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Fluoride occurrence in the groundwater in a coastal region of Andhra Pradesh, India

Rao, N. Subba, Rao, P. Surya, Dinakar, A., Rao, P. V. Nageswara, Marghade, Deepali
Appl Water Sci 2017 v.7 no.3 pp. 1467-1478
Ongole, alkalinity, alluvium, anion exchange, anthropogenic activities, bicarbonates, calcium, calcium carbonate, climate, coasts, fluorides, groundwater, hydrogeochemistry, leaching, minerals, pH, rivers, rocks, saline water, sodium, solubility, India
Fluoride (F⁻) content varies from 0.60 to 1.80 mg/L in the coastal region between Chirala and Ongole of Andhra Pradesh, India. It exceeds the threshold limit of 1.20 mg/L in 20 % of the total groundwater samples. The aim of the present study is to assess the controlling factors of F⁻ content. The study area experiences a dry climate and is underlain by Charnockite Group of rocks over which the river and coastal alluvium occur. The results of the study identify the four factors that control the high F⁻ content. First one is related to alkalinity, leading to active dissolution and leaching of F⁻-bearing minerals, which supports the positive correlation of F⁻ with pH and HCO₃ ⁻. A longer water residence time in the clays is the second factor, which activates not only solubility and dissolution of F⁻-bearing minerals, but also anion exchange between F⁻ and OH⁻. Third factor is a result of higher Na⁺ due to impact of saline water, ion exchange between Na⁺ and Ca²⁺, and precipitation of CaCO₃. This reduces the Ca²⁺ content, causing dissolution of CaF₂ to maintain the chemical equilibria, which is supported by positive correlation between Na²⁺ and F⁻. The influence of anthropogenic activities is the last factor, which acts as an additional source of F⁻. Thus, the shallow groundwater shows higher content of F⁻ and the hydrogeochemical facies also support this hypothesis. The study suggests the remedial measures to reduce the F⁻ content.