Natural Attenuation of Acid Mine Drainage by Various Rocks in the Witbank, Ermelo and Highveld Coalfields, South Africa

In the Karoo coalfields, mining operations that release acid mine drainage (AMD) are threatening groundwater resources. An important parameter controlling the extent and severity of AMD impact is the natural attenuation of rocks in response to AMD. Very little is known about such response of Karoo rocks in saturated and unsaturated conditions, a research gap filled by this paper. Laboratory column leach experiments were used to study the responses of different rock types from the Witbank, Ermelo and Highveld coalfields of South Africa. The results of the experiments show that rock samples with high quartz mineral content offered little attenuation of AMD and those consisting of plagioclase and clays offered some attenuation, while those with high carbonate mineral content, neutralized and reduced the concentration of heavy metals from AMD. The presence of oxygen increased the rate of heavy metal removal from AMD. For all rock samples under saturated and unsaturated conditions, the sulfate and chloride concentrations of AMD remained unchanged after the leaching process, showing that these parameters are conservative and can be used as potential natural tracers of AMD movement in the subsurface. The research shows that laboratory leach tests may be used to rank the various rock types in the Karoo coalfields in terms of their capacity to buffer the impacts of AMD. Such a ranking could inform policy and decision makers in regard to handling and storage of AMD-generating wastes, as well as to location and design of AMD waste storage facilities.