A preliminary investigation of lithogenic and anthropogenic influence over fluoride ion chemistry in the groundwater of the southern coastal city, Tamilnadu, India

A total of 72 groundwater samples were collected from open wells and boreholes during pre- and post-monsoon periods in Tuticorin. Samples were analyzed for physicochemical properties, major cations, and anions in the laboratory using the standard methods given by the American Public Health Association. The fluoride concentration was analyzed in the laboratory using Metrohm 861 advanced compact ion chromatography. The geographic information system-based spatial distribution map of different major elements has been prepared using ArcGIS 9.3. The fluoride concentration ranges between 0.16 mg/l and 4.8 mg/l during pre-monsoon and 0.2-3.2 mg/l during post-monsoon. Alkaline pH, low calcium concentrations, high groundwater temperatures, and semiarid climatic conditions of the study area may cause elevated fluoride concentrations in groundwater, by increasing the solubility of fluoride-bearing formations (fluoride). Linear trend analysis on seasonal and annual basis clearly depicted that fluoride pollution in the study area is increasing significantly. Fluoride concentrations showed positive correlations with those of Na+ and HCO3- and negative correlations with Ca2+ and Mg2+. The alkaline waters were saturated with calcite in spite of the low Ca2+ concentrations. Northwestern parts of the study area are inherently enriched with fluorides threatening several ecosystems. The saturation index indicates that dissolution and precipitation contribute fluoride dissolution along with mixing apart from anthropogenic activities.