Cost-effective aquifer protection: Verifying the combination of vulnerability mapping with wellhead protection area delineation methods in the Cape Flats, South Africa

Aquifer vulnerability assessments are essential for identifying areas at risk of groundwater contamination and for guiding land use management to prevent aquifer degradation. In this study, we propose a cost-effective approach to characterize groundwater vulnerability by combining and simplifying existing methods. Our focus is the Cape Flats Aquifer (CFA) in the Western Cape, South Africa, where groundwater quality has declined over the past decade due to urbanization and agricultural activities. We assessed vulnerability using the DRASTIC-L method and delineated wellhead protection areas (WPAs) through a steady-state MODFLOW model with reverse particle tracking (MODPATH). Given the subjectivity of DRASTIC-L and the resource intensity of numerical modelling, we developed a simplified alternative by removing redundant parameters (DRASTL) and integrating it with an analytical WPA delineation method. The resulting hybrid method, DRASTL-WPA, offers a faster and more accessible approach to vulnerability mapping. Our findings show high vulnerability across the CFA, with lower values in higher elevation zones. The DRASTL method yields more conservative results, though with less precision in identifying hotspots. The analytical WPA method aligned closely with MODPATH outputs when wells were more than 12 km from complex boundaries, indicating its suitability in simpler hydrogeologic settings. Overall, the combined method provides a practical framework for groundwater protection in data- or resource-limited regions, though refinement is needed for complex terrains.