Seepage Relationships Between Lake Chad and the Chad Aquifers

Abstract

AbstractLake Chad, a closed basin lake, is the only large surface‐water body in the African Sahel. Future water resource development in the Chad Basin requires an understanding of the hydrogeology of the basin. In this paper we examine the hydraulic relationship between the lake and the phreatic aquifer of the Chad Formation. Electrical resistivity data from 30 surveys were combined with over 60 open well measurements to construct a water‐table map of the southwestern quadrant of the lake. The map indicates that the Lake Chad water level is at a higher hydraulic level than the phreatic aquifer and that the ground‐water flow is southwest, away from the lake. The average measured seepage rate into the phreatic aquifer in the southwestern and southern parts of the lake was 7.1 × 10−3 m/d (median = 1.3 × 10−3 m/d) which is about 21% of the annual water input to the lake. This measured seepage flux (9.96 × 109 m3/yr) can account for about 107% of the annual solute input to the lake. Solute transport model simulations indicate about 32% (15.3 × 109 m3/yr) of the input water and 152% (2.87 × 1012 g/yr) of the total solute input can be removed by ground‐water recharge (seepage) from the lake. These results provide an explanation to account for the freshness of Lake Chad's water despite its surficial closed basin geologic setting: solutes delivered to the lake via surface‐water inflow leave the lake via ground‐water recharge. These results also indicate recharge from the lake (∼ 1010 m3/yr) represents an enormous amount of water available as a ground‐water resource in the African Sahel.