Factors influencing the high fluoride concentration in groundwater of Vellore District, South India

Most of the arid and semi-arid zones of the Indian subcontinent experience serious health problems due to high concentration of fluoride in drinking water. The Vellore District of Tamil Nadu suffers from high concentration of fluoride in water. However, most of the past studies in this region focused on tannery-related pollution and not on fluoride contamination. The present study attempts to identify the factors influencing the origin and spatial distribution of fluoride in the district. From the observed hydrochemical results of 68 well samples in the context of water level, well depth and hydrochemical parameters, F⁻ concentration showed increasing trend in the presence of Na⁺ and HCO₃ ⁻. This is due to the alkaline nature of groundwater that favors the dissolution of F⁻-rich minerals. The occurrence of high fluoride in Na–HCO₃ type of water confirmed this hypothesis. However, Ca²⁺ showed an insignificant correlation with F⁻. The high Na/Ca ratio (>1) in 73 % of the samples and the result of Na/Na+Cl plot suggest the occurrence of cation exchange in the study area. The major source of F⁻ was identified as products of the weathering and the dissolution of fluorites, amphiboles and micas present in the geological formations in the study area. The positive relationship between NO₃ ⁻ and F⁻ in few wells located in agricultural fields suggest possible source of F⁻ from the application of fertilizers. More than 25 % of the samples had higher values of fluoride than the permissible limit of drinking water according to Indian standards. Spatial distribution of fluoride showed a higher concentration in the southwest part of the study area, namely, Thirupathur and Vaniyambadi. This study shows that contamination was high in certain parts of Vellore District and the quality of water must be maintained by resorting to appropriate treatment and management strategies.