Hydrogeochemical Processes Regulating the Groundwater Quality and Its Suitability for Drinking and Irrigation Purpose in a Changing Climate in Essaouira, Southwestern Morocco

Groundwater is vital for water supply and environmental protection, especially in semi-arid and desert regions. An integrated assessment, focused on the combined use of Water Quality Index (WQI), Irrigation Water Quality Index (IWQI), geochemical and isotopic (δ18O, δ2H and δ3H) tools, was performed in the Cenomanian–Turonian aquifer during the campaigns 2017, 2018, 2019, and 2020. Hydrogeochemical analysis reveals that the groundwater is of mixed Ca–Mg–Cl, Ca–HCO3, Ca–Cl, and Ca–SO4 types, with a dominance of the first type. The analysis of the abundance of the main cations and anions shows the dominance of Ca2+ > Na+ > Mg2+ > K+ for cations and the dominance of Cl− > HCO32− > SO4− > NO3− for anions. The WQI in the Cenomanian–Turonian aquifer was divided into 32.8% (good areas), 44.3% (poor areas), 21.3% (very poor areas) and 1.6% (areas unsuitable for consumption). The IWQI showed that 43% of the studied samples have a high to severe restriction level, while 57% of the studied samples were placed in the low to moderate restrictions for irrigation use. The piezometry of the study area showed that the water flows generally from northeast to northwest and northeast to southwest. Groundwater mineralization in the Cenomanian–Turonian aquifer system is controlled by dissolution, including dissolution of evaporite minerals and reverse cation exchange. Stable isotope signatures (δ18O, δ2H) indicate that the groundwater samples are of meteoric origin without significant evaporation. The most of spring has tritium < 1TU. This could be explained by a reduction in annual precipitation recorded in the study area. The recharge altitude of the aquifer was estimated between 375 to 1275 m, following an altitudinal gradient of 0.26% per 100 m.