Groundwater nitrate pollution in the Essaouira Basin (Morocco)

The Essaouira Basin (Morocco) contains a multi-layered aquifer situated in fractured and karstic materials from the Middle and Upper Cretaceous (the Cenomanian, Turonian and Senonian). Water percolates through the limestone and dolomite formations of the Turonian stage either through the marls and calcareous marls of the Cenomanian or through the calcareous marly materials of the Senonian. The aquifer system may be interconnected since the marl layer separating the Turonian, Cenomanian and Senonian aquifers is thin or intensively fractured. In that case, the water is transported through a network of fractures and stratification joints. This paper describes the extent of the nitrate pollution in the area and its origin. Most of the wells and drillholes located in the Kourimat perimeter are contaminated by nitrates with some concentrations over 400 mg l −1 . Nitrate contamination is also observed in the surface water of the Qsob River, which constitutes the natural outlet of the multi-layered complex aquifer system. In this area, agriculture is more developed than in the rest of the Essaouira Basin. Diffuse pollution of the karstic groundwater body by agricultural fertiliser residues may therefore partially explain the observed nitrate pollution. However, point pollution around the wells, springs and drillholes from human wastewater, livestock faeces and the mineralisation of organic debris close to the Muslim cemeteries cannot be excluded. Le bassin d'Essaouira (Maroc) renferme un système aquifère multicouche contenu dans le Crétacé moyen et supérieur (Cénomanien, Turonien et Sénonien). Les circulations d'eau entre étages adjacents sont possibles à travers des niveaux marneux peu épais et intensément fracturés. La présence des nitrates dans des proportions extrêmes reste problématique ( 400 mg l −1 parfois). Les origines classiques des nitrates (industrie, agriculture, etc.) ne peuvent pas expliquer, à elles seules, une telle abondance. Une description de l'extension spatiale de la contamination par les nitrates ainsi qu'une tentative d'explication des origines de ces sels sont présentées dans cet article. Une comparaison avec les eaux de surface est également ébauchée.