Hydrochemistry with special reference to fluoride contamination in groundwater of the Bongo district, Upper East Region, Ghana

Abstract

This study was conducted to understand the hydrochemistry of groundwater in the Bongo district and unravel the source of fluoride in the groundwater using an integrated hydrochemical analysis and multivariate geostatistical analysis. A total of thirty (30) borehole water samples were collected in various communities in the district during the dry season. The district is dominated by Upper Birimian (Paleoproterozoic) metavolcanics and granitoids known as the Bongo granitoids. Hydrochemical facies identified in the area include Ca–Na–HCO₃ (70%) and Ca–Mg–HCO₃ (30%) water types. The Ca–Na–HCO₃ water types are hosted in fractured bedrocks of the Upper Birimian metavolcanics and the K-feldspar rich Bongo granitoids, whereas the Ca–Mg–HCO₃ water types are within the Upper Birimian volcanic/metavolcanic sequences. All the hydrochemical parameters show acceptable concentrations for drinking purposes except fluoride (1.71–4.0 mg/L). The high fluoride concentrations in the groundwater are largely due to intense dissolution of the Bongo granitoids, which contain biotite and muscovite as the dominant fluoride-bearing minerals. The pH, Ca²⁺, SO₄²⁻, HCO₃⁻ and CO₃²⁻ concentrations have weak positive correlations with F⁻ concentrations of the groundwater implying some dependent relationship and different source for the fluoride. Principal component analysis performed on the hydrochemical data resulted in three principal components (PCs), which explain 76.251% of the total variance. The three PCs represent the dominant processes influencing the groundwater chemistry, which include water–rock interaction, mineral dissolution, and ion exchange reactions, respectively, with water–rock interaction as the most dominant process. However, anthropogenic sources such as the use of phosphate fertilizers cannot be precluded from contributing to the groundwater fluoride contamination.