Abstract

Hydrogeochemical evaluation of aquifers belonging to Chimakurthy granitic pluton complex reveals wide spatial and temporal variation in F^- distribution. F^- concentration in groundwater of different aquifers varies from < 0. 50 to 9.84 mg/l. Among three sampled episodes, the mean value of F^- is high in July 2010 and 61% of samples have values above maximum permissible limit as per the Bureau of Indian Standards. In April 2008, 44% and in February 2009, 41% of samples were not fit for drinking purposes due to enrichment of F^-. Northern half of the area is almost free from F^- but south central part, which has witnessed igneous activity with mafic intrusive form the core area of F^- mineralization. Cent percent testing of all the groundwater structures in fifteen villages show uneven distribution of F^- even within limited  terrain. F^- show close positive correlation with pH (0.57), Na⁺ (0.68), HCO₃^- (0.61) and distinct negative correlation (-0.31) with Ca²⁺. The groundwater facies is of sodium bicarbonate chloride type, sodium magnesium bicarbonate type and calcium magnesium bicarbonate type. Among 23 analyzed trace elements, Zn is the most dominant (mean 3200 µmg/l). Trace elements concentrations do not exhibit any distinct control over F^- contribution to groundwater. F^- content in rock samples varies drastically from below detectable limit (BDL) to 492 ppm whereas in soil it varies between (BDL) and 612 ppm. F^- concentration of rock-soil-groundwater does not synchronize depicting latter enrichment of F^- into formation water. Ratios, indices and plots indicate multiple mechanisms were responsible in F^- absorption into groundwater. Water-rock interactions, ion exchange followed by evapotranspiration have facilitated the F^- mineralization of groundwater. Na⁺/Cl^- ratio of >1in almost all the samples of three sampling sessions establish silicate weathering. Ca²⁺/Mg²⁺, Na⁺/Ca²⁺, Ca²⁺, Mg²⁺ and HCO₃^- ratios authenticate carbonate dissolution is responsible for alkali earths addition. Sodic rich and calcium depleted water together with balanced alkalinity has enabled F^- enrichment. Presence