Shallow Quaternary groundwater in the Lake Chad basin is resilient to climate change but requires sustainable management strategy: Results of isotopic investigation.

Within the Lake Chad Basin, the unconfined Quaternary aquifer offers permanent and easy access to water resources. This transboundary regional aquifer is shared by Chad, Niger, Nigeria and Cameroon and extends over ~500,000 km. Climatic conditions and repeated droughts as well as the intensification of agriculture in the region have multiple negative impacts on the aquifer such as changes in groundwater level and its quality. Being a strategic water resource for the whole Chadian region, the groundwater potential of the Quaternary aquifer must be better characterized and understood to evaluate its resilience to climate change and anthropogenic impact. Stable isotopes and tritium of the water molecule were used to estimate water origin and residence time at the regional scale and to elucidate the interconnections between the different hydrological and hydrogeological components. Results show active recharge processes to the Quaternary aquifer as well as dynamic connections with surface waters (both river courses and wetlands) but also indicate less dynamic behavior of the Quaternary groundwater resource in some areas of the region. Based on the isotopic investigations, the Quaternary aquifer in the Chad basin was found to be resilient to climate change but its hydrogeological specificities (dependence to surface water from the upstream basins and transboundary nature of its structure) can make it prone to inadequate management strategies.