Hydrogeochemical processes and contaminants enrichment with special emphasis on fluoride in groundwater of Birbhum district, West Bengal, India

The hydrogeochemistry of groundwater in rural parts of Birbhum district, West Bengal, India, has been studied to understand the contaminants and prime processes involved in their enrichment with a focus on F- concentration. The lithological units consist of Quaternary alluviums with underlying Rajmahal basaltic rocks of Middle Jurassic age. Groundwater occurs in the alluviums, weathered residuum and fracture zone of Rajmahal rocks. Studies show elevated concentration of Cl-, SiO2, Fe and F-; excess Cl- is attributed to anthropogenic inputs, SiO2 is ascribed to high degree of weathering of silica rich host rocks, and high Fe is due to the interaction of water with Fe-rich sediments under reducing condition. The F- concentration is found high (>1.20 mg/L) mainly in water from Rajmahal rocks revealing a lithological control on F- enrichment. The weathering of silicates and ion exchange are the leading controlling processes for major ions in groundwater. The F- enrichment is due to the dissolution of F--bearing minerals and perhaps also through anion exchange (OH- for F-) on clay minerals at high alkaline conditions; precipitation of CaCO3 favours CaF2 dissolution leading to elevated F- concentration. CaHCO3, the dominant water type, contains low F- while NaHCO3 and NaCl types exhibit high F- concentrations. Among the three spatial associations, Cluster-1 and Cluster-2 are CaHCO3 type; Cluster-3 shows NaHCO3 and NaCl waters with low Ca2+ and Mg2+ and high Na+ contents. Cluster-1 and Cluster-2 waters are, in general, drinkable barring the elevated Fe content, while Cluster-3 water is unsafe for drinking due to the high F- concentration.