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