A comprehensive overview of groundwater salinization and recharge processes in a semi-arid coastal aquifer (Essaouira, Morocco)

Study region: The downstream part of the Essaouira basin, Morocco, Northwestern Africa. Study focus: This study combines multivariate statistical analyses, stable isotopes, hydrogeochemical, hydrogeological, geological and remotely sensed data to gain a better understanding of the hydrological and groundwater salinization processes in a coastal aquifer. Hydrogeochemical data from 223 samples as well as stable isotope data collected during 10 sampling campaigns between 1990 and 2020 were compiled and subjected to a comprehensive analysis. Groundwater recharge obtained from the stable isotope approach was checked using a water balance method based on remote sensing data of actual evapotranspiration. New hydrological insights for the region: Seawater intrusion was detected only for the period of 2009–2020, nearby the ocean, whereas hyper-salinity due to evaporite dissolution occurred throughout the entire period (1990–2020) primarily in the southern part of the aquifer. Additionally, the isotopic mass balance model shows that aquifer recharge from the Ksob river area is mainly located in the northeastern region. Furthermore, based on average values of stable isotopes of rainfall and groundwater, preliminary results show that only months of November, December and January with rainfall higher than 54 mm contribute to groundwater recharge. The water balance approach confirmed these results, but also revealed a lower recharge in February, with a monthly average precipitation of 46 mm. The outcomes of this research enabled the construction of a comprehensive conceptual 3D model of the Essaouira coastal aquifer. The insights from this research are important to guide measures for sustainable water resources management.