Groundwater quality and its implications for domestic and agricultural water supplies in a semi-arid river basin of Niger

In the River Goulbi Maradi Basin (RGMB), groundwater is a vital source of drinking water and plays a central role in the region's socio-economic development. The quality and suitability of groundwater for irrigation and drinking-water remain inadequately understood. We examine hydrochemical analyses of 35 groundwater samples from the shallow alluvial (17) and underlying Continental Hamadien (CH) sandstone (18) aquifers and evaluate these against standard measures of their suitability for drinking water (World Health Organization (WHO) guideline values) and irrigation (i.e., sodium adsorption ratio, sodium percentage, and the residual sodium carbonate). Hydrochemical facies are principally of Na-HCO3 and Na-Cl types. Bivariate plots combined with saturation indices and electrical conductivity monitoring suggest that the main hydrogeochemical processes influencing groundwater quality are cation exchange in the CH aquifer and solute leaching from soils during focused recharge in the alluvial aquifer. 76% (13/17) of groundwater samples from the alluvial aquifer were suitable for irrigation compared to 38% (6/16) of the samples from the CH. The identification of high fluoride concentrations exceeding the WHO drinking-water guideline value (> 1.5 mg/L) in 33% (6/18) of samples from the CH aquifer and 18% (3/17) in the alluvial aquifer, and their respective attribution to the release of fluoride of geogenic origin through cation exchange and local use of fluorapatite fertilisers, provide valuable insight into efforts to address the on-going challenge of fluorosis in the Maradi region of Niger and more widely across African drylands. The health consequences of the widespread observation of Mn in concentrations exceeding the new WHO guideline value (0.08 mg/L) in the alluvial aquifer (6/9 samples), often alongside elevated Fe concentrations, are unclear.