Geochemical controls on fluoride concentrations in groundwater from alluvial aquifers of the Birbhum district, West Bengal, India

•Fluoride concentration occurs up to 13 mg/l in shallow alluvial aquifer.•Ion-exchange is the dominant in hydrogeochemical process.•Intercalated zeolitic clay within aquifer sand is the main source of F−.•Sorption and de-sorption mechanism of clay leads to mobilization of F−.

Groundwaters with high fluoride (F−) concentration up to 13.61 mg/L occur in some parts of the Quaternary alluvial aquifer of Birbhum district, West Bengal, India. The high F− groundwater zones are mainly located in the discharge areas of Dwarka river basin, especially in places where groundwater occurs at a depth of 24-30 m within alluvial sediments, consisting of micaceous sand, silt and clay. Soil depth samples were collected from the 3 bore holes drilled up to about 30 m depth for geochemical analysis. Total F− in the sedimentary material from the drilled holes measured about 400 to 450 mg/kg at deeper depth in CS1 and CS3 but no abnormal values found in CS2. XRD, XRF and total F− analyses of sediments from aquifer zone reveal that, the presence of intercalated zeolitic clay within the aquifer sand is the major source of F− in groundwater. Furthermore, geochemical investigations and laboratory analysis emphasize that sorption and de-sorption of intercalated zeolitic clay at different pH levels along with ion-exchange are the major mobilising factors for F− in pre- and post-monsoon seasons. Most of the groundwater having fast recharge component either from flowing canals or rainfall, contains relatively depleted δ18O content (− 5 to − 4‰). However, δ18O content has no correlation either with Cl− or F− that shows more than one contaminant sources.