Fluoride occurrence in groundwater in the Northern region of Ghana

Abstract

37The presence of fluoride in groundwater in parts of the Northern region of Ghana at levels above 1.5 mg/ L, the WHO guideline, value has exposed the population in the fluoritic communities to fluoride-related health hazards. Piper graphical classification, principal component analysis (PCA) and thermodynamic calculations were used as an approach to gain insight into the groundwater chemical composition and to help understand the dominant mechanisms influencing the occurrence of high fluoride waters. Inverse distance weighting interpolation (IDW) and spatial join procedure were used to examine the relationship between the underlying geology of the study area and fluoride distribution. The fluoride concentration in 357groundwater samples from the area ranged between 0.0 and 11.6 mg/L, with a mean value of 1.13 mg/L. Six groundwater types were identified for the area: Ca-Mg-HCO3, Ca-MgSO4, Na-Cl, Na-SO4, Na-HCO3 and mixed water type. PCA performed on the groundwater chemical data resulted in 4 principal components (PCs) explaining 72% of the data variance. The PCs represented the predominant processes controlling the groundwater chemistry in the study area which include, mineral dissolution reactions, ion exchange processes and evapotranspiration processes. PHREEQC calculations for saturation indices of the groundwater samples indicated they were largely saturated with respect to calcite and under-saturated with respect to fluorite, suggesting that dissolution of fluorite may be occurring in the areas where it is present. A review of the PCA results and an evaluation of the equilibrium state of the groundwater based on the saturation indices suggest that the processes controlling the overall groundwater chemistry in the area also influenced the fluoride enrichment. This predominant processes include the dissolution of the mineral fluorite, anion exchange processes (F / OH-) involving clay minerals and evapotranspiration processes. Elevated fluoride levels in the study area were found to occur predominantly in the Saboba and Cheriponi districts and also in the Yendi, Nanumba North and South districts. These areas are underlain by the Obossom and Oti beds, comprising mainly of sandstone, limestone, conglomerate, shale, arkose and mudstone. Results of the conducted hydrochemical analysis show that aside the boreholes with elevated concentrations of fluoride (beyond 1.5 mg/ L), groundwater in the study area based on the parameters analysed is chemically acceptable and suitable for domestic use.