All that glitters... Acid mine drainage: The toxic legacy of gold mining in South Africa

Acid mine drainage coats the hippos at Hippo Dam at the Krugersdorp Game Reserve.

Credit: 

© Stephan du Toit, 2011

Mining communities in the Witwatersrand goldfields are often built near, or on top of, tailings piles that contain uranium dust.

Credit: 

© Stephan du Toit, 2011

Pumps at the Grootvlei Mine in South Africa's Easter Basin before pumping ceased in early 2011.

Credit: 

South African Water Research Commission

In 1947, in a dolomite cave 50 kilometers northwest of Johannesburg, South Africa, anthropologist Robert Broom discovered a 2.05-millionyear- old adult australopithecine skull, one of the oldest and most important hominid fossils ever found. The caves of the Sterkfontein Valley have since produced a treasure trove of australopithecine fossils along with evidence of the early use of stone tools and fire, prompting UNESCO in 1999 to name it the Cradle of Humankind World Heritage Site. For 2 million years, the site has preserved evidence of human origins and evolution — but the activities of modern humans are now threatening its existence.

In the Witwatersrand goldfields, 30 kilometers south of the caves, more than a century of mining 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. 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. “It unfortunately makes a beautiful sulfuric acid,” says Jay Barton, a geochemist and board member of South Africa’s Council for Geoscience, that nation’s equivalent of the U.S. Geological Survey.

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. However, last year, it was finally found within the site, where it caused a fishkill, triggering public outcry and focusing the debate on the larger issue of acid mine drainage in the Witwatersrand and throughout South Africa.

Scientists estimate the volume of acid mine drainage from 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. Many say the action will be too little, too late.

“The department of mines sat on this for a decade,” Barton says. “Now it’s a dangerous situation.”

Witwatersrand Gold Rush

The name Witwatersrand, literally “White Water Ridge” in Afrikaans, is ubiquitous in South Africa. The namesake ridge, often called the Rand, passes through Johannesburg. This highly erosion- resistant quartzite ridge forms the north side of the Witwatersrand goldfields, a 300-kilometer-wide ancient sedimentary basin containing the metropolis of Johannesburg with a population of 3.8 million. The ridge also forms a continental divide. To the south, the Vaal River watershed drains to the west, crossing most of the country before flowing into the Atlantic Ocean. To the north, the Limpopo watershed drains into the Indian Ocean.

Two billion years ago, the ridge was on the northern shore of a shallow inland sea where gold-bearing sediment — that would later become part of a 7,000-meterthick rock formation aptly named the Witwatersrand Supergroup — was being deposited. Today, the lithified conglomerate forms seams, or reefs, in the Witwatersrand goldfields, or basins as they are called, including the Western, Central and Eastern basins. Other goldfields in the Witwatersrand include the Carletonville, Klerksdorp, Welkom (or Free State) and Evander goldfields, as well as a new basin that is currently being developed called the South-Rand Basin or Burnstone Project.

Since the 1886 discovery of gold in the Central Basin, 40,000 metric tons — 30 percent of the gold in the world — have been extracted from the mines of the Witwatersrand, which is why the South African currency is also named the Rand. The two deepest mines in the world are found in the Witwatersrand. The Mponeng and TauTona gold mines, owned by AngloGold Ashanti, currently extend nearly 4 kilometers below the surface, where temperatures reach 60 degrees Celsius. Nearly 800 kilometers of interconnected tunnels and shafts have been drilled and dug, the deepest of which are expected to reach 4.5 kilometers in a few years.

Toxic Environmental Legacy

Such technological and engineering feats have allowed the extraction of immense wealth for South Africa, but also have left behind a toxic legacy.

“Over 120 years, in more than 120 mines, companies simply mined, they did not address the long-term environmental impacts, especially the impacts on water,” says Mariette Liefferink, CEO of the Federation for a Sustainable Environment, an environmental organization in Johannesburg. “They maximized their profits and externalized the costs.”

Nearly 400 square kilometers of tailings dumps are scattered in, around and under the city of Johannesburg. The dumps, or slimes, as they are called, contain waste rock, pyrite and thousands of tons of uranium dust. When it rains, the tailings dumps, especially those that are being reworked for secondary gold recovery, produce sulfuric acid.

With pH levels as low as two (pure water is neutral with a pH of seven), the sulfuric acid mobilizes aluminum, heavy metals and uranium from the rock, and produces iron oxide as a byproduct. When this toxic brew flows into groundwater, streams and rivers, it destroys ecosystems and taints water in a region where freshwater is already in short supply.

