A methodology to quantify the risks of urbanisation on groundwater systems in South Africa
Van Rooyen, Johanna Margaretha
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Each year, the urbanised population grows exponentially and due to this growth, cities are forced to expand beyond their manageable borders resulting in greater pressure on the surrounding urban environment. Many South African towns or cities are dependent on surface water for water supply. These resources are slowly being depleted and the dependence on groundwater resources is becoming increasingly important. Due to increased mining, industrial and agricultural activities in South Africa the surface water and groundwater environments have become vulnerable to contamination. This study aimed to develop a methodology in which the risks of urbanisation can be quantified. The conceptualisation and qualitative site impact and risk assessments focused on any environmental changes. The urban environment was sub-divided into three distinct areas and analysed separately in order to detect possible groundwater impacts of the water flowing through the urban area. Upstream river flow gauge chemistry by way of tri-linear grouping (piper diagram) of the major anions and cations showed already impacted water due to mining activities north of Potchefstroom. Uranium concentrations in the downstream surface water showed negligible impact as the concentrations remained within the relevant standards over time. Total coliform bacteria concentrations were found to be well above the acceptable levels and these high concentrations are an indicator of water purification inefficiency. Different qualitative risk assessment approaches i.e. the Environmental Risk Assessment method, Fuzzy Logic and the Depth to water, recharge, Aquifer media, Soil media, Topography, Impact of the vadose zone media and Conductivity of the aquifer (DRASTIC) approach risk assessments were compared and used to determine the most effective way to determine the most likely risks associated with urbanisation. Different modelling tools namely, analytical element modelling method (AEM) using the Visual AEM program, a finite difference numerical modelling method using Processing Modflow v.8 (PMWin) were evaluated, however due to insufficient data, an analytical approach had to be developed. This approach incorporated logical steps and associated processes to serve as a guide for future urban hydrogeological investigations. A case study (namely Potchefstroom) was used to test the developed methodology. The developed methodology provides a step by step approach to urban risk assessment, even in areas where there is insufficient data.