The Witwatersrand Legacy
The Witwatersrand is a low, sedimentary range of hills, at an elevation of 1700—1800 meters above sea-level, which runs in an east-west direction through Gauteng in South Africa. The discovery of gold in the Witwatersrand led to the Second Boer War and ultimately the founding of South Africa. The Witwatersrand goldfields have more than a century of mining and has left the region littered with mounds of waste, known as tailings dumps, and underlain by a deep underground network of abandoned mine shafts, which are gradually filling with water. These mines as a result of highly efficient production methods may operate at depths in excess of 3000 meters where the specific geological conditions are suitable.
Today, the mines are producing less and less gold and more and more sulfuric acid. The metamorphic rock of the mining district contains abundant pyrite (iron disulfide or fool’s gold), which reacts with oxygenated rainwater or groundwater.
Pyrite exposed to the atmosphere during mining and excavation reacts with oxygen and water to form sulfate, resulting in acid mine drainage. This acidity results from the action of bacteria, which generate their energy by oxidizing ferrous iron (Fe2+) to ferric iron (Fe3+) using oxygen. The ferric iron in turn attacks the pyrite to produce ferrous iron and sulfate. The ferrous iron is then available for oxidation by the bacterium; this cycle continues until the pyrite is depleted.
Waste from gold mines constitutes the largest single source of waste and pollution in South Africa and there is wide acceptance that Acid Mine Drainage (AMD) is responsible for the most costly environmental and socio-economic impacts. While South Africa has made significant progress in shifting policy frameworks to address mine closure and mine water management, and the mining industry has changed their practices to conform to new regulations, vulnerabilities in the current system still remain. The pollution reality of gold mining waste is illustrated by a case study in the West Rand area, where decant from gold mines started in 2002. Potential receptors of the pollution in the case study area include neighboring property owners, a game reserve and, further afield, the Cradle of Humankind World Heritage Site.
The Witwatersrand is not the only region in South Africa where acid mine drainage is a problem, but it is currently the area of greatest concern. In 2002, near the town of Krugersdorp, acid mine drainage from an abandoned mine welled up and began pouring out on the surface. Since then, about 15 million liters a day of acid mine drainage have been spilling out — some of it into a stream that flows north toward the Cradle of Humankind World Heritage Site. In the last few years, acid mine drainage has been known to be encroaching on the heritage site and has already dissolved a 16,000-cubic-meter void in the calcium carbonate rocks outside the protected site.
Drainage from the abandoned mines in the Witwatersrand goldfields alone could reach 350 million liters per day, threatening the Vaal River and Limpopo River watersheds that supply freshwater to hundreds of thousands of people. They also say the problem will likely persist for decades, if not centuries. Despite warnings from the scientific community as early as the 1950s, the South African government only recently established a committee to investigate acid mine drainage, after it reached the Cradle of Humankind.
In December 2010, a technical task force appointed by the committee reported that the effects of acid mine drainage in the Witwatersrand include contamination of groundwater used for human and agriculture use; serious negative ecological impacts; regional impacts on major river systems; flooding in low-lying areas; and increased seismic activity (as the subterranean hydrology adjusts to the influx of water).
The government has budgeted funding to implement some preliminary remediation efforts by February 2012, including pumping some of the mines most at risk for flooding and performing basic water treatment. The full cleanup will cost billions, however, and little real progress has been seen so far.
For further information: http://www.earthmagazine.org/earth/article/48e-7db-9-17