Groundwater recharge processes in the Lake Chad Basin based on isotopic and chemical data
Sara Vassolo, Maike Gröschke, Paul Koeniger, Christoph Neukum, Laila Seehausen, Lilli Witt, Mélanie Ronnelngar, D. Djoret
Abstract
Abstract 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 ( 18 O, 2 H, 3 H) and carbon ( 14 C) 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, 3 H measurements and 14 C 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 3 H 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 δ-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 δ-values (δ 18 O –0.78 to 7.45‰, δ 2 H –13.6 to 30.8‰). Recharge is caused by surface water that undergoes evaporative processes before percolation. Groundwater ages of 600–4,150 years, estimated from 14 C analyses combined with high SO 4 concentrations, along the Bahr el Ghazal indicate that recharge was caused by residuals of the Mega Lake before it dried out completely.