Hydrogeochemical and isotopic characterization of the groundwater in the Chari-Baguirmi depression, Republic of Chad

The Chari Baguirmi groundwater in the Republic of Chad is undergoing a steady deterioration both qualitative and quantitative due to climatic (severe droughts) and anthropogenic (overexploitation) constraints. However, this major water resource has not been to date the object of any comprehensive scientific investigation. To fill this gap and consider a sustainable exploitation, a hydrodynamic and hydrochemical study was undertaken. This aquifer is located south of Lake Chad and displays a significant natural piezometric depression. Piezometric campaign on an area of 70,000 km(2), bounded by Lake Chad, the Chari River, the mountains of Guera and the dunes of Harr, was conducted to understand the hydrodynamics of the groundwater. Water samples were collected on 124 points scattered over the study area. Hydrochemical and isotopic analyses (O-18 and H-2) were achieved on all samples. The interpretation of hydrochemical data was done using the Piper diagram, the multivariate analysis (hierarchical ascending classification), the index of base exchanges and ratios of Na/Cl, Na/SO4, Cl/SO4, Br/Cl, Sr/Ca. The hydrochemical results coupled with groundwater isotopes data allowed to understand the processes that govern the mineralization and the origin of groundwater salinity. These investigations allowed on the one hand to differentiate between two poles of water mixing and second, to demonstrate that the mineralization process and origin of salinity in the center of depression are both related to the lithology of the geological formations crossed by groundwater flow and climatic events causing evaporation of water from the aquifer.