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Controls on Groundwater Chemistry in Rural Southwest Niger, West Africa
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In a region characterized by frequent drought and rapid population growth, an understanding of the processes causing spatial and temporal variations in groundwater chemistry is crucial to sustain potable groundwater supplies in rural southwest Niger, West Africa. Seventy-one groundwater samples were collected at 14 locations during a 6-month dry period in three rural communities and were analyzed for major ion and trace element concentrations and physical parameter values. The objectives were to 1) identify and characterize the natural and anthropogenic factors controlling spatial and temporal variations in groundwater chemistry and 2) determine whether the extent to which these factors affect groundwater chemistry increases or decreases over time. Graphical analyses, such as Piper, Durov, and Stiff diagrams categorized the water chemistry of the study area as Ca-Mg-HCO3 and a Gibbs plot indicated that the main source of dissolved solids is weathering of silicate minerals in both fresh and weathered granitoids. A principal component analysis (PCA) was used to determine that the degree of aquifer weathering and fracturing and the presence or absence of a laterite layer explain 52 % of the data variances. A hierarchical cluster analysis (HCA) demonstrated that samples with similar groundwater chemistry cluster by spatial proximity, degree of aquifer weathering and fracturing, presence or absence of laterite, or land cover. A Regional Kendall Test (RKT) indicated that the impact of the dissolution of Ca and Mg-rich silicate minerals on groundwater chemistry increased significantly (p < 0.05) throughout the study area. In areas with laterite, the influence of laterite weathering on groundwater chemistry reduced significantly. Nitrate and Cl concentrations were partitioned by land cover and subjected to an analysis of variance (ANOVA), revealing that concentrations of both variables are greatest in areas of high-population densities. These analyses have led to a better understanding of the mechanisms controlling groundwater chemistry in a weathered-granitoid aquifer in rural southwest Niger.