South Africa plans to set up a chain of pumping stations and treatment plants to prevent toxic liquids building up in defunct gold mines beneath Johannesburg from reaching dangerous levels.

Immediate action is needed because acidic mine water is expected to reach environmentally critical levels under Johannesburg by June 2012, government officials and scientists told reporters on Tuesday.

Water has already started leaking from abandoned mines west of Johannesburg in the so-called Western Basin.

Work in the Western Basin is immediate, Thibedi Ramontja, chief executive officer of the Council for Geoscience, said after the briefing.

The dangerous cocktail of chemicals has been building up in mine shafts dug more than a century ago to tap one of the world's largest gold deposits and stretching scores of kilometres under Johannesburg.

Water accumulating in shafts and tunnels has been reacting with rocks formed about 2.8 billion years ago and triggering chemical reactions that produced sulphuric acid, heavy metals, toxins and radiation.

The government commissioned a team of experts to investigate the problem and will release its findings on Thursday.

If the pumps are in place by March 2012, we will protect that environment-critical level, Henk Coetzee, one of the experts, told the briefing, referring to Johannesburg.

Acid mine drainage has plagued derelict mines globally for decades but most of the damage has been in remote areas.

The problem for Johannesburg is that the city was built over its gold mines and the land that will be affected is occupied by some of the country's biggest firms and some of its most densely packed townships and suburbs.

The toxic liquid is now about 500 metres below the surface in the Central Basin under Johannesburg and about 700 metres below the surface in the Eastern Basin, water ministry officials said.

Independent studies show the water is rising on average by about 15 metres a month.

The government plans to set up a series of pumping stations and water treatment facilities across the country to prevent the water from harming a key centre of Africa's largest economy, Planning Minister Trevor Manuel told the briefing.

My understanding is that there is no one magical pump. There is a series of pumping activities that you need to do. You are talking of a distance of 200 kilometres (120 miles), said Manuel, a former finance minister.

Manuel said the government had been talking to mining firms about funding the project but would not estimate the cost of the clean-up. Many of the original mining firms closed or were taken over decades ago.

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