Hydrological and geochemical evaluation of fluoride-safe groundwater from Rampurhat-II block in fluoride-laden Birbhum district, West Bengal, India: a multivariate statistical approach

It has been reported earlier that Birbhum district is often contaminated with groundwater fluoride and many studies have been conducted to decipher that. Our present study area (Rampurhat-II block) is fluoride-safe (<1.5 mg/L) and located in fluoride-laden Birbhum district. The present investigation mainly focused on the reason behind the occurrence of a low concentration of fluoride (0.24-0.37 mg/L) in groundwater fluoride contaminated Birbhum district. It is very interesting to know that groundwater from the study area contains fluoride in groundwater below the permissible limit (1.5 mg/L) though the aquifer sediments contain fluoride bearing minerals (mainly fluorite). To understand the anomaly, multivariate statistical analysis (Pearson correlation matrix, factor analysis and Agglomerative Hierarchical Clustering) has been performed for the physiochemical data of the collected groundwater samples. Moreover, the mineral phase's equilibrium studies have been done to recognise the actual scenario of water-rock interaction in the aquifers of the present study area. The dissolution of carbonate minerals (mainly calcite) in saturated to super-saturated conditions hinders the dissolution of the fluoride bearing minerals resulting in fluoride-safe groundwater. Again, ion-exchange (between Ca2+/Mg(2+)and Na+/K+) is one of the principal mechanistic pathways in the aquifers which maintain the lower content of fluoride in the groundwater.