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Rising acid mine water could be 'catastrophic' for Johannesburg

Millions of litres of highly acidic mine water is rising up under Johannesburg and, if left unchecked, could spill out into its  streets some 18 months from now, Parliament's water affairs
portfolio committee heard on Wednesday.

The acid water is currently about 600 metres below the city's surface, but is rising at a rate of between 0,6 and 0,9 metres a day, water affairs deputy director water quality management Marius Keet told MPs.

"[It] can have catastrophic consequences for the Johannesburg central business district if not stopped in time. A new pumping station and upgrades to the high-density sludge treatment works are urgently required to stop disaster," he warned.

"This environmental problem is second [in South Africa] only to global warming in terms of its impact, and poses a serious risk to the Witwatersrand as a whole. At the rate it is rising, the basin [under Johannesburg] will be fully flooded in about 18 months."

She said the rising mine water had the same acidity as vinegar or lemon juice, and was a legacy of 120 years of gold mining in the region.

Acid water is formed underground when old shafts and tunnels fill up. The water oxidises with the sulphide mineral iron pyrite, better known as fool's gold. The water then fills the mine and starts decanting into the environment, in a process known as acid mine drainage.

Keet said the problem was not just confined to Johannesburg, which is located atop one of several major mining "basins" in the Witwatersrand, known as the Central Basin.

In 2002, acid mine drainage had started decanting from the Western Basin, located below the Krugersdorp-Randfontein area. The outflow had grown worse earlier this year after heavy rains,
prompting his department to intervene.

However, a lack of treatment capacity in the area "compelled in-stream treatment as a short-term intervention".

This intervention saw the department pouring tons of lime, an alkali, into the Tweelopies Spruit in an effort to neutralise the acid mine water. This had led to problems with the resulting sludge that had formed in the water course.

The region's Eastern Basin, below the town of Nigel, was also threatened. The last working mine still pumping out water in the area was Grootvlei.

Keet said that if the mine stopped pumping, acid water would start decanting into the town "within two to three years".

Water affairs is currently taking legal action against the mine, after it allegedly failed to comply with a departmental directive to treat the pumped water before discharging it.

On stopping the growing threat below Johannesburg, Keet said about R220-million was needed to establish pump stations, pipelines and treatment works.

(source: http://www.miningweekly.com/article/rising-acid-mine-water-threat-to-joburg-2010-07-21)