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Hydrodynamic, hydrochemical and isotopic characterization of the waters of the quaternary aquifer of Bahr-El-Ghazal, north central Chad.

Located in the middle of the Sahel, the study area is an integral part of the Bahr-El-Ghazal region. It is crossed by a fossil valley, which has a geological history similar to that of rivers that disappeared under the influence of climate change where evaporation phenomena were intense. From the hydrogeological point of view, the free water table of this region is housed in sedimentary geological formations of different natures. It passes from the sand tablecloth to that of the fluviolacustrine clays. In some cases, the latter confer particularly bad chemical qualities to the water, even though groundwater is the main source of drinking water for the local population. For this reason, a study aimed at understanding the different mechanisms that govern the chemical functioning of the aquifer of the Bahar-El-Ghazal region has been carried out. It is with this in mind that a piezometric study is being carried out in the first instance. This reveals the presence of some hydrodynamic anomalies. Thus, a shallow depth of the body of water is observed in the Harr and in the sandy formations of the extreme West. However, underground flows oriented from these two zones towards the central part characterize a deepening of the water level. Subsequently, a hydro-chemical study was undertaken. As regards the chemical facies, they also show an evolutionary character. They pass from calcium and magnesian bicarbonate facies to chloride or sodium and potassium sulphate facies in the piezometric depression. The values of the isotopic composition of the studied groundwater vary from -4.86 to 4.04‰ and -35.60 to 14.50‰ and delta 2H respectively.