UIS Commission on Karst Hydrogeology and Speleogenesis

Involving one hundred years of well-intended anthropogenic destruction, which is destroying a unique karst groundwater system and habitat

by Mike Buchanan

August 2008
The city of Johannesburg in South Africa had its beginnings 1886 when gold was discovered on the Witwatersrand by an Australian prospector named George Harrison.
Most gold mines in the Johannesburg (JHB) area ceased operation in the late 1970s. These closed mines were considered as future potential reserves. Interspersed amongst them were a few productive mines, which continued to pump out invading groundwater. The mining houses had long relied upon the South African government to help finance the back pumping of ingressing groundwater to enable access to continuous supplies of gold from deeper gold rich seams. At the time many leading academics and others were unaware that the water they were back pumping came from the abundant, dolomitic rock that surrounds Johannesburg. Furthermore, these groundwater reservoirs were of a great depth and area, thus containing an immense water reserve.
In 1995 the government discontinued its pumping subsidy and allowed the un-used voids to fill with groundwater. By 2002 the groundwater table essentially restored itself to the pre mining days and started overflowing from natural springs at the lowest altitude, which is located 1.5km north of the Krugersdorp Game Reserve (KGR), west of Johannesburg. Output or decant was recorded at 15 -30 ml per day with seasonal variation. It was only when fish, birds and then mammals started dying within the KGR that anyone sat up and took notice. On investigation the decanting water was found to contain elevated levels of sulphates of 5300mg/l (Council for Geosciences readings). The Environmental Protection Agency primary (enforceable) maximum contaminant level for sulphates in drinking water is 500 mg/l. On closer investigation the findings reported a plethora of heavy metals including high uranium, lead 1,2,3 (as a result of both geological dissolution and that of uranium dissolution), molybdenum, manganese, zinc and other unacceptably high heavy metals. The actual total volume of overflow water appears to be increasing which suggests leakage from the old storage dams.
The sulphate levels were directly attributed to the dissolution of the gold bearing conglomerate which includes pyrites and the latter mentioned heavy metals. Diverse bacteria, particularly acidithiobacillus were being held responsible for the elevated sulphate levels which caused the discharging groundwater to have a pH of around 2.0. The bacterium most often responsible for iron and sulphur consumption/dissolution is Thiobacillus ferrooxidans, but usually other species also occur.
A honeycomb of thousands of kilometres of interconnected mine shafts, tunnels and voids under JHB and the entire Witwatersrand created a diversionary storm water drainage system for the greater JHB catchment. Many mega litres of storm water had been inadvertently allowed to pass through this now expansive subterranean drainage system which decants west of JHB. As a result of this diversionary flow during winter months (lowest rainfall) the entire process is slowed right down, allowing for mega litres of groundwater to be impacted by the acidic exudation of huge and expanding colonies of acidithiobacillus. This continuing process now sees the water being contaminated by diverse species of Thiobacillus bacteria. These organisms are producing corrosive plumes which emanate from over 200 mineshafts and now all the contaminated rivers around JHB. Although this is a centrally important in mine management, and absolutely crucial to the problems which arise in karst sites, it is an extremely complex and not fully understood phenomenon.
The dolomite Karst surrounding JHB is highly interconnected. It is hypothesized to be
a) one of the globe?s oldest (c. 2300 billion years old)
b) overlain by the Bushveld Igneous Complex (BIC) and
c) being vastly expansive covering an area of 500km x 250km which could mean that this is a unique kind of regenerating (post the BIC cover) Karst system.
The total catchment is much larger by the mere virtue of the system being so vast. Catchments involved are the Limpopo River Catchment to the northeast and the Vaal River Catchment to the south west. Both of which provide water to South Africa and her neighbours - Botswana, Namibia, Mozambique. The Limpopo based karst is a cross boundary karst as it is shared with Botswana.
A mean average of approximately 20 mega liters per day of contaminated groundwater has flowed into rivers and karst systems from the mining voids of JHB over the past six years with little or no intervention to launder the water within the voids. The overflowing fluvial flow disappears via a sunken stream or swallet some three kilometers downstream the now smitten KGR. Unfortunately this contaminated AMD water is being returned to the dolomite karst aquifers in a highly contaminated form. Hypotheses have been leveled at the South
African authorities as to the outcomes in years to come from a karst health, structural and water quality point of view to no avail. Offers of assistance from the international community have also been turned away as the Government appears to believe that there is not any cause for concern. It is a nice example of the Ostrich syndrome.
The action being taken is only dealing with the over-flowing water which is processed and sold at a considerable and profitable price by government sanctioned companies. There appears to be no awareness at all that this water is only a very small part of the total groundwater reservoir and there is inevitably a great diversity of underground flows through the karst. Thus the current program serves opportunistically to increase revenue, but has no impact at all on reducing the problem and probably exacerbates it. Regrettably the ostrich response means that it is not possible to do any further research or even to properly monitor what is happening
Many hundreds of viable wild cave habitats, including the world famous Cradle of Humankind World Heritage fossil site at Sterkfontein are in direct conflict with the ongoing decant and associated complications. It is really very clear that unless appropriate action is taken, there will be irreversible detrimental damage to the world Heritage Site and to the greater Witwatersrand and her people, not to mention the detriment which will play out and also damage South Africa?s unsuspecting neighbours. This will include the belated impact to the world famous Kruger National Park which is in a direct surface flow path of the Limpopo River and the associated Oliphant?s River catchments.
1. Nordstrom, D.K. & Southam, G. 1997. Geomicrobiology of sulfide mineral oxidation. In Geomicrobiology: Interactions between microbes and minerals, Reviews in Mineralogy, 35, 361-390.