Controlling factors of fluoride in groundwater in a part of South India

Abstract

Fluoride (F⁻) is the most important element for human health. Hydrogeochemical survey was conducted to probe into the controlling factors of F⁻ in the groundwater of a semi-arid part of South India. The study area comprises of F⁻-bearing minerals (hornblende, biotite, and apatite) in the Precambrian rocks, sandstones in the Upper Gondwana rocks, and clays in the Quaternary formations. Forty-seven percent of the total groundwater samples have the higher F⁻ content than its permissible limit of 1.50 mg/L prescribed for drinking purpose. The chemical data of the groundwater is analyzed to assess the geochemical processes dominating the F⁻-bearing groundwater, using Piper and Gibbs diagrams, correlations, saturation indices, ionic ratios, and multivariate analysis. Piper diagram shows that the groundwater quality is characterized by Na⁺-HCO${}_3^{-}$ and Na⁺-Cl⁻types, while the Gibbs diagrams suggest that the groundwater chemistry is mainly controlled by water-rock interactions and is subsequently modified by human activities. The F⁻ groundwater is positively correlated with pH, total dissolved solids, Mg²⁺, Na⁺, K⁺, HCO${}_3^{-}$, Cl⁻, SO${}_4^{2-}$, and NO${}_3^{-}$ and negatively with Ca²⁺, representing the influences of geogenic and anthropogenic origins on the groundwater system. Geochemical ratios and saturation indices indicate that the processes of mineral dissolution, ion exchange, and evaporation mainly govern the high F⁻ groundwater, while the chemical fertilizers cause for the enrichment of F⁻in the groundwater. Cluster and principal component analyses further support the above findings.