The first Gauteng City-Region Observatory (GCRO) Provocations series lecture was launched focusing on a topical issue that affects everyone in Gauteng. Prof. Terence McCarthy from the Wits School of Geosciences spoke passionately about the decanting of acid mine drainage in the Gauteng City-Region and amongst other matters, its impact on water quality.

The looming threat of the decanting of acid mine water in Johannesburg and neighbouring municipalities has received considerable media coverage over the last few months, but what has been lacking is a concise explanation of the origin of the problem, how it may unfold and which areas are potentially at risk.

“It is in all our interests that there is a general understanding of the nature of the problem,” says McCarthy.

According to McCarthy, Witwatersrand gold occurs in layers of pebbly rock called conglomerate. In the Johannesburg region, only two or three such layers contained economically extractable gold. These layers extend from Randfontein in the west to Boksburg in the east, and they dip down from the surface at an angle of about 30 degrees. The average thickness of the individual layers that were extracted was about one metre.

Mining involved extraction of the conglomerate rock, which was then taken to surface where the gold was extracted. The remaining waste rock was then dumped, forming Johannesburg’s iconic mine dumps.

“Mining therefore created open spaces below ground – the mine void. In the course of mining, water-bearing fractures were intersected and water flowed from these openings into the mine workings. This water was collected and pumped to the surface via the mines’ access shafts. By continuous pumping the miners ensured that the void remained filled with air,” explains McCarthy.

The mines on the Witwatersrand began to close in the late 1950s due to declining profits. Closure of a mine meant that pumping ceased and water started accumulating in the deeper underground workings. From there it began to flow into the adjacent mines, which took up the pumping responsibility of their defunct neighbour.

Says McCarthy: “Eventually, only one operating mine was left – East Rand Propriety Mines Ltd. (ERPM), which is situated on the eastern extremity of the Witwatersrand mining belt in Boksburg. This mine therefore had to carry the burden of pumping out water that flowed from all of the defunct mines, which required it to pump an average of 40 million litres of water per day.”

When ERPM finally ceased pumping in October of 2008 the void began to fill. The water level currently lies at a depth of about 600 metres below the surface, with the average rate of rise of the water level at about 15 metres per month. “At this rate, the void will be completely filled in about two and a half years from now,” warns McCarthy.

The relief from this possible disastrous situation will come once the void has filled and the decant will commence. Decant will occur because the mine void and openings connected to it such as shafts and collapsed areas occur at a variety of elevations and water will flow into the void in higher areas and decant at low points. The lowest large opening connected to the void is the Cinderella Shaft of ERPM in Boksburg, and if free flow of water through the void was possible, decant would take place only at this shaft. However, lateral flow of water through the void is most likely restricted, which means that the water level in the void will stabilise at different levels across the mining belt, resulting in multiple decant points.

The major zone of risk is the region where the workings are relatively shallow – within about 200 metres of the surface, and extends along the outcrop of the Main Reef and from there in a southerly direction over a width of about of about 500 metres.

“As a rough guide, it more or less follows Main Reef Road and the M2 motorway in central Johannesburg. There is a second risk zone associated with the Kimberley Reef. This reef was only sporadically mined and the zone is not continuous. Outside of these two zones, the flooding risk is low to negligible. The water that accumulates in the mine void is acidic and contains high concentrations of dissolved sulphate and heavy metals and is toxic and corrosive,” says McCarthy.

According to him the solution to the problem is relatively simple and involves the establishment of pumping stations to pump the water to surface for basic treatment so that the water level does not rise to the surface. The depth at which the water level should be stabilised needs to be carefully considered.

To ensure the security of the Gold Reef City underground facility, the water level would need to be maintained at a depth of at least 250 metres below surface at this location. Lateral flow of water in the mine void is restricted, so for safety, it is suggested that two pump stations be established, one in the Germiston area and one at Florida, and that the water level be maintained at 300 metres below surface at these two points.

“It must be accepted that pumping will have to continue indefinitely. Initially, this will involve considerable financial outlay to establish the pumping and treatment facilities with an ongoing cost to maintain the pumping and water treatment,” adds McCarthy.

“It should be noted that much of this expense will not be new, as the government has for decades been paying pumping subsidies to mines to cover the cost of pumping inflow from defunct, adjacent mines. In the long term, the quality of the pumped water will improve to the point where the water will become saleable and the pumping operations will then probably generate a profit.”

The GCRO Provocations Series is a vehicle for leading academics to summarise ‘the state of the art’ in their discipline in an accessible and argumentative form, making it understandable and useful to policy-makers and citizens.

Article courtesy of Wits, visit: www.wits.ac.za