ACID MINE DRAINAGE AND ITS POTENTIAL IMPACT ON THE WATER RESOURCES IN THE WATERBERG COALFIELD, SOUTH AFRICA

Abstract

South Africa currently ranks ninth in the world for proven coal reserves that have been estimated to last for over 200 years. Coal constitutes 77% of the primary energy needs in the country, with the Waterberg Coalfield estimated to hold about 40% of the remaining South African coal reserves. Coal deposits in the study area are largely composed of shales, mudstones, siltstones and sandstones which host coal-containing clay minerals, quartz, carbonates and sulphides; the most abundant sulphide mineral is pyrite. Once mining operations commence, the sulphide minerals are exposed to atmospheric oxygen which promotes degradation of hydrous sulphides into iron and sulphuric acid causing acid mine drainage (AMD). Acid-base accounting (ABA) was used to determine the balance between the acid-producing potential (AP) and neutralising potential (NP). From the analysis the net neutralising potential (NNP) was determined to classify a sample as potentially acid-producing or non-acid producing. X-ray diffraction (XRD) and X-ray fluorescence (XRF) were used as other sources of characterisation for geological samples where the mineralogical and whole-rock analyses are determined. Acid mine drainage does not only result in the generation of acid but is accompanied by decreased pH and increased values of specific conductance, dissolved metals and sulphate. Inductively coupled plasma analysis was applied to determine the release of the heavy metals as well other constituents which can be detrimental to the environment. Samples were collected from the interburden and overburden as well as the coal seams. Acid-base accounting results show that the interburden and coal samples have the higher risk of producing acid upon oxidation when compared with overburden samples which have a higher concentration of neutralising minerals that can neutralise the acid produced through sulphide minerals as oxidation takes place. Analysis of kinetic test results indicated the long-term behaviour of different samples, with the electrical conductivity (EC) and pH changing over time. Samples with lower pH continued to produce more sulphate, while calcium continued to increase until it was depleted from the samples. The water demand will increase as mining continues in the area, with inter-catchment transfers identified to overcome local water-scarcity issues. Acid mine drainage poses a major threat to water resources, both groundwater and surface water.