Characterisation of the deep aquifers of South Africa - the bushveld igneous complex, crystalline basement rocks and dolomite formations

[摘要] During periods of drought, as South Africa has experienced recently, groundwater boreholes are explored to identify alternative sources of water. However, much emphasis is placed on the shallow groundwater system with minimal information available on the deep aquifer system. The aim of this dissertation is to identify potential deep aquifer systems in South Africa and characterise their potential as potable water sources. This dissertation concentrates on three types of aquifer systems, the crystalline basement aquifers, Bushveld Igneous Complex and dolomite aquifers. In order to characterise deep aquifer systems, an understanding of basic hydrogeological terminology and characteristics of the shallow aquifer system need to be devised first. The methodology that is required is 1) to determine the defining depth between shallow and deep aquifer systems, 2) identify possible deep aquifer systems in South Africa, 3) identify the groundwater quality of shallow aquifer systems and infer characteristics to the deep aquifer system, depending on the geologies encounters, 4) identify any structures that may contribute to groundwater flow and the origin of the deep groundwater, 5) analyse thermal spring data as they provide an insight into the groundwater quality at depth, 6) characterisation of potential deep aquifer systems, and 7) determine the possible threats to the aquifer systems and provide mitigation procedures to prevent any contamination from occurring.The defining depth between shallow and deep aquifer systems from a South African perspective is 300 mbgl. Based on the research carried out, all of the above-mentioned aquifer systems are generally classified as secondary aquifer systems in which the groundwater is typically found in fractures and cavities within the bedrock. The volumes of groundwater stored and transported will depend on the degree of fracturing and weathering if the bedrock. The dolomite aquifers in South Africa are considered to be the highest yielding aquifer system. In terms of groundwater quality, the dolomite aquifers have the most pristine groundwater condition of the three, provided that no contamination has occurred. A signature fingerprint with deep groundwater aquifer systems is that the salt content increases with depth, which is possibly due to the long residence time within the aquifer systems. The characteristics of the shallow aquifer system have been inferred to characterise the deeper aquifer systems based on similar geologies. However, the deep aquifer systems are subjected to lower recharge rates than the shallow aquifer system and may be at risk for overexploitation. On the contrary, the deeper aquifer systems are less susceptible to contamination from human activities at the surface. The main identified threat to deep groundwater aquifer systems is from the mining activities, which as we know in South Africa, with mined depth at least 3200 mbgl. The existing legislation that is currently in place focuses more on the shallow aquifer system than the protection of the deep aquifer system. Hence, more investigations are required to develop suitable legislation and tools to manage contamination in deeper aquifer systems.In conclusions, although there may be potential deep aquifer systems in South Africa, the volumes of groundwater at this stage cannot be determined as there is minimal information available. It is possible that the groundwater at depth comprises a high salt content which may require treatment. It is suggested that more investigations be carried out to determine the volumes of groundwater available within each aquifer system at depth. In this regard, the mining activities and deep drilling projects could assist by collecting hydrogeological information during the exploration and operation phases.

[发布机构] University of the Free State