Contribution of Hydrochemical and Isotopic Tracers to the Investigation of Water Resources in Ouham Watershed (Lake Chad Sub-basin)

The Central African Republic (CAR) occupies a strategic position in the Lake Chad basin since most of the water feeding the various tributaries of the Chari River, the principal water source for Lake Chad, comes from its territory. Indeed, the north of CAR, and in particular the Ouham prefecture, at the head of the entire endorheic watershed of Chad, benefits from favorable rainfall conditions. Sampling campaigns and in situ measurements were conducted for chemical and isotopic elements. The results show that the waters are relatively acidic, with pH ranges of 5.53 and 6.56 and low mineralization. The chemical type of the waters is bicarbonate, calcareous, and magnesian. The geochemical process is controlled by two mechanisms (i) 62% water/rock exchange and (ii) 38% precipitation water input. The values of log pCO2 (0.06 atm or log pCO2 = − 1.5 atm) indicate that it is CO2 contribution under open-system conditions to an unlimited gas reservoir, which is the soil atmosphere. The average contents are for oxygen-18 of − 3.83 ± 0.2‰ and deuterium of − 19.36 ± 0.9‰, with a deuterium excess of 10.99‰. The water table recharge is recent. It is done directly by precipitation infiltration without significant evaporation. This recent contribution of water is confirmed by SiO2 and HCO3− values, which are the essential components of the soluble load. Alkalis and alkaline piles of the earth (16% of the total weight), and finally, the low contents in anions (Cl−, SO42−), translate well to the exclusively crystalline character. The quality and the potential of groundwater as a water supply resource are confirmed, bearing in mind the local rainfall (1500 mm/year). Plus, indications of the possibility for the development of the exploitation are provided.