Geochemical and geostatistical appraisal of fluoride contamination: An insight into the Quaternary aquifer

Abstract

Fluoride (F−) in groundwater poses a severe public health threat in the Dwarka River Basin (DRB) of West Bengal, India, where many cases of fluorosis have been reported. This research evaluates the spatial distribution patterns of major cations and anions, delineates zones of high F− concentrations within alluvial sediments of the DRB, and identifies both the sources and the geochemical processes responsible for the release of F− to groundwater. A total of 607 groundwater samples were collected from shallow and deep tube wells located within the DRB, encompassing an area of 435 km2 and including 211 villages. Fluoride levels range from 0.01 to 10.6 mg/L, and high concentrations (>1.5 mg/L) are restricted to isolated areas within the basin (occurring within nine of the villages and comprising 4.3% of the samples collected). The high-fluoride areas are characterized by mostly Na-HCO3 type groundwater, where the abundance of cations and anions are Na+ > Ca2+ > Mg2+ > K+ and HCO3– > Cl− > SO42− > F− > NO3− > Br−, respectively. Analyses of the groundwater geochemistry and sediment mineralogy suggest that fluoride is released to groundwater primarily through the hydrolysis of albite and biotite; however, the resulting alkaline conditions are also favorable for release of fluoride from weathered biotite and clay minerals through anion exchange (OH– in groundwater replacing F− within the mineral structure). Multiple linear regression models show that fluoride concentrations can be predicted from the measures of other dissolved constituents with a high degree of accuracy (R2 = 0.96 for high fluoride samples and R2 = 0.8 for low fluoride samples).