ESTIMATING THE SPATIAL VARIABILITY OF GROUNDWATER RECHARGE IN THE SAHEL USING CHLORIDE

Thirteen interstitial water chloride profiles have been obtained from the unsaturated zone of the coastal Quaternary aquifer in Senegal over depth intervals ranging between 7.0 and 35.5 m; three of these profiles reached the water table. A regional study of 119 dug wells was also carried out over an area of some 1600 km2 for the measurement of chloride, as well as other parameters, in samples taken from the water table. These two sets of data have been used, together with rainfall input chemistry collected over a 3 year interval to derive regional estimates of recharge for this area typical of the Sahel margin (mean annual rainfall 1970-1990 around 280 mm). Recharge estimates range from 13000 to 1100 m3 kM-2 year-1. It is concluded that the conjunctive use of interstitial water profiles and regional shallow groundwater chemistry provide an inexpensive technique for recharge estimation which can be widely applied in sedimentary terrains.