Groundwater recharge, salinization, and mixing mechanisms between the shallow and fossil aquifers of the Atlantic Sahara, Morocco

The multi-aquifer system of the Dakhla region, in the hyper-arid Atlantic Sahara, was investigated to understand the primary recharge mechanisms and identify the sources contributing to the groundwater salinization. Groundwater samples were collected and analyzed for their chemical and stable isotopes (δ2H and δ18O) composition. The results show that the groundwater quality ranges from brackish (up to 4700 µS/cm) to saline (15,810 µS/cm). Ionic ratios and saturation indices indicate that evaporite dissolution is the primary process driving groundwater mineralization, whereas the Cl−/Br− molar ratio demonstrates that saline intrusion is negligible. Water isotopes reveal two distinct groundwater types: paleo-recharged water in the deeper aquifers and modern meteoric water in the shallow aquifers. Overall, salinization is mainly governed by water–rock interactions under thermal effect, coupled with ion exchange and rock weathering across all aquifers. Additionally, groundwater in the Plio-Quaternary aquifer shows signs of evaporation before infiltration. These processes collectively degrade the groundwater quality, rendering it unsuitable for domestic and agricultural uses. This study presents the first hydrogeochemical model in the region, establishing a scientific foundation for sustainable groundwater management and protection against overexploitation and pollution in the Dakhla coastal area.