Hydrogeological dynamics of a large alluvial aquifer in the dryland Iullemmeden Basin of Niger

In drylands, groundwater is often the only perennial source of freshwater to sustain domestic water supplies, irrigation and aquatic ecosystems. This study examines the pathways and dynamics of groundwater discharge and recharge in the Dallol Maouri, a fossil tributary of the Niger River. Groundwater levels rose steadily over the last half century (1969 to 2020) by 20 to 30 cm per decade. Recharge derives from seasonal rainfall and groundwater flow from regional aquifer systems. Stable isotope ratios of O and H together with anthropogenic gases (CFC-12, SF6) indicate a meteoric source of groundwater that mostly has residence times exceeding 50 to 70 years. Recent high-frequency monitoring of groundwater levels reveals not only the seasonality and a first-pass estimate of the magnitude of groundwater recharge in the valley (160 to 340 mm/year) but also the dynamics of discharge to the Niger River, smallholder irrigation and direct evapotranspiration.