Seawater intrusion into coastal aquifers from semi-arid environments, Case of the alluvial aquifer of Essaouira basin (Morocco)

Marine intrusion phenomenon is the main phenomenon threatening the groundwater quality in coastal aquifers around the world. This phenomenon is generally caused by the overexploitation of aquifers, decline in the piezometric level and the rise in sea level under the climate change effect. There are several approaches to study and assess the marine intrusion phenomenon. For this study, the crossing of piezometric, hydrochemical and isotopic approaches was adopted to highlight the state of this phenomenon within the Plio-quaternary aquifer of the Essaouira basin. The couples (Na, Cl), (Ca, Mg), (Br, Cl), (δ2H, δ18O), (δ18O, Cl) were determined for 24 samples capturing the shallow aquifer of the Essaouira basin. The piezometric approach shows that negative piezometric levels are registered. The ionic ratios Br/Cl close to 1.5 and 1.7‰, Na/Cl close to 0.86, Mg/Ca and SO4/Cl weak showed that the seawater begins to invade the freshwater of the Plio-quaternary aquifer of Essaouira basin. This intrusion demonstrated by ionic ratios and corroborated by isotopic approach and the combined use of oxygen-18 contents and chlorides has a mixing rate of 15.9% at the well 11/51, 14.5% at the sample 45/51, 13.2% at the well 149/51, 13.3% at point O114 and 12.8% at the well O94. However, the results of the hydrogeochemical and isotopic approach suggest intrusion up to 2 km from the sea; this reflects a warning sign about the groundwater deterioration of the study area.