Groundwater Characterization Using Hydrochemical and Isotopic Techniques in the Upper Lake Chad Basin in the North and North-East of the Central African Republic

Water resources in the upper Lake Chad basin in the north and north-east of the Central African Republic are poorly monitored, and their hydrochemical characteristics are poorly understood. The present study was carried out with the aim of characterizing the water sampled and gaining a better understanding of the origin of groundwater mineralization, using a combination of hydrochemical and isotopic methods. The results show that the water presents bicarbonate-calcium, bicarbonate-calcium-magnesium, bicarbonate-sodium and bicarbonate-potassium facies, with an evolution towards chloride and nitrate types, reflecting superficial inputs and a probably very short residence time in a relatively humid region of the basin. Ionic strength values ranged from 3.10-4 to 132.8.10-4 atm, with a mean value of 62.8.10-4 ± 60.910-4 atm, which is overall higher than the atmospheric value (3.16.10-4 atm). These results suggest that the system is open to soil CO2. Water stable isotope analyses revealed that tritium contents varied from 0 to 3.9 TU in the wet season and from 0 to 4.7 TU in the dry season. The joint analysis of hydroclimatic, hydrochemical and isotopic approaches enabled us to establish a diagnosis of the vulnerability of aquifers in the Lake Chad basin. Thus, recourse to non-conventional resources, such as the use of purified wastewater for agriculture, must be considered a priority in order to avoid a situation of severe water shortage.