Introduction South Africa is a country blessed with an abundance of minerals. Coal, which is the main source of energy in South Africa, is one of these abundant minerals. In 1997, 68% of the energy consumption of Southern Africa was coalbased (Energy Information Administration 2000). In South Africa coal resources occur in 19 coalfields (Erasmus et al. 1981). The most important coalfields in the Mpumalanga Province are the Springs-Witbank coalfield and the Highveld coalfield. A map of these coalfields is shown in Fig. 1. Coal mining has been ongoing in the Mpumalanga Area since 1870 (Barker 1999). Initially, all the mines around Witbank were shallow. Bord-and-pillar mining was used throughout. (First bords (streets) are driven, leaving supporting coal between. This is followed by cross drives, connecting the bords. Supporting coal is left behind as rectangular pillars to support the weight of the strata overlying the coal seams). During the 1970s, wide-scale expansion leads to the development of mega-mine structures south of the existing coalfields (Vermeulen PD Investigation of water decant from underground collieries in Mpumalanga. Unpublished MSc, Thesis (2003), UFS, Bloemfontein, South Africa). Almost all of these developments were associated with power generation or the utilisation of the coal by the chemical industry. Water in South African Collieries As far as mine water is concerned, complexities that arise from mining are: P. D. Vermeulen B. H. Usher Sulphate generation in South African underground and opencast collieries Received: 24 May 2005 Accepted: 30 August 2005 Published online: 13 December 2005 Ó Springer-Verlag 2005 Abstract Deterioration in the water quality results from coal mining, and the significant impact on the envi- ronment in South Africa has been reported (Funke in SA Water, Sci Technol 15:115–144; 1983). Pyrite in with South African coal can be oxi- dised to yield acidic and high salinity water. This study deals with the rate at which sulphate could be produced for collieries under different condi- tions. Four collieries in the Witbank coalfield were selected for this investigation. The boreholes were investigated with an in-situ multi- parameter probe, and then sampled at different times with a specific depth sampler to determine the chemical composition of the water in the collieries. Water level measure- ments and volumetric calculations were done to determine influx rates. Daily sulphate generation rate for underground mining was found to be in the range of 0.4–2.7 kg/ha, depending on the availability of oxygen and the residence time of the water (the time available for neu- tralising agents present to react) in the mining void. The exclusion of oxygen due to flooding and the presence of calcareous carbonates influences the observed sulphate generation. Keywords Coalmines Æ Underground Æ Opencast Æ Sulphate generation Æ Flooding Æ South Africa Environ Geol (2006) 49: 552–569 DOI 10.1007/s00254-005-0091-2 ORIGINAL ARTICLE P. D. Vermeulen (&) Æ B. H. Usher Institute for Groundwater Studies, University of the Free State, 339 Bloemfontein 9300, South Africa E-mail: VemeulenD.SCI@mail.uovs.ac.za