Geochemical and isotopic (oxygen, hydrogen, carbon, strontium) constraints for the origin, salinity, and residence time of groundwater from a carbonate aquifer in the Western Anti-Atlas Mountains, Morocco

Groundwater in many arid basins, particularly in developing countries, is the only available water resource that sustains local communities. Yet, information on the basic hydrological parameters and the sustainability of the groundwater exploitation are often lacking. This study investigates the origin of groundwater from the Lower Cambrian carbonate aquifer of the Lakhssas Plateau in the Anti-Atlas Mountains of southwestern Morocco. The study aims to reveal the origin of the groundwater, salinity sources, and the residence time of the water. The study is based on a comprehensive geochemical and isotopic (oxygen, hydrogen, carbon, and strontium) investigation of groundwater from different parts of the basin. The hydrochemical and isotopes results indicated three types of groundwater in the Lakhssas Plateau: (1) thermal water in the southern part of the basin with solute composition that reflects dissolution of calcium–sulfate and calcium carbonate minerals; (2) low-temperature groundwater at the southern margin of the basin with low salinity (chloride content up to 100 mg/L) and chemical composition that is expected from equilibrium with limestone–dolomite rocks; and (3) low-temperature groundwater in the northern, western, and eastern margins of the basin with a wide range of salinity (chloride up to 800 mg/L). The different water types had also different stable isotope composition; the thermal water was depleted in 18O and 2H (δ18O as low as −7.6‰) relative to the southern (−5.9 to −5.3‰) and northern waters (−5.7 to −3.8‰). The differences in δ18O and δ2H between the southern and northern waters are related to elevation that induced fractionation of oxygen and hydrogen isotopes in recharge water originated from coastal moisture. The data suggest that the high salinity in groundwater from the northern, western and eastern margins of the Lakhssas Plateau is related to the presence of schist rocks in these areas. The distinctive low Na/Cl and Br/Cl ratios, coupled with high silica contents and high 87Sr/86Sr rat