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

This paper reports the results of higher F− and \( {\text{HCO}}_{3}^{ - } \) 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−, \( {\text{HCO}}_{3}^{ - } \) 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 \( {\text{HCO}}_{3}^{ - } \) 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 \( {\text{HCO}}_{3}^{ - } \) are eminent in the northern and central zone of the study area due to the impact of lithology. The higher values in pCO2 versus \( {\text{HCO}}_{3}^{ - } \) plot indicate higher residence time which favours more water–rock interactions, which further increase the F− concentrations in groundwater. \( {\text{HCO}}_{3}^{ - } \) 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 Ca2+ that promotes increase in F−concentration in the groundwater.