Combining quantity and quality controls to determine groundwater vulnerability to depletion and deterioration throughout South Africa

Groundwater vulnerability is normally associated with the potential for contamination, for example the DRASTIC approach, but the increasing importance of groundwater in human, agricultural, and industrial systems, implies that groundwater vulnerability is dependent on a range of other parameters and particularly parameters impacted by climate change. A robust method for evaluating groundwater vulnerability therefore requires the incorporation of multiple indices into one holistic evaluation. In this study, a model for groundwater vulnerability in South Africa was developed, calculating the combined weighted averages of nine spatial datasets; namely: (1) mean annual precipitation, (2) mean annual surface temperature, (3) tritium distribution in groundwater, (4) potential evaporation, (5) aquifer type and yield, (6) terrain slope, (7) electrical conductivity, (8) population density and (9) cultivated land use. The model was run through 36 different weighting scenarios and indicates that on average 22.4% of South Africa's groundwater is predicted to have a very high vulnerability to depletion in quantity and deterioration in quality (12.2% very low; 35.4% low; 16.8% moderate and 13.1% high). A single weighting scenario (M-I/C-2/P-2) was chosen that showed good alignment to the previous assessments that predicted that 20.9% of groundwater resources in South Africa have very high vulnerability (1.5% very low; 35.7% low; 24.9% moderate and 17.0% high). The model was adjusted to reflect climate conditions 50 years into the future, resulting in a 3% increase in very high vulnerability areas and a 6% decrease in low vulnerability areas. The results suggest that large areas of South Africa have high groundwater vulnerability, especially in areas where groundwater supplements domestic supply and agriculture. The implementation of this method to evaluate groundwater vulnerability across diverse natural environments and political boundaries will supplement comprehensive groundwater vulnerability assessments and the development of effective groundwater management policy.