Caractérisation hydrodynamique, hydrochimique et isotopique du système aquifère de Kourimat (Bassin d'Essaouira, Maroc)

Abstract

The synclinal basin of Kourimat, subdivided in two sub-basins (Igounzar in the North, and Zeltene in the South), is situated in the oriental part of Essaouira basin shelters a carbonate aquifer system contained in the limestones and dolomitic limestones of the Cenomanian and the Turonian. These ground waters represent the only drinking water and irrigation resource for the region. The geological formations outcropping in the studied area, start with the clay and marly formations of the terminal Jurassic in the south of Zelten watershed, and end with the sandy and loamy Quaternary formations in coastal areas in the Low Qsob watershed. The Middle and Upper Cretaceous (marl, fractured limestone, and dolomites) represent the rest of the stratigraphic series especially in the areas of the Igrounzar watershed (up to 85% of the outcrops) and in Zelten (up to 50%). The field occupation is not very significant. It basically consists of three types: 1) cultures (essentially cereals) where it is possible (on the marly soils of the Cretaceous, the Quaternary and the Eocene); 2) uncovered areas corresponding to the fissured and karstic limestone outcrops of the Cretaceous and the Jurassic formations, and 3) forests (coniferous trees) in the downstream part of the catchments area (Plio-quatemary soils and mountainous reinforcements of the Western High Atlas). Like for other undeveloped areas of Morocco, the Mekala basin is also subjected to an intensive deforestation (wood for heating and cooking). The study undertaken on the Cenomano-turonian aquifer has allowed us to characterize it from the hydrodynamical, hydrogeological and hydrogeochemical point of view. The piezometric map of the two regrouped aquifer levels shows water flow senses from the SE to the NW. The hydraulic gradient varies from upstream to downstream, being weak n the central zone relative to the best hydrodynamic characteristics. The time evolution of the piezometry shows annual and seasonal fluctuations in connection with the variations of precipitations. Hydrodynamic characteristics have been calculated by means of the interpretation of data of pumping tests, performed in the region by the General Hydraulic Administration of Morocco. On the hydrochemical point of view, results of chemical analyses have allowed us to determine the chemical facies of waters of the two aquifers, their interaction and their quality for the human consumption and their use in agriculture. To better understand the functioning modalities of the Mekala-Kourimat aquifer system, an isotopic study has been performed that showed the variations relative to the altitude of the recharge zones of the different channels. The altitudinal gradient in 18O, estimated to 0,25%» every 100 m confirms the validity of this result at a regional scale. The oxygène-18/deutéhum relationship (δ2H = 7,74*(δ18O) + 10,83) indicates an Atlantic system of the precipitation without evaporation. This suggests a quick infiltration, also confirmed by the importance of the fracturation developed in limestones and dolomitic limestones of the Cenomano-turonian aquifer of Meskala-Kouhmat.