the sewage effluent water from the Randfontein and Mogale City sewage plants.
The Western Basin Mine Void is the first and smallest of four mining voids that will be decanting onto surface within the next few years, the others being the Eastern Basin (Nigel area), Central Basin (Johannesburg area) and the Far Western Basin (Westonaria to Carletonville area). The water decanting from the Western Basin unfortunately enters the most environmentally sensitive of all the downstream areas (the Cradle of Humankind World Heritage Site).
In summary, the most important/critical issues relating to the decanting of water from the Western Basin Mine Void is as follows:
- Poisoning of a large groundwater resource, the Zwartkrans aquifer underlying the Cradle of Humankind World Heritage Site, used by many people as a sole drinking water resource.
- Danger of the N14 road of collapse in the general area where the road crosses the Tweelopiespruit.
- Poisoning of animals in the Krugersdorp Game reserve by drinking the only source of water available to them, the water flowing in the Tweelopiespruit.
- Loss of caves and fossil sites in the Cradle of Humankind World Heritage Site due to the continuing dissolution of carbonate rock by the acidic mine void water.
Please note that these paragraphs are merely a short summary of the core issues and by no means represent the full scenario relating the decanting of water from the Western Basin Mine Void.
06 February 2009