South Africa's city of gold turning into swamp of acid as mines leak

South Africa's city of gold, Johannesburg, may soon start being eaten away by acidic water flowing from the mines that created its astronomical fortunes.

Mines dug more than a century ago that stretch about 25 miles along one of the world's largest gold deposits have reached their water storage limit.

They will start leaking a toxic cocktail of chemicals in the coming months, independent experts and government officials are warning.

If left unchecked, acidic mine water is expected to foul up works near the country's famed Apartheid Museum, flood basements in downtown Johannesburg and then seep into the streets of the city of about four million people.

"The threat of acid water decanting from old mine workings is a real and present danger. It poses a threat to our economy, environment, health and history," Terence McCarthy, a professor of geosciences at the University of Witwatersrand, said.

Acid has leaked from derelict mines the world over for decades. Most of the damage has been in remote areas. But Johannesburg was built over its gold mines and is now home to four million people and some of the country's biggest firms.

About three years ago, the last pump removing water from the mines stopped, signalling an end to a gold rush that brought wealth to a few while hundreds of thousands of black Africans went deep underground to dig.

• South Africa: Anger at inaction over mines

Then the water began to accumulate in the massive underground cavities, reacting with rocks formed about 2.8 billion years ago and producing sulphuric acid, heavy metals, toxins and radiation.

The water, once several hundred metres underground, has been rising at an average rate of 15 metres per month, with the void expected to fill up completely in less than three years.

The leakage problems will be small at first and grow more costly the longer action is delayed, experts said.

Mr McCarthy urged immediately setting up two pump and treatment stations along the main gold reef to keep the water at least 300 metres below the surface.

"The solutions are expensive, though not technically daunting - and must be implemented in a matter of months," he said.

The government acknowledges urgent action is needed but has given little indication it will do anything before the acid water reaches underground facilities in Johannesburg.

A report from a government-appointed team of experts planned for release in early January has yet to see the light of day, prompting the opposition Democratic Alliance to claim delays are putting lives at risk.

Along with finding a way to solve the problem, the government has yet to decide how to pay for it. It cannot pass the bill on to firms since ownership of mines has changed hands so often and many firms have vanished.There is not enough gold left to make it commercially viable for a new firm to go in and pay for a clean-up.

The environment ministry warned of the escalating costs of inaction about three years ago, saying in a report: "If the threat from acid mine drainage is not solved in the short to medium term, it is likely to persist for centuries to come."