Fluoride-contaminated groundwater of Birbhum district, West Bengal, India: Interpretation of drinking and irrigation suitability and major geochemical processes using principal component analysis.

The present research work is confined to a rural tract located in the north-western part of Birbhum district, West Bengal, India. Chemical analysis of the groundwater shows the cations is in the order of Na+ > Ca2+ > Mg2+ while for anions it is HCO3─ > Cl─ > SO42─ > NO3─. The F─ concentration was found to vary from 0.01 to 18 mg/L in the pre-monsoon and 0.023 to 19 mg/L in post-monsoon period. 86% of samples show low F─ content (<0.60 mg/L) whereas, 8% exhibit elevated concentration of F─ (>1.2 mg/L) mainly in the central and north-central parts of the study area at a depth of 46 to 98 m. The prime water type is CaHCO3 succeeded by F─-rich NaHCO3 and NaCl waters. The suitability analysis reveals that the water at about 81% of the sampling sites is unsuitable for drinking and at 16% of sites unsuitable for irrigation. The alkaline nature of the water and/or elevated concentration of Fe, Mn and F─ make the water unsuitable for potable purposes while the high F─ and Na+ contents delimit the groundwater for irrigation uses. Multivariate statistical analysis suggests that chemical weathering along with ion exchange is the key process, responsible for mobilization of fluoride in groundwater of the study area.