Integrated approach to groundwater quality assessment in the Plio-Quaternary aquifer of Essaouira Basin, Western Morocco

The objective of the present study was to determine the geochemical processes that govern the groundwater mineralization within the Plio-Quaternary aquifer of Essaouira Basin in Western Morocco, and to assess the groundwater quality in this aquifer. 61 groundwater samples collected in June 1995 and June 2015 were analyzed for their physicochemical properties, and 7 samples collected in October 2007 and 20 samples collected in May 2018 were analyzed for stable and radioactive isotopes. The resulting hydrogeochemical data indicate that the dominant facies type is Na-Cl. Based on ionic ratios, saturation indices, and the Gibbs diagram, the geochemical phenomena responsible for groundwater mineralization in the region of interest are dissolution, evaporation, and cation exchange triggered by seawater intrusion. The contribution of dissolved halite, gypsum and/or anhydrite, calcite, and dolomite to the groundwater mineralization is corroborated by the positive correlations of those minerals with (Na+ + Cl-), (Ca2+ + SO42-), (Ca2+ + HCO3-), and (Ca2+ + Mg2+), respectively. Contamination with nitrate (contents > 50 mg/L) was observed in the majority of the water samples, which can be explained by tourist activity, livestock activity, and domestic utilization of the water. Upon comparing the groundwater quality with World Health Organization standards, more than half of the water samples were found to exceed the desired threshold for consumption fixed by WHO. Based on the Wilcox and USSL diagrams, the groundwater was generally found to be suitable for plants, even through the groundwater was highly saline and had high levels of Na. According the isotope analysis, the aquifer recharge is supported by the infiltration of oceanic precipitation (evaporation is negligible), and the groundwater is of modern age. This study provides the data needed to effectively manage and protect groundwater resources in the area studied.