Physico-chemical quality and origin of groundwater of an aquifer under semi-arid climate. Case of the Barremo-Aptian aquifer of Essaouira basin (Morocco Occidental)

The study area is part of the semi-arid areas of Morocco. It is located about thirty kilometers south of the Essaouira city. It extends between the Tidziwadi to the north and the Amsittene anticline to the south with a surface of 250 km². According to the hydrogeological investigations, the waters of this region is circulating in the formations of Barremian and Aptian age. These are formed of alternating marls, Marno-calcareous to traces of gypsum, limestone fractured fossil and sandstone. Given the importance of this aquifer in the populations supply in drinking water, the study of the water quality is therefore necessary. Of this fact, 26 water points capturing the Barremo-Aptian aquifer were sampled in June 2015 and 3 other in October 2007 for chemical analyzes and isotope. The results of the approach hydrochemical conditions have helped to distinguish between two facies types: Ca-Mg type-Cl representing the downstream part of the aquifer and the type Ca-Mg-HCO_3 governing the upstream part. 50% of the waters from the Barremo-Aptian aquifer displayed an average quality, the rest shows a quality poor to very poor. The bad and very bad quality generally result from the presence of nitrates with concentrations that greatly exceed the standards of potability. The isotopic study shows that the Barremo-Aptien aquifer recharge is ensured by the direct infiltration, without evaporation, of precipitation of oceanic origin. This recharge is performed at an altitude of 600 m asl, to the East of the study area, where are flush with the formations of the Barremian and Aptian. The Barremo-Aptien aquifer presents old waters indicating a low rate of renewal.