SEEPAGE RELATIONSHIPS BETWEEN LAKE CHAD AND THE CHAD AQUIFERS

Lake Chad, a closed basin]lake, is the only large surface-water body i n the African Sah el. Future water resource development in the Chad Ba sin requires an understanding of the hydrogeology of the basin. In thi s paper we examine the hydraulic relationship between the lake and the phreatic aquifer of the Chad Formation. Electrical resistivity data f rom 30 surveys were combined with over 60 open well measurements to co nstruct a water-table map of the southwestern quadrant of the lake. Th e map indicates that the Lake Chad water level is at a higher hydrauli c level than the phreatic aquifer and that the ground-water flow is so uthwest, away from the lake. The average measured seepage rate into th e phreatic aquifer in the southwestern and southern parts of the lake was 7.1 X 10(-3) m/d (median = 1.3 X 10(-3) m/d) which is about 21% of the annual water input to the lake. This measured seepage flux (9.96 X 10(9) m(3)/yr) can account for about 107% of the annual solute input to the lake. Solute transport model simulations indicate about 32% (1 5.3 X 10(9) m(3)/yr) of the input water and 152% (2.87 X 10(12) g/yr) of the total solute input can be removed by ground-water recharge (see page) from the lake. These results provide an explanation to account f or the freshness of Lake Chad's water despite its surficial closed bas in geologic setting: solutes delivered to the lake via surface-water i nflow leave the lake via ground-water recharge. These results also ind icate recharge from the lake (similar to 10(10) m(3)/yr) represents an enormous amount of water available as a ground-water resource in the African Sahel.