Groundwater recharge processes in the Lake Chad Basin based on isotopic and chemical data

The Lake Chad Basin is Africa's largest endorheic basin. Because water supply for the rural population and most of the urban population depends on groundwater, assessment of groundwater recharge is crucial. Recharge sources for the upper Quaternary aquifer are precipitation, rivers, and swamps. Using water chemistry, and environmental (O-18, H-2, H-3) and carbon (C-14) isotopes, recharge processes can be assessed and groundwater ages roughly estimated. For this purpose, more than 1,000 samples from groundwater, surface water and precipitation were analysed for hydrochemistry and environmental stable isotopes. Furthermore, 3H measurements and C-14 values of dissolved inorganic carbon for groundwater from the northeastern part of the Basin are included in the evaluation. The environmental isotope distribution shows recent recharge from precipitation north of Lake Chad (Kanem Region), where very low 3H values indicate occurrence before the 1960s bomb peak. Focused recharge from fresh river water is typical for Salamat Region in south Chad and the Komadugu Yobe wetlands between Nigeria and Niger. Slightly high d-values in water occur in the Waza Logone area between Chad and Cameroon. Groundwater along the Lake Chad shore and the Bahr el Ghazal corridor show high d-values (d(18)O -0.78 to 7.45%o, d(2)H -13.6 to 30.8%o). Recharge is caused by surface water that undergoes evaporative processes before percolation. Groundwater ages of 600-4,150 years, estimated from C-14 analyses combined with high SO4 concentrations, along the Bahr el Ghazal indicate that recharge was caused by residuals of the Mega Lake before it dried out completely.

Automated summary

The assessment of groundwater recharge in the Lake Chad Basin, Africa's largest endorheic basin, is crucial due to its importance for both rural and urban water supply. By analyzing water chemistry, environmental stable isotopes, and carbon isotopes, the recharge processes and groundwater ages can be estimated. The results show that recent recharge occurs from precipitation in the north of Lake Chad, while focused recharge is observed in the Salamat Region and the Komadugu Yobe wetlands. Surface water undergoing evaporative processes is the main source of recharge along the Lake Chad shore and the Bahr el Ghazal corridor, and the groundwater ages in the latter area indicate residuals from the Mega Lake.