Groundwater recharge from Lake Chad

The open waters of Lake Chad are reasonably free of salt (∼120–320 mg liter−1) even though it is a shallow, old, closed-basin lake located in the highly evaporative environment of sub-Saharan Africa. It is possible that the lake remains relatively fresh due to significant removal of water and solutes by seepage into the groundwater beneath the lake. Directly measured seepage rates (seepage meters and core tracer methods) through the sandy lake bottom are 1.15 × 10−1 m d−1. The regional water table derived in this study indicates the lake to be at a higher hydraulic elevation than the phreatic aquifer, and measurements of the groundwater flow direction determined from minipiezometric nests confirm that flow is SW, away from the lake. Preliminary calculations indicate that seepage plays a significant role in water and solute budgets, since it removes as much as 18% of the annual water input and 40–160% of the annual salt input to the lake.