Dali Mondal, Srimanta Gupta and Anindita Mahato
Present research work undertakes fluoride (F-) hydrogeochemistry in the unconfined aquifer of Purulia (Block II) integrated with geospatial approach. Based on the geospatial analysis, it has been observed that high F- in groundwater mainly occurs in the formation having a litho-assemblage of saprolitic zone between fractured granite gneiss and weathered mantle. Groundwater with high F- concentration is mainly associated with Ca-Cl, Mixed Ca- Mg-Cl indicating dominance of evaporation effect. Positive correlation with pH, HCO3 - and Na+ indicates dissolution of fluoride bearing mineral such as fluorite is the main responsible mineral for F- contamination. PHREEQCI geochemical modeling also indicates that F- in groundwater can be increased as a result of precipitation of CaCO3, CaMg(CO3)2 and CaSO4, 2H2O at high pH, which removes Ca2+ from solution allowing more fluorite to dissolve. Since the increase in Na+ concentration increases the solubility of F- bearing minerals, the geochemical processes leading to increase in Na+ and decrease in Ca2+ concentrations play an important role in Fenrichment in groundwater. Apart from fluorite dissolution cation exchange (Na+ for Ca2+) accompanied with anion exchange (OH- for F-) may also be the important processes by which muscovite and biotite minerals (containing fluorine at the OH- sites) may contribute to F- enrichment in groundwater
