Aqueous geochemistry of fluoride enriched groundwater in arid part of Western India

Fluoride-enriched water has become a major public health issue in India. The present study tries to evaluate the geochemical mechanism of fluoride enrichment in groundwater of western India. Total 100 groundwater samples were collected for the study spreading across the entire study area. The results of the analyzed parameters formed the attribute database for geographical information system (GIS) analysis and final output maps. A preliminary field survey was conducted and fluoride testing was done using Hach make field kits. The fluoride concentration ranges from 0.08 to 6.6 mg/L (mean 2.4 mg/L), with 63 % of the samples containing fluoride concentrations that exceed the World Health Organization (WHO) drinking water guideline value of 1.5 mg/L and 85 % samples exceeding the Bureau of Indian Standards (BIS) guidelines of 1 mg/L. The study also reveals high concentration of nitrate that is found to be above WHO standrads. The dominant geochemical facies present in water are Na-Cl-HCO₃(26 samples), Na-Ca-Cl-HCO₃(20 samples), Na-Cl (14 samples), and Na-Ca-Mg-Cl-HCO₃(11 samples); however, sodium and bicarbonate being the major component in all the water types of 100 samples, which in fact has a tendency to increase fluoride concentration in water by dissolving fluoride from fluorite. The thermodynamic considerations between the activities of calcium, fluoride, and bicarbonate suggest that fluoride concentration is being governed by activity of calcium ion. X-ray diffraction analysis of sediments reveals calcite and fluorite are the main solubility-control minerals controlling the aqueous geochemistry of high fluoride groundwater. The results indicate that the fluoride concentration in groundwater is mainly governed by geochemical composition of rocks, such as metamorphic granites and sedimentary rocks, alkaline hydrogeological environment, climatic conditions, high temperature and lesser rainfall, and geochemical processes such as weathering, evaporation, dissolution, and ion exchange.