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 100mg/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 800mg/L). The different water types had also different stable isotope composition; the thermal water was depleted in ¹⁸O and ²H (δ ¹⁸O 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 δ ¹⁸O and δ ²H 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 ratios (up to 0.713) in the saline groundwater provide additional evidences for the link between salinity and the schist rocks. In contrast, the thermal water had relatively low ⁸⁷Sr/ ⁸⁶Sr ratio (0.7089), which is identical to the secular seawater Sr isotope ratio during the Early Cambrian period and presumably reflects interaction with the Early Cambrian carbonate and sulfate aquifer rocks. In the northern and southern groundwater, the ⁸⁷Sr/ ⁸⁶Sr ratios were higher and correlated with the Mg/Ca ratios, inferring mixing between the Early Cambrian limestone and other rocks with higher ⁸⁷Sr/ ⁸⁶Sr, such as the schist rocks. The radiocarbon data showed ¹⁴C activities ranging from 6 pmC in the thermal water to 62 pmC in the southern water. Age-modeling, corrected for dissolution of the carbonate rocks with dead carbon, simulated mean residence time of 10-15ka BP for the thermal water and a range of 0 to 3ka BP for the northern and southern waters, depending on the used models. The integration of the data enables us to establish a conceptual model for the origin of groundwater in the Lakhssas Plateau: (1) recharge to the aquifer from relatively heavy-isotope depleted recharge water, presumably during wetter conditions about 10-15ka BP. The recharge water interacted at high depth with limestone and calcium sulfate minerals and emerged to the surface as thermal water at the southern part of the basin; (2) more recent recharge from coastal moisture originated from the Atlantic Ocean. The stable-isotope composition of groundwater was controlled by the elevation of their recharge areas: recharge at higher elevation, particularly in the southern margin resulted in lower δ ¹⁸O and δ ²H values; (3) the recharge water interacted with the carbonate aquifer rocks, particularly with calcite and dolomite minerals. In areas of exposure of schist rocks, the water-rock interaction induced salinization of the groundwater. Overall, our data reveal that the limited water resources in this semi-arid zone of Morocco could be in some parts less renewable and also saline. Future exploitation of this basin will have to account the salinity factor and the suggested contribution of water recharged some thousands years ago.