This study investigates the Batha endorheic basin in Chad, situated east of the Lake Chad basin in the arid to semi-arid Sahelian zone. This region has not yet undergone comprehensive geological and hydrogeological studies. More broadly, the transition zone between semi-arid and arid climates has been minimally explored. This research aims to evaluate the resources and dynamics of this multi-layered system using a combined geology-hydrogeology-hydrochemistry-isotopes approach. The multilayer system includes sedimentary layers (Quaternary, Pliocene, and Eocene) over a crystalline basement. A piezometric investigation of the system shows a general SE–NW groundwater, indicating an interconnection between all layers. Hydrochemical analyses identifies four main facies (calcium-bicarbonate, sodium-bicarbonate, sulphate-sodium, and mixed), primarily controlled by water–rock interaction with secondary influences from base-exchange and evaporation. Saturation indices indicate that these waters are close to equilibrium with the calcite-Mg phases, gaylussite and gypsum. Stable isotopes (oxygen-18 and deuterium) categorize groundwater into three groups: ancient water, recent and older meteoric water mixtures affected by evaporation, and mixtures more heavily impacted by evaporation. Tritium contents reveal three groups: current rainwater, modern water, and sub-modern water. These results indicate that ionic and isotopic differentiations cannot be strictly linked to specific layers, confirming the interconnected nature of the Batha system. The observed heterogeneity is mainly influenced by lithological and climatic variations. This study, though still limited, enhances significantly the understanding of the basin’s functioning and supports the rational exploitation of its vital resources for the Batha area’s development. Future investigations to complete the present study are highlighted.

Groundwater resources in semi-arid crystalline bedrock regions are essential, yet challenging to locate due to pronounced subsurface heterogeneity. In the Abéché Basin (Chad), traditional prospecting methods achieve low drilling success rates (~ 40%). This study demonstrates the effectiveness of an integrated 2D and 3D Electrical Resistivity Tomography (ERT) approach for improving groundwater exploration in this urban Sahelian setting. We acquired 20 two-dimensional (2D) profiles and seven three-dimensional (3D) survey blocks across ten neighborhoods. Validation against eight boreholes confirmed a success rate of 75% (yield > 1 m³/h), significantly outperforming traditional methods. A significant negative correlation was established between bulk resistivity and yield. ERT identified low-resistivity anomalies (< 200 Ω·m) associated with fractured zones proximal to temporary watercourses, delineating preferential recharge pathways. This work provides a validated, replicable framework that positions 3D ERT as a robust tool for sustainable groundwater mapping in complex crystalline aquifers, directly supporting water security goals in Sahelian urban areas.