Characterization of Groundwater Physico-chemical Parameters in Lake Chad Polders: The Case of the Bol-bagassola Polder

Located southwest of Lake Chad (Bol-Bagassola), the study area is characterized by a Sahelian-type climate underlain by more recent sedimentary cover. The aim of this manuscript is to assess the overall impact of agricultural activities on groundwater quality in the polders (Bol and Bagassola), in order to gain a better understanding of agricultural pollution. To achieve this objective, a water sampling campaign was carried out to study hydrodynamics and hydrochemistry. As a result of this study, piezometric levels are not sufficiently heterogeneous. The pH is around neutral with an average of 7.68, and water temperatures vary between 25.3°C and 32.5°C. Conductivities ranged from 334µS/cm to 9800µS/cm, exceeding the maximum WHO/Chad standards limit of 2500µS/cm. Most of the variables (chemical parameters) are below the drinking water quality limits defined by WHO/Chad standards, with the exception of the open wells (P1, P2) at Malkoura, which show very high concentrations of potassium, sulfate, sodium, chloride, ammonium and magnesium, and the high iron signature observed in boreholes F9 and F12 at Talia. These variables (chemical parameters) enabled us to define the bicarbonate calcic and magnesian facies, followed by chloride sodium and potassium or sulfate sodium for the single F9 borehole at Malkoura. The predominance of bicarbonate facies can be explained by the dissolution of evaporites in the reservoir.