Department of Earth Sciences

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Browsing Department of Earth Sciences by Subject "Aquifers -- South Africa -- Cape Flats"

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    Developing a cost-effective method to characterise regional aquifer vulnerability in the Cape Flats
    (Stellenbosch : Stellenbosch University, 2024-03) Hunink, Brynn Matthew; Chow, Reynold; Woehling, Thomas; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Aquifer vulnerability assessments are important tools in avoiding aquifer deterioration as they indicate how likely an aquifer is to be adversely affected by land-based, anthropogenic activities.” The aim of this study is to develop a simple, cost-effective method to characterise aquifer vulnerability by combining and modifying existing methods. These methods are then applied in the same study area to validate the cost-effective method. The Cape Flats Aquifer (CFA), located in the Western Cape, South Africa, is used as a case study due to concerns over the deterioration of groundwater quality to the well-utilised aquifer during the past decade. Assessment of aquifer vulnerability in the CFA is achieved through a combination of DRASTIC-L aquifer vulnerability mapping and wellhead protection area (WPA) delineation. DRASTIC-L is used to identify areas that are most vulnerable to contamination. Steady-state wellhead protection areas are delineated using finite difference numerical modelling (MODFLOW) and reverse particle tracking (MODPATH) to determine the capture zones for major abstraction wells. However, the complex, expensive, and time-consuming nature of numerical modelling highlights the need for simplified, cost-effective methods that can be applied across South Africa. Inherent limitations and redundancies with the DRASTIC-L method that were identified by previous studies and within this study resulted in the development of the modified DRASTL method. Additionally, a new method for cost-effective capture zone delineation using a combination of GIS and analytical equations is evaluated. The results of DRASTL and the newly developed method for wellhead protection area delineation are combined to cost-effectively characterise aquifer vulnerability in the CFA, which is referred to as the DRASTL-WPA method. Results from DRASTIC-L indicate that the CFA is very vulnerable. The DRASTL method shows a similar spatial trend to DRASTIC-L; however, DRASTL is associated with a higher average vulnerability but a lower tendency for very low or very high vulnerabilities. It can be seen that a very similar output can be produced with fewer input parameters; however, validation is required to evaluate the accuracy of the methods for the study area. The cost-effective WPA delineation method yielded an intersection of greater than 70% with the results from MODPATH given that an abstraction well is ~8 km away from a bedrock outcrop, and ~12 km away from the combined effects of bedrock outcrops and river boundaries. This is likely due to the inability of GIS software to simulate complex hydrogeological boundary conditions, as well as limiting assumptions of homogeneity and isotropy. Additionally, one can expect an average 3.7 times overprediction of the capture zone size. Several capture zones in the CFA correspond with the highest aquifer vulnerability highlighted by the results of DRASTIC-L and DRASTL. Thus, it is suggested that land use practices are monitored or restricted within these capture zones. Additionally, the capture zones delineated by the numerical model or DRASTL-WPA can be used to define areas where in-situ remediation or beneficial management practices should be implemented. Moreover, the numerical model can help with groundwater management planning in the CFA to ensure the sustainable use of groundwater resources.