Acid mine drainage threatens to turn Gauteng into a vinegary broth, but also offers project opportunities

Acid mine drainage (AMD), and the failure of the government to address it, could reach the ecologically dangerous level of 150 metres below the surface in February 2012 and can start flooding an area close to the central business district of Boksburg at a tempo of 57 megalitres per day.

This is according to Marius van Biljon, who registered a master’s degree in Geohydrology and researched AMD on behalf of the old JCI in the Western Witwatersrand, and according to Rapport. (Source: Rapport, 21 November 2010)

Mariette Liefferink, chief executive officer of The Federation for a Sustainable Environment, told The Project Manager: “Waste from gold mines constitutes the largest single source of waste and pollution in South Africa.

“There is wide acceptance that AMD is responsible for the most costly environmental and socio-economic impacts.

“Gold mining waste was estimated to account for 221 million tonnes, or 47% of all mineral waste produced in South Africa – making it the largest, single source of waste and pollution,” she added.

Production of AMD may continue for many years after mines are closed and tailings dams decommissioned.

AMD is not only associated with surface and groundwater pollution, degradation of soil quality, for harming aquatic sediments and fauna, and for allowing heavy metals to seep into the environment, says Liefferink.

Long-term exposure to AMD-polluted drinking water may lead to increased rates of cancer, decreased cognitive function and appearance of skin lesions.

Furthermore, heavy metals in drinking water could compromise the neural development of the fetus, which could result in mental retardation.

Rehabilitation

Rehabilitation of the Witwatersrand goldfields will have to address the pumping and treatment of AMD long after mine closure.

It will further have to include the removal of polluted soil and sediment, soil replacement or soil amelioration, revegetation and biodiversity re-establishment, including the reintroduction of aquatic biota, Liefferink told The Project Manager.

Phytoremediation is an effective treatment in minimising AMD from mine tailings storage facilities and in rehabilitating polluted soils with plants and trees tolerant to high sulphate loads and heavy metals.

West Rand Basin

The Western Basin was the first to flood with AMD. It is the smallest of the mining basins.

Decant began in 2002, and resulted in devastating consequences. Since 2002, crisis management has followed.

All aquatic biota were wiped out in the Tweelopiespruit. It resulted in the Tweelopiespruit being declared a Class V river i.e. a high acute toxic hazard river.

Since the decant of AMD occurs at an intercontinental divide, it impacts upon the Wonderfonteinspruit (Vaal River catchment) to the south.

The mean values for the sediment of the Wonderfonteinspruit exceed not only natural background concentrations but also levels of regulatory concern for cobalt, zinc, arsenic, cadmium and uranium – with uranium and cadmium exhibiting the highest risk coefficients.

It will require remediation of the wetlands, employing passive and active treatment systems.

Dams, such as the Robinson Lake and the Lancaster Dam, as well as pits such as the Lindum Pit, the CPS Pit, the West Wits Pit (which contributes 30% of AMD to the Western Basin) and the Millsite Pit, will have to be rehabilitated by the removal of the sediment or the filling in or capping of the pits, Liefferink told The Project Manager.

East Rand Basin

Pumping and treatment of AMD – and the remediation of the Blesbokspruit – are called for. The sediment contains elevated levels of heavy metals and sulphates.

Central Basin

Pumping and treatment of AMD has to be implemented with urgency.

There is significant dust pollution and water pollution from the tailings dams within the Central Basin.

Footprints of re-mined tailings dams ought to be rehabilitated.

Wetlands also have to be remediated.

Far West Rand and Kosh Basin

Many dolomite compartments have been dewatered over the years, resulting in sinkhole formation and ground stability problems. Vigorous groundwater and stability monitoring systems must continue for some time after mine closure.

After closure, the mine workings will flood and the dolomite aquifers will largely recover to pre-mining levels.

No management options are in place to cope with contaminated decant water yet, warned Liefferink.

Sinkholes have been historically filled with uraniferous tailings. When pre-mining flow patterns and volumes of water are restored, tailings will be remobilised into the groundwater systems.

Wetlands

Wetlands can act as metallic pollutant sinks or sponges for heavy metals. Wetlands downstream of the Witwatersrand’s mining areas provide some protection to downstream water users, but cannot be relied upon indefinitely.

They act as pollution sinks, primarily because of the hydrological and chemical conditions that exist within them. In order for them to continue to act as sinks, these conditions must be maintained.

Plausible changes in these conditions have been observed to release trapped contaminants, resulting in downstream water pollution.

Many wetlands saturated with heavy metals are now secondary sources of contamination, Liefferink told The Project Manager.

The processes that trap metals have, in some areas, trapped potentially economic concentrations of gold. These gold concentrations could well partially fund the vital rehabilitation of these wetlands.

This process may be beyond the technical and financial scope of small operators, but can be developed as a public-private partnership involving local communities

Additional funding for such a rehabilitation programme must be sourced using the polluter-pays principle.

Source apportionment tools do exist to assist with the identification of pollution sources, said Liefferink.

Tailings dams and waste rock dumps

There are more than 270 tailings dams in the Witwatersrand Basin, covering approximately 400 square kilometres in surface area, containing six billion tonnes of pyrite tailings.

Tailings dams cannot be maintained in a reducing (oxygen-free) environment.

Tailings dams generate AMD, and the impact will continue for hundreds of years after mining operations cease.

Most of the tailings dams are unlined, unvegetated and were historically placed on dolomite. This allows heavy metal seepage, including uranium (50 tonnes per annum within the West and Far West Rand Basins), and storm water runoff into the receiving surface and groundwater.

Tailings dams will have to be removed from dolomite by re-mining or capping in order to prevent the generation of AMD.

There are also many waste rock dumps scattered within the Witwatersrand gold fields. They have very large inventories of fine material and are much more permeable to oxygen than tailings dams.

The secondary source of contaminants that remain in the soil after a dump has been removed appears to be universally ignored, and it is assumed that removal of the dump removes all potential for pollution from that site.

Footprints

Long-term risks (quantity and quality) are associated with post-closure seepage from footprints below removed dumps

Footprints contain toxic and radioactive heavy metals, and are acidic. The gold mined is associated with sulphide minerals that, on exposure to atmosphere, oxidate and produce sulphuric acid.

Footprints leach highly acidic waters; capillary action raises the acidity from underlying sulphide-containing materials up through the overlying soil profile.

Acidity mining through soil causes chemical and physical composition changes.

A key aspect for rehabilitation is to ensure the source of the acidity is removed so that recontamination will not occur.

Footprints can be treated with limestone.

For this limestone to be effective, however, it has to be incorporated throughout the acid soil horizon. This should be done by deep ploughing, said Liefferink.

The soils will be high in metal toxicities, and residential developments or agricultural usage of the land is not advisable.

Liefferink concluded that the contaminant sources must be eliminated or minimised. Once pollution inputs are
reduced, rehabilitation can commence.

A long-term monitoring and management programme must be implemented.

A rehabilitation case study

The Aveng Group has the only two full-scale water reclamation reference plants treating AMD water in the country: the eMalahleni Water Reclamation Plant for AngloCoal / BHP Billiton and the water reclamation plant in the Middelburg area for Optimum Coal – this according to Harry Singleton, general manager of the water management and treatment division of the E+PC Engineering and Projects Company Limited.

The eMalahleni plant has, since September 2007, produced 25 megalitres per day of high-quality drinking water that AngloCoal sells to the eMalahleni municipality as potable water.

The Optimum plant produces 15 megalitres of potable water a day; plans exist to sell this it to the Hendrina Municipality.

Both plants recover in excess of 99.7% of the AMD water entering them.

Aveng built and operates the above plants on behalf of its clients, Singleton told The Project Manager.

The high precipitation reverse osmosis technology used in the plants was developed by Keyplan, a wholly owned subsidiary of Aveng, and can be applied to AMD water in general, whether in the coal or gold mining sectors.

The immediate challenge is for the appropriate stakeholders to agree on a practical mechanism to apportion the responsibility fpr treating AMD.

Some indication from the Department of Water and Environmental Affairs on whether it sees AMD as a future potential source of ‘new’ potable water would assist in compiling a holistic solution to the problem, says Singleton.

Rehabilitation deadline and costs

The typical lead time to design and build a custom-designed water reclamation plant of the size of the eMalahleni or Optimum Coal plants is in the order of 18 months, excluding environmental impact assessments and any other regulatory requirements, says Singleton.

The Aveng Group offers a mobile plant option available in two megalitres per day modules that can be put on the ground in a shorter period (eight to 10 months) as an emergency measure and can then be scaled up over time into a full-size plant.

The critical path, however, is often determined by the time it takes to design and build not the water reclamation plant, but by the infrastructure around the plant.

Many factors must be accounted for prior to costing: besides the costs of erecting a water reclamation plant, associated infrastructure costs (related to pipelines, reservoirs, pump stations, utilities etc.)can cost as much as, if not more, than the water reclamation plant itself.

The cost of the water reclamation plant itself depends on factors such as the quality of the water to be treated and the standard to which the water must be treated.

The question is: Will it be used for consumption as potable water, or simply treated and released into the environment or used as agricultural or industrial water? 

Fanie Heyns