Paleochannel groundwater discharge to the River Niger in the Iullemmeden Basin estimated by sub- surface geophysics and piezometry mapping

Abstract In drylands groundwater is often the only perennial source of freshwater to sustain domestic water supplies and irrigation. Knowledge of the pathways and dynamics of groundwater discharge and recharge is essential to inform sustainable and rational management of limited water resources. The lower valley of the Dallol Maouri in Niger represents a large fossil tributary (i.e. paleochannel) of the River Niger and drains groundwater regionally from the Iullemmeden Basin through coarse-grained Quaternary sediments. The objective of this paper is to quantify groundwater discharge within this paleochannel using piezometry and surface geophysics (TDEM : Time Domain Electromagnetic, MRS : Magnetic Resonance Sounding). TDEM and MRS experiments were conducted at 21 sites along 3 transects show respectively the mean thickness of Quaternary alluvium varies from 11 m to 18 m with the effective porosities ranging from 20% to 33% and hydraulic conductivity of 2 × 10-3 m/s. Dense piezometric surveys along drainage channel reveal hydraulic gradients of 0.2 ‰ to 0.3 ‰ that indicate Darcy fluxes of 1,000 to 2,000 m3/day (dry season, i.e. minimum value). Paleochannel discharge, which currently provides baseflow to the River Niger is the focus local demand to increase access to water for drinking, livestock watering, and irrigation.