Geochemical and isotopic investigation of the aquifer system under semi-arid climate: case of Essaouira basin (Southwestern Morocco)

Detailed hydrochemical and stable isotopic data of groundwater were used to understand the geochemical processes occurring in the Wadi Ouazzi basin in Southwestern Morocco. Hydrogeochemical investigations show that the evaporite dissolution (halite and gypsum), followed by cation-exchange reactions with reservoir matrix and by sea spray, constitute the main processes that control groundwater mineralization. Data inferred from 18-oxygen and deuterium isotopes in groundwater samples indicated recharge with modern rainfall. Water characterized by lower O-18 and H-2 values is interpreted as recharged by non-evaporated rainfall originating from Atlantic Ocean. Tritium contents, ranging between 0.18 and 3.43 TU, indicate that groundwater in both aquifers derive from post- and pre-nuclear recharge. The study of the carbon isotopes made it possible to estimate the water residence time of the two aquifers studied. Moreover, the calculated ages range from the modern to the late Pleistocene with the dominance of the modern age waters.