The role of local perched aquifers in regional groundwater recharge in semi-arid environments: evidence from the Cuvelai-Etosha Basin, Namibia

The influence of local perched aquifers on groundwater recharge in semi-arid areas at a regional scale remains inadequately studied, especially in southern Africa where data scarcity limits the understanding of recharge and groundwater flow processes. The objective of this study was to understand the interaction between localized perched aquifers and an underlying regional aquifer in the Cuvelai-Etosha Basin in Namibia. The connection between the perched and regional aquifers was assessed based on an extensive dataset of hydrochemical and isotope (delta O-18 and delta H-2) measurements for groundwater from both shallow and deep hand-dug wells as well as boreholes. Ephemeral rivers and pans/depression landforms were analyzed separately. Recharge was calculated using the chloride mass balance method. It was found that groundwater originates from precipitation that undergoes strong evapotranspiration and water-rock interaction, especially carbonate dissolution. Overlapping chemical and isotopic compositions suggest a common or similar origin of groundwater in the perched and regional aquifers in the pan/depression hydrotope. No such similarity was established for the ephemeral river hydrotope, suggesting that ephemeral rivers contribute negligibly to regional groundwater recharge. This has important implications for groundwater management and helps to understand recharge processes in similar semi-arid environments.