[miningmx.com] — AN expert report on acid mine drainage (AMD) has sketched a gloomy scenario of how the legacy of 120 years’ mining in the Witwatersrand had developed a real threat to the area’s people and environment.
The report, released on Thursday by the department of water affairs, described how the issue of AMD’s was brought to the boil in September 2002 when acidic mine water started to flow from an abandoned shaft in the Randfontein area of the Witwatersrand’s Western Basin.
A similar situation has since been developing in the Central Basin (Johannesburg area) and the Eastern Basin (Springs-Nigel) area, with rising effluent expected to reach an environmentally critical level of 150m below surface – where AMD starts to flow into surrounding groundwater or surface water systems – in the Central Basin by June 2012.
WESTERN BASIN
“Flooding of the underground mine workings, representing more than 120 years of mining in the Witwatersrand, culminated in the decant of AMD from the smallest basin in late-August 2002,’ read the report. “Flooding commenced when the last mine to shut down in this basin ceased pumping.
“The volume of decant has peaked at around 60 mega litres per day (Ml/d).during wet summer rainfall seasons. More typical is a decant rate of around 15 to 20 Ml/d. Basic treatment of this water currently permits the release of around 12 Ml/d into the Crocodile and Marico drainage system.’
The report said existing pumping and treatment capacity was inadequate to effectively manage the impact of AMD, with the excess volume flowing untreated into the receiving aquatic environment.
As an immediate priority, the report suggested that a neutralisation plant be established with a capacity of 20 Ml/d. “This is required to supplement the existing treatment capacity operated by mines in the area and the upgrade of mine water pumping facilities accordingly.’
CENTRAL BASIN
In the largest basin, the water level has been rising at an average rate of 0.59m per day since July 2009. By end-November 2010, mine water reached a level of 510m below surface at the Catlin shaft of Simmer & Jack. It is expected that the rising water level would reach the surface by March 2013.
“By this time, however, it will have sterilised still exploitable gold reserves located at a depth of less than 400m below surface,’ read the report. “Of greater concern is that it will not only have flooded the shallower underground tourist facilities at Gold Reef City, but also compromised the shallow groundwater resource associated with the dolomitic strata located to the southeast of Johannesburg.’
“The problems posed by acid mine drainage will have implication far into the future, with impacts likely to continue for many years…”
The report stated that the flooding of the Central Basin has also been associated with an increase in seismic activity, with a higher frequency of earth tremors in the area following the cessation of pumping at ERPM in October 2008.
As a remedy, a pumping facility with a capacity of 60 Ml/d is required in one or more of the existing mines shafts, coupled with a neutralisation plant with the same capacity.
EASTERN BASIN
The situation in the Eastern Basin has been complicated due to uncertainty over whether pumping would continue at Aurora Empowerment System’s Grootvlei mine. The mine has historically maintained the mine water level at a depth of around 700m below surface in its Number 3 shaft.
“While continuation of this pumping regimen is at risk of failure owing to financial constraints, an early casualty of the situation was the treatment of the raw mine water prior to its release into the Blesbokspruit and a Ramsar-listed wetland,’ read the report.
“Treatment of the mine water has not occurred for some time. The cessation of pumping will result in flooding of the pump station within 30 days, after which the mine water will rise to its decant level and decant in or close to the CBD of Nigel on the East Rand.’
It recommended that the pumping capability in Number 3 Shaft of Grootvlei be secured as soon as possible, while the existing treatment plant at this locality be returned to service.
“The volumes of water to be managed may be reduced by the timely implementation of ingress management measures, with a resultant reduction in operating costs. The design of the pump and treat systems will need to take this into account.
The report further warned that flooded mines were not the only sources of AMD in the Witwatersrand. Mine residues would also have to be monitored to reduced AMD impacts on the environment.
“The problems posed by AMD will have implication far into the future, with impacts likely to continue for many years,’ read the report. “The process of management of these impacts will therefore need to continue, with ongoing assessments and adaption as conditions change.’