“The iron oxidizes on exposure to air and precipitates along the flow path, leaving a bright orange trail on riverbeds and banks,” writes Terence McCarthy, a geochemist at the University of the Witwatersrand in a 2011 commentary in the South African Journal of Science. McCarthy is a member of the technical task force appointed by the committee on acid mine drainage to investigate the problem and suggest solutions.

A bigger problem than the acid mine drainage from the tailings piles at the surface, however, is the acid filling the underground mines. When mine operations are active, pumping is required to prevent mines from flooding. However, as gold has become depleted or economically unviable to recover, mining companies have abandoned the mines and shut down the water pumps. When pumping ceases, mines begin to fill up with water. For the past few decades, the government has been issuing pumping subsidies to mining companies to offset the cost of pumping water that flows in from adjacent closed mines. However, when all of the mines in a goldfield close, McCarthy says, no one remains to pump and the government must step in.

In 1996, Harmony Gold shut down a depleted mine in the Western Basin and, two years later, shut down the large pumps that had previously kept groundwater levels in the mine below the depth of mining operations. In a 1998 report to the Department of Mineral Resources, the company predicted — based on the rate of water flow into the mine — that contaminated water would reach the surface in four years. By late August 2002, as predicted, acid mine drainage had filled the mine and, like a bathtub with the faucet left running, the water began to overflow.

By 2002, Harmony had transferred the mine to Rand Uranium, in which Harmony holds a 40 percent stake. Under South African law, the company was not required to have a mine closure plan in place, but a company that took over a mine, whether it was operational or not, was responsible for its environmental liability. However, “this [law] has proved impossible to enforce — it delayed government intervention in the Western Basin — which has prompted the government to get directly involved,” McCarthy says.

Acid mine drainage flows out, or decants, from the lowest surface opening of a mine. In the Western Basin, it first surfaced from an old well on a farm near the town of Krugersdorp that no one knew was connected to the mine, which illustrates part of the problem: The complex hydrology underlying the Witwatersrand goldfields is poorly understood, making it difficult to predict where and when acid mine drainage will ultimately surface.

The Western Basin has now been decanting 15 million to 20 million liters per day — equivalent to about 100 twoliter bottles per second — for more than nine years. A Rand Uranium water treatment facility at the Krugersdorp site, which adds lime to neutralize the acid and then discharges the treated water into local waterways, has a capacity to treat 12 million liters per day. But during wet seasons, peak flow rates have reached 60 million liters per day. The untreated acid mine drainage pours into a stream called the Tweelopiespruit, which flows through the Krugersdorp Game Reserve — where hippos have been coated in rusty residue — and into the subterranean dolomitic aquifer of the Cradle of Humankind World Heritage Site.

The situation in the Western Basin may presage an even bigger problem, however. The task force reports that in the largest mining district, the Central Basin, pumping ceased in 2008 and the water level in the mines is rising at about 12 meters per month. At this rate, it will reach the surface and decant by March 2013. Pumping in the Eastern Basin, which has already experienced induced seismicity, ceased early in 2011. If there is no intervention, the task force reports, acid mine drainage will surely decant. They are not sure where the acid will rise to the surface, but they suspect the Eastern Basin is connected to the underground mine workings in the town of Nigel, about 60 kilometers southeast of Johannesburg.

How to Solve the Problem

As the water levels in the mines have risen, so has the ire, as politicians, scientists, environmentalists, mining companies and taxpayers debate what action to take next.

“The main issue is not whether there is a technical, viable treatment for acid mine drainage — there are many — but who will pay,” Liefferink says.

The task force suggests that an environmental levy on active mines be considered as a way to fund the cleanup of legacy damage, as occurs in the U.S. However, after nearly a century of mining, during which hundreds of companies and their myriad subsidiaries operated in the region, tracking down the parties originally responsible for the acid mine drainage is difficult, Barton says.

Even if responsible parties could be identified, Liefferink adds, a legacy of lax environmental regulation, especially under apartheid, makes it difficult to hold former mine owners financially responsible for damage caused by mine closure. “The new mining companies are like the last man standing,” she says. “They now have to carry the cost of 120 years of irresponsible mining; but the gold-mining industry is in decline and [these new mining companies] do not feel they can pay.”

Indeed, said Rand Uranium CEO John Munro in testimony before the National Assembly in June: “Burdening new companies with the sins of the past is simply unsustainable. These companies will collapse. So not only will we not have a solution, we won’t have mining in the Central, Western and Eastern basins. So we lose a whole lot of jobs.”

Munro instead supports a scheme to sell the treated water to other companies for industrial use. Plans for such endeavors are being explored by some of the mining companies; however, because the cost to treat the water currently exceeds the price of potable water, there is little financial incentive to treat the water as part of a commercial venture at this point. Others have suggested that some form of publicprivate partnership will ultimately be needed to solve the problem. But taxpayers may balk.

“The taxpayers do not want to pay because the public feels they should not have to pay for pollution that they did not cause and from which they have not reaped benefits or profits,” Liefferink says.

McCarthy notes, however, that the government did profit from mining through profit-sharing schemes and taxes, which funded roads, schools, universities and hospitals that benefit current citizens.

“It is incorrect to say that citizens of today do not benefit from the past mining revenue and hence should be absolved from paying for the damage,” McCarthy says. “It is immoral to expect current mining companies to pay for the damage caused by previous mining companies.”

Government Action

In February 2011, the acid mine drainage committee approved several of the short-term flood management and water treatment measures recommended by the task force. These include pumping out water from a point below the old mine workings to prevent it from mixing with shallower, clean groundwater, and controlling the flow of water into the mines by capping or sealing openings.

The already-contaminated water must also be treated and monitored. The task force notes in the report that water quality is not fit “for untreated discharge to the environment or direct use.” The first line of treatment — neutralizing the acid by adding lime and removing some of the heavy metals — is the lowest-cost option and could be implemented quickly, according to the task force. But this requires storage facilities for the sludge and does not address the water’s high salinity, which limits its use.

Longer-term treatments will need to include desalinization, through one of several treatment methods involving chemical precipitation, membranes (such as reverse osmosis), ion exchange or biological sulfate removal. All methods produce clean, potable water, but are costly and produce waste products of their own, the task force notes.

Neutralization treatment will cost up to 5 Rand (less than $1 USD) per cubic meter (1,000 liters). More thorough treatments, like reverse osmosis or ion exchange, remove more heavy metals, but could cost up to 15 Rand (a little more than $2 USD) per cubic meter. If 350 million liters per day will require treatment, the costs add up. Pumping and managing the influx of water into the mines are expensive as well, especially if undertaken indefinitely.

According to McCarthy, the government can use some of the money that formerly funded pumping subsidies to pay for pumping and water treatment. In the long term, the total cost remains unknown, but current estimates are in the billions. “It will cost a colossal amount of money to treat this water,” Barton says.

The Department of Water Affairs has so far allocated 225 million Rand ($33 million USD) to implement some of the task force’s recommendations, including installing the necessary pumps and constructing water treatment plants in each basin, and building infrastructure to transport water. In April 2011, the department appointed the Trans Caledon Tunnel Authority to implement the approved recommendations and gave it a deadline of February 2012. As of July, construction had begun on a canal in the Central Basin, called the Florida Canal, to direct water away from the mines and reduce the inflow of water into the old mine workings. The government had also established a monitoring committee to monitor water quality and the rate at which acid mine drainage is rising in each basin.

The Future

Challenges remain, however. The task force emphasizes that the initial efforts are needed to avert a catastrophe and that many more steps must be taken in the future to solve the problem.

Although the government is currently focused on reducing inflow into the mine voids and treating the polluted mine water, the Department of Mineral Resources is starting to develop future management strategies to deal with abandoned, ownerless mines and to close mines in an environmentally sustainable way, especially when they are highly interconnected, as in the Witwatersrand.

“A problem like this needs a two- to three-year lead time and we only have a matter of months,” Barton says. “It will be addressed, but there will be a lot of damage before it is addressed adequately.”

According to environmentalists, the action the government is taking is too little, too late. Liefferink says the recommended first line of treatment — neutralization with lime — merely raises the pH of the water without removing enough of the heavy metals, sulfides or uranium, thus leaving behind a toxic and radioactive sludge that will need to be stored somewhere and treated again in a few years.

“The gold used to be seen as a blessing,” Liefferink says, “but now we realize it is a curse.”

Funding for future pumping and water treatment has not yet been set aside. Given the global economic crisis, acid mine drainage must compete for limited financial resources with a host of other serious issues currently facing South Africa, including unemployment, improvements to the townships, HIV/ AIDS, rising food prices and persistent energy shortages.

“The problem could be solved,” Barton says, “but there is neither the money nor the political inclination at this time.”

Sara E. Pratt

Pratt is an associate editor at EARTH.

Friday, September 23, 2011 - 19:00

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