Contrasting Salinity Patterns and Spatiotemporal Groundwater Dynamics in Complex Endorheic Aquifer Systems: Insights from Chemical and Isotopic Tracers

Understanding geochemical dynamics and salinity patterns in aquifer systems of endorheic basins is crucial for water resource management in arid and semi-arid climates. These environments, often characterized by intense agriculture and limited water availability, face significant challenges due to water scarcity and elevated groundwater salinity. This study investigates the geochemical processes shaping salinity patterns in interconnected shallow and deep aquifers within a structurally complex endorheic basin. A comprehensive dataset of groundwater samples from 213 wells across two aquifer systems in Bahira, central Morocco, was analysed for major ions and stable isotopes. Known for agriculture, Bahira faces notable issues of water scarcity and high groundwater salinity. The results highlight contrasting salinity levels, with the shallow aquifer exhibiting extreme salinity (EC up to 60,000 µS/cm) due to enhanced evaporation and soil leaching, whereas the deep aquifer maintains relatively lower EC values (500 to 3,000 μS/cm). Spatial analysis reveals a west-to-east salinity gradient driven by recharge variability and hydrogeological connectivity. Geochemical data underline the critical role of water-rock interactions, gypsum dissolution, and ion exchange in controlling salinity. Stable isotope analyses corroborate these findings, demonstrating evaporative enrichment and distinguishing between local recharge sources for the shallow aquifer and regional contributions from high-altitude precipitation in the deep aquifer. These insights enhance understanding of the hydrogeochemical dynamics in endorheic basins, emphasizing the interplay of climatic, geological, and anthropogenic factors in shaping groundwater quality. The findings offer broader implications for sustainable water management in similar arid environments worldwide.