2011 Abstract Volume - World Water Week

The Impact of Mining on the Water Security of theWitwatersrand, Gauteng, South AfricaAuthor:Keywords:Dr. François Durand, University of Johannesburg, South Africawater security, karst aquifer, acid mine drainage, gold mines, uraniumIntroduction/Problem IdentificationMore than 40% of the population in South Africa live on the Witwatersrand, the economic hub ofSouth Africa. The Witwatersrand is situated on top of an extensive dolomite deposit, adjacent to avast gold repository which poses a unique and profound challenge to the conservation of the ecologyand human health. Johannesburg was founded in 1886 to accommodate tens of thousands of peopleconverging on the newly discovered gold reef which turned out to be the richest gold deposit in theworld. Over a few years many towns were established along the Main Reef as it was traced along thelength of the Witwatersrand. The towns grew into a continuous strip of habitation stretching overmore than a hundred kilometres. Settlement patterns were driven by opportunism and avarice thatmatched the spawning of mines and industries on the Witwatersrand. Urbanisation took place in ahaphazard and reactive way rather than with careful planning with the eye on long term sustainability.Analysis/Results and Implications for Policy and/or ResearchThe rock ore is brought to the surface where it is ground to a powder and treated with chemicals toextract the gold. The remaining slimes is then pumped to slimes dams where it dries. At present thereare over 270 slimes dumps on the Witwatersrand, covering an area of approximately 400km 2 . In thepast slimes dumps were built on the dolomite itself because this caused it to be more stable due tothe draining of the water directly into the karst system below. This strategy caused the pollution ofthe vast groundwater resources in and around the Witwatersrand with acid mine drainage. The karstaquifer is large enough to supply an urban area many times the size of the Witwatersrand if it werenot for the continued and worsening pollution of this important resource.Due to the karst aquifer which occurs in the dolomites, the gold mines have to be dewatered constantlyin order to allow deep mining to take place in the adjacent or underlying gold-bearing WitwatersrandSupergroup. The pumping of massive amounts of groundwater from kilometres underground intosurface streams turned small streams into sizeable rivers or in some instances clogged waterways withmine silt to form swamplands. Groundwater levels and flow were also affected adversely, one beingthe drying up of the springs from which the name Witwatersrand (“white water ridge”) was derived.Sinkholes started to form over a large area due to this extraction of water.Pyrite occurs in gold-bearing quartzite in the Witwatersrand Supergroup. During the mining processpyrite is exposed to water and oxygen which results in the formation of sulphuric acid. Sulphuricacid, in turn reacts strongly with the silt, gravel and exposed rock it comes into contact with whichreleases and mobilises metals contained within. This effluent, containing sulphuric acid, sulphate saltsand metals is called acid mine drainage (AMD). The run-off from slimes dumps and rock dumps onthe Witwatersrand which enters the surface water bodies and groundwater typically contains AMD.Most of the mines on the Witwatersrand were closed down after they reached their operational limits.After the pumping ceased the mine void, which in the West Rand alone has a volume of over 45million cubic meters, was flooded with groundwater allowing the water table to return to its original20 Workshop 1: Cities in a 3-D Landscape Perspective – Hidden Risks