Impacts of hydrogeochemical processes and anthropogenic activities on groundwater quality in the Upper Precambrian sedimentary aquifer of northwestern Burkina Faso

This study investigates the hydrogeochemical and anthropogenic factors that control groundwater quality in an Upper Precambrian sedimentary aquifer in the northwestern Burkina Faso. The raw data and statistical and geochemical modeling results were used to identify the sources of major ions in dug well, private borewell and tap water samples. Tap waters were classified as Ca-HCO3 and Ca-Mg-HCO3 types, reflecting the weathering of the local dolomitic limestones and silicate minerals. Dug well waters, with a direct contact with various sources of contamination, were classified as Ca-Na-K-HCO3 type. Two factors that explain 94% of the total variance suggested that water-rock interaction was the most important factor controlling the groundwater chemistry. Factor 1 had high loadings on pH, Ca2+, Mg2+, HCO3 (-), SO4 (2-) and TDS. These variables were also strongly correlated indicating their common geogenic sources. Based on the HCO3 (-)/(HCO3 (-) + SO4 (2-)) ratios (0.8-0.99), carbonic acid weathering appeared to control Ca2+, Mg2+, HCO3 (-) and SO4 (2-) acquisition in the groundwater. With relatively lower Ca2+ and Mg2+ concentrations, the majority of dug well and borewell waters were soft to moderately hard, whereas tap waters were considered very hard. Thus, the dug well and, to a lesser extent, borewell waters are likely to have a low buffering capacity. Factor 2 had high loadings on Na+, NO3 (-) and Cl-. The strong correlation between Na+ and NO3 (-) and Cl- implied that factor 2 represented the anthropogenic contribution to the groundwater chemistry. In contrast, K+ had moderate loadings on factors 1 and 2, consistent with its geogenic and anthropogenic sources. The study demonstrated that waters from dug wells and borewells were bacteriologically unsafe for human consumption, and their low buffering capacity may favor mobility of potentially toxic heavy metals in the aquifer. Not only very hard tap waters have aesthetic inconvenient, but their consumption may also pose health problems.