The chemical characteristics of acid mine drainage with particular reference to sources, distribution and remediation: The Witwatersrand Basin, South Africa as a case study

Abstract

Abstract Water quality in the immediate vicinity of mine tailings in and around Johannesburg, South Africa was investigated. Pollution is derived primarily from Au mine tailings dumps that are disused or are undergoing retreatment to extract remaining Au, and is dispersed by way of groundwater plumes. These discharge into perennial streams in the area. Pollution manifests itself in the form of low pH (>2) and high SO 4 2 - concentrations (exceeding 7,000 mg L−1 in some cases). Water quality improves away from the tailings area. Pollution loads were found to be higher at the end of the rainy season, due to a rise in the water table and hence increased groundwater seepage. Polluted groundwater usually has low Eh (300 mV) and pH (2–3), and high EC (up to 8 mS cm−1 in some instances). Oxidation of Fe occurs as the groundwater emerges on surface, further lowering pH, and establishing an Fe(III)-Fe(OH)3 redox equilibrium, which operates for many kilometres downstream. Various processes that occur increase compositional heterogeneity in the water, amongst which are evaporation, dissolution of efflorescent crusts and dilution by unpolluted water. Wetland environments are characterised by high pH and low Eh, and appear to be influenced by a sulfide–sulfate redox system, under which SO 4 2 - and metals are removed, and pH increased. Lakes in the mining area have normal water quality, which arises from a combination of metal removal by wetlands (most lakes have wetlands at their inlets) and dilution by rain and unpolluted groundwater. They offer a potential method for passively treating polluted water arising from tailings dumps.