Changes in aquifer properties along a seasonal river channel of the Niger Basin: Identifying groundwater recharge pathways in a dryland environment

Abstract

In drylands of tropical Africa, groundwater plays a fundamental role in alleviating food insecurity and adapting to the effects of climate change. Substantial uncertainty persists in the renewability of groundwater resources in drylands and recharge pathways through the surface geology. Here we characterize the architecture and hydrogeological properties of alluvium and underlying sandstone and crystalline basement rocks along the ephemeral River Goulbi de Maradi in the Iullemmeden Basin of Niger using Magnetic Resonance Soundings (MRS), Time-Domain Electromagnetic (TDEM) soundings, and borehole lithological data. Considerable variations in lithological facies and hydrophysical properties are found along a series of 5 transects perpendicular to the seasonal/ephemeral river channel and adjacent plateaux of the Continental Hamadien (CH) sandstone. The CH aquifer comprises a pebbly sand facies upstream and sandstone clay facies downstream with Farak-type sandstones located at the base of the two facies. Consistent with these variations in facies, the geophysical parameters decrease from 19%, 390 ms, and 800 Ω m upstream to 3%, 160 ms, and 10 Ω m downstream, respectively for effective porosity, relaxation time, and resistivity. The transmissivity and specific yield estimated from the decline of MRS longitudinally also vary from upstream to downstream. The combined use of surface geophysics constrained by lithological borehole logs provides vital insight into groundwater replenishment in this dryland environment.