Determining the recharge mode of Sahelian aquifers using water isotopes

It is proposed that the drainage network plays an important role in the recharge process of the fractured aquifers in the African Precambrian shield and that the fractured aquifer system is likely to be hydraulically continuous; this contrasts with most previous studies, which suggested direct recharge by rainwater percolation. Two areas were selected in Niger for the study of the aquifer recharge process using isotopic analyses of water (2H, 18O, 3H). The first area, centred on the village of Kobio, is the 21-km2 drainage basin of the Lomona intermittent stream, some 60 km southwest of Niamey (the capital city of Niger). The second area, in the vicinity of Niamey, represents a portion of the Niger basin, draining a surface area many orders of magnitude larger than the Lomona basin. The mean 18O composition of water from all wells in the Kobio aquifer provides evidence for recharge by evaporated water. This is confirmed by the concomitant increase of 18O content with rising static water levels as recharge proceeded. Tritium data suggest progressive aging of the Kobio aquifer water in the flow direction inferred from static water levels, with a down-gradient depletion of 18O composition, suggesting that 'enriched' recharge water is progressively mixed with 'depleted' aquifer water. Recharge by a reach of the Lomona is proposed to explain these results. In the Niamey area wells, the 18O time series clearly define an injection of evaporated water from the surface into the fractured aquifer. This recharging plume of evaporated water most likely originates from the Niger River. Thus, isotope data for two drainage basins of very different sizes indicate that aquifers are recharged by water from the rivers and that the flow regime of surface waters controls the recharge process.