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. This study aims to fill the research gap regarding the response of Karoo rocks in saturated and unsaturated conditions. Laboratory column leach experiments were conducted to analyze the responses of various rock types from the Witbank, Ermelo, and Highveld coalfields of South Africa. The results indicated that rock samples with a high quartz mineral content provided minimal attenuation of AMD, while those containing plagioclase and clays demonstrated some attenuation. Rocks with high carbonate mineral content effectively neutralized and reduced heavy metal concentrations in AMD. Additionally, the presence of oxygen significantly enhanced the rate of heavy metal removal. Importantly, sulfate and chloride concentrations in AMD remained unchanged under both saturated and unsaturated conditions, suggesting potential as natural tracers of AMD movement. This research presents a ranking of rock types based on their buffering capacity against AMD impacts, offering valuable insights for policymakers and decision-makers regarding adequate management of AMD-generating wastes and the design of waste storage facilities.