Groundwater resource directed measures in karst terrain with emphasis on aquifer characterisation in the cradle of humankind near Krugersdorp, South Africa

Abstract

Karst terrain present a highly heterogeneous aquifer system created and organised by groundwater flow which make them very different from other aquifers. Consequently karst aquifers are highly vulnerable to contamination due to the rapid transport of pollutants in conduit systems. The karst of the Cradle of Humankind World Heritage Site, play a major role in assimilation or carrying of acid mine drainage, sewage effluent return flow and agricultural fertilisers, emphasising the need for a sustainable balance between utilisation and protection. Groundwater Resource Directed Measures (GRDM) recognises the need for groundwater to be included in the integrated management of water resources and is regarded as a strategy and approach to implement the National Water Act (Act 36 of 1998). However, the application of Resource Directed Measures in highly utilised karst terrain remains a challenging task. The level of confidence of GRDM assessments in karst depends on our ability to adequately characterise the dual- recharge, flow and storage processes. In this context, karst terrains do present some common characteristics in their groundwater flow conditions, which allow us to use common exploration and exploitation methods to study its structure and functioning. Despite the World Heritage status and ongoing exploitation, the karst of the Cradle of Humankind was not yet systematically investigated nor is it understood. This study has been undertaken to determine the current hydrogeological status of the karst aquifer and to investigate methodologies for GRDM assessments in karst terrain. As infiltration of contaminated water has altered the chemical composition of the natural waters of the karst system it was important to determine pollution indicators and to assess whether the pollution could be attributed to distinct anthropogenic sources. Cluster analysis, in conjunction with conventional graphical displays and distribution maps provide a consistent and reliable method for delineating sources that influence groundwater chemistry. It has been proven from the hydrograph response graphs and isotope investigation that the karst aquifer represents both slow diffuse and rapid concentrated allogenic and autogenic recharge mechanisms. The groundwater flow has been influenced by dykes creating compartments isolated hydrogeologically from each other, suggesting a highly complex karst system. Based on the conceptual findings it is obvious that GRDM assessments in karst aquifers need a much more comprehensive understanding of the underlying groundwater system compared to homogenous aquifers. Reserve determination in karst is further influenced by the uncertainties concerning inflows (sinking streams), outflows (abstraction) and baseflow where springs comprise the source of numerous streams. The investigation revealed the potential shortcomings inherent from the lack of site-specific data and recommended a better understanding of the interaction between ground- and surface water while identifying the most vulnerable areas to exploitation. In return this will greatly improve the significance of the GRDM assessments and account for better decision making of the development of karst groundwater resources in the Cradle of Humankind and other karst terrain.