Managed aquifer recharge (MAR) site suitability in the Nelson Mandela Bay: the application of multi-criteria decision analysis techniques

The Nelson Mandela Bay (NMB) recently experienced a severe drought where dam levels dropped to a record low of less than 10% of total storage, about 6% of which was inaccessible. This resulted in an increase in the number of boreholes across the area. Unlike surface water, groundwater has a delayed response to droughts where groundwater levels continue to decrease beyond the drought period. Managed Aquifer Recharge (MAR) is proposed as a sustainable and effective solution for groundwater resource management and to build resilience against climate change. This study presents MAR site suitability mapping using four different multi-criteria decision analysis (MCDA) techniques, which is the first step towards MAR implementation in the fractured aquifer of NMB. Soil texture, geology, vadose zone, water quality, slope and hydraulic conductivity are six effective criteria chosen based on their importance for MAR implementation in fractured aquifers. Data from various sources were collected, processed and reclassified according to the Boolean Logic, Fuzzy Logic, Weighted Linear Combination (WLC) and Analytical Hierarchy Process (AHP) to produce suitability maps for MAR site selection. Constraint mapping was applied before the reclassification and processing of the selected criteria using Land Use Land Cover to exclude water bodies, built-up areas, and farms. The final suitability maps show that 5.45% (53.05 km2), 15.71% (152.03 km2) and 13.57% (132.18 km2) were suitable to very suitable according to the Boolean logic, WLC, and AHP methods, respectively. The final overlays for all techniques indicate that suitable sites are located towards the south-eastern region of the NMB. The application of MCDA and integration with GIS proved to be effective at delineating suitable sites for MAR in NMB. Future work should focus on fracture analysis to understand groundwater flow as well as hydrogeological modelling within the complex fractured aquifer systems of the NMB area.