High-Fluoride Groundwater By N Subba Rao

Fluoride  is essential for normal bone growth, but its higher concentration in the drinking water poses great health problems and fluorosis is common in many parts of India. The present paper deals with the aim of establishment of facts of the chemical characteristics responsible for the higher concentration of F⁻ in the groundwater, after understanding the chemical behavior of F⁻ in relation to pH, total alkalinity (TA), total hardness (TH), carbonate hardness (CH), non-carbonate hardness (NCH), and excess alkalinity (EA) in the groundwater observed from the known areas of endemic fluorosis zones of Andhra Pradesh that have abundant sources of F⁻-bearing minerals of the Precambrians. The chemical data of the groundwater shows that the pH increases with increase F⁻; the concentration of TH is more than the concentration of TA at low F⁻ groundwater, the resulting water is represented by NCH; the TH has less concentration compared to TA at high F⁻ groundwater, causing the water that is characterized by EA; and the water of both low and high concentrations of F⁻ has CH. As a result, the F⁻ has a positive relation with pH and TA, and a negative relation with TH. The operating mechanism derived from these observations is that the F⁻ is released from the source into the groundwater by geochemical reactions and that the groundwater in its flowpath is subjected to evapotranspiration due to the influence of dry climate, which accelerates a precipitation of CaCO₃ and a reduction of TH, and thereby a dissolution of F⁻. Furthermore, the EA in the water activates the alkalinity in the areas of alkaline soils, leading to enrichment of F. Therefore, the alkaline condition, with high pH and EA, and low TH, is a more conducive environment for the higher concentration of F in the groundwater.