Major ion signatures for identification of geochemical reactions responsible for release of fluoride from geogenic sources to groundwater and associated risk in Vaniyar River basin, Dharmapuri district, Tamil Nadu, India

Fluoride is a common constituent in most of the rocks and the release of fluoride into groundwater is the major problem in several parts of the world. The present study was carried out with the objective of identifying sources and mechanism of release of fluoride from the rocks and its impact on human health in Vaniyar river basin, Dharmapuri district, Tamil Nadu. Groundwater samples were collected from 44 wells once in 2 months from June 2011 to August 2014 and analyzed for major cations and anions using an ion chromatograph. The possible reasons for the increased concentration of fluoride in groundwater of the area were investigated by taking into account of the agricultural practises and the major rock types in the area. The observed order of the dominance of cations in groundwater in the study area is Na+ > Ca2+ > Mg2+ > K+ and that of anions is Cl− > HCO3− > SO42− > CO3− > F−. The fluoride concentration in groundwater of this region ranges from 0.15 to 6.14 mg/l. Out of 569 samples analysed, fluoride concentration exceeded the maximum permissible limit of the drinking water standards of BIS 1.5 mg/l in 396 samples. Many people living in this area are suffering from dental and skeletal fluorosis due to prolonged use of fluoride-rich groundwater for drinking. Concentration of fluoride in groundwater is relatively low in wells located in charnockite region, whereas the concentration of fluoride in groundwater is high in the deep wells in regions comprising of epidote hornblende biotite gneiss. The major source of fluoride in groundwater in the area is due to leaching of fluoride-bearing minerals. Fluoride ions from biotite and hornblende minerals leach into the groundwater and contribute high fluoride concentration in this region. The concentration of fluoride in groundwater increases when the water level is deep indicating higher release of fluoride from greater depth. The geochemical reactions that control the release of fluoride from fluoride-bearing minerals such as biotite and hornblende into groundwater were identified. Removal of high concentration of fluoride from drinking water is necessary in this region and measures should be taken to supply water after removal of fluoride for the betterment of the livelihood in the area.