Controls on groundwater quality and dug-well asphyxiation hazard in Dakoro area of Niger

In the Dakoro area of Niger, hand-dug wells are commonly used for village water supply. Several cases of asphyxiation during well digging have occurred, but the source and composition of the gas (suspected to be CO₂, H₂S, or CH₄) remain uncertain. In addition, data on water quality in the region are sparse. We sampled ten dug wells and five deeper boreholes for analyses of solute chemistry and core samples from one borehole for sediment chemistry. With respect to inorganic solutes, groundwater quality was generally good, although one dug-well sample exceeded the drinking-water standard for NO₃^-. Solute chemistry tended to vary between dug wells (commonly Ca-HCO₃ or Ca-SO₄ waters, with elevated Si and circumneutral pH) and boreholes (typically Na-HCO₃ waters, with lower Si and alkaline pH). These compositions variously appear to reflect dissolution of calcite, gypsum, and silicates; pyrite oxidation; and cation exchange. Some dug wells and boreholes had high acetate concentrations, low SO₄-S relative to total S, and/or a rotten-egg odor, all of which are consistent with reduction of SO₄^2- to H₂S. This reaction is likely in Marine Series shales that are relatively high in S and organic C. Consequently, monitoring for H₂S and other gases during well digging is warranted.