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

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, therefore, 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 near-surface geophysics (TDEM: Time Domain Electromagnetics, MRS: Magnetic Resonance Sounding). TDEM and MRS experiments, conducted at 21 sites along 3 transects, show the thickness of the saturated Quaternary alluvium varies from 7 to 19 m with estimated effective porosities ranging from 18 to 38% and a hydraulic conductivity of 0.6–3 × 10 –3 m/s. Dense piezometric surveys along drainage channel reveal hydraulic gradients of 0.2–0.3‰ that generate Darcy fluxes of 1000–2000 m 3 /day (dry season, i.e. minimum value). Paleochannel discharge, which currently provides baseflow to the River Niger, is the focus of local demand to increase access to water for drinking, livestock watering, and supplementary irrigation.