Multi-isotopic approach to identify groundwater flow paths, relationships with rivers and sources of contaminants in the Regional Quaternary Aquifer of the Lake Chad Basin.

The regional Quaternary aquifer of the Lake Chad Basin represents a major resource crossing several countries in the Sahel region. It is intensively used for many purposes, including drinking water supply, domestic uses, breeding and irrigation. However, this resource already undergone the climate change effects and the strong pressure of human activities. For instance, some authors shown that in Cameroon, the NO₃^- contents have increased by a factor of six between 1993 and 2013. It is therefore urgent to propose pragmatic water management strategies in this region, which will be based on a total understanding of the aquifer functioning. To achieve this objective, major ions chemistry and isotopic tools were combined. It is the first time that a such multi-proxy study is conducted in the region. Chemical analyses revealed that the geogenic influence is marked by dissolution, hydrolysis of silicate minerals, and possible cation exchanges. The nitrates content up to 1500 mg/l confirm the high impact of anthropogenic activities on this resource. Water stable isotopes show the contribution of surface water in addition to actual rainwater, in the recharge of the aquifer while ³H and ¹⁴C highlight that the Quaternary aquifer gathers both recent and old groundwater (~1500 years). Above all, these radioactive tracers helped to identify the different flow paths: a shallower one (under an active recharge from river and rainwater), a deep one (with the lowest concentrations in ³H and ¹⁴C, more protected from surface pollutants and dominated by geogenic processes) and an intermediate level attesting a mixing between the deep and shallower levels. Finally, thanks to δ¹⁵N-NO₃ and δ¹⁸O-NO₃ three main sources of nitrates were identified: (i) deposits of nitrogenous organic matter (animal excrement, manure, household waste); (ii) naturally synthesized nitrate in the soil; and (iii) nitrate from nutrients of the paleo-sea. A conceptual model of the regional Quaternary aquifer can then now be proposed in 02 points: (i) the existence of distinct flow paths with the deep one which is strategic especially for drinking purpose and (ii) the dependence of the aquifer to perennial rivers.