Where is PFAS risk to groundwater highest in South Africa?

Groundwater is a vital resource in water-scarce South Africa, but faces significant anthropogenic pollution threats. Per- and polyfluoroalkyl substances (PFAS), widely used chemicals with high environmental persistence, have been detected in South African surface and groundwater, often exceeding drinking water limits. Our study assessed groundwater vulnerability to PFAS pollution across South Africa using a modified DRASTIC method, termed DRASTIL. The DRASTIC method, which considers hydrogeological factors like Depth to groundwater (D), Recharge (R), Aquifer media (A), Soil type (S), Topography (T), and Impact on vadose zone (I), was adapted by removing a redundant parameter (hydraulic Conductivity, C) and adding Land use (L). Land use was incorporated due to its critical role in providing spatial context for expected PFAS pollution sources. The methodology involved a weighted overlay of these seven parameters within a Geographic Information System (GIS), utilizing existing hydrogeological data and land use data linked to potential PFAS sources. Calibration was performed using existing PFAS measurements in surface water and boreholes. Results indicate that the Western Cape, particularly Cape Town, is highly vulnerable due to a combination of urban areas, agriculture, high recharge, shallow groundwater, and permeable geological features. Similar high vulnerability was observed in parts of Free State, North West, and Gauteng, influenced by mining, agriculture, urban areas, and karst aquifer systems. Overall, over 36 % of South Africa's area is classified as moderately to extremely high-risk for PFAS groundwater pollution. Gauteng, despite being the smallest province, has over 25 % of the country's population and over 60 % of its area in the high-risk category. The Western Cape and North West also show high proportions of vulnerable areas (50.4 % and 49.6 %, respectively). These findings highlight the urgent need for targeted management strategies and monitoring of PFAS concentrations in identified high-risk areas to safeguard South Africa's crucial groundwater resources.