Origin of high fluoride in groundwater of the Tuticorin district, Tamil Nadu, India

This paper reports the results of higher F − and HCO_3^ - concentrations and its response to high pH level in a hard rock terrain in Tamil Nadu, India. About 400 groundwater samples from the study area were collected from a period of four different seasons and analysed for F − , HCO_3^ - and other major cations and anions. The key rationale for the higher fluoride and bicarbonate in the study area is the soaring rate of the leaching fluoride-bearing minerals and weathering processes. Fluoride and HCO_3^ - ranges from BDL to 3.30 mgl −1 and 12 to 940 mgl −1 , its concentrations are lower for the period of SWM and it increases during POM and reaches to a maximum in PRM. Higher dissolution is observed in the NEM season due to rainfall impact. Spatial distribution and factor score show that the higher concentrations of F − and HCO_3^ - are eminent in the northern and central zone of the study area due to the impact of lithology. The higher values in pCO 2 versus HCO_3^ - plot indicate higher residence time which favours more water–rock interactions, which further increase the F − concentrations in groundwater. HCO_3^ - is linearly correlated with F − which indicates that these ions were consequent from the weathering influences. At the same time, poor correlation of F − with pH could possibly be due to the increase of alkalinity follow-on from the swell of bicarbonate level with very low Ca 2+ that promotes increase in F − concentration in the groundwater.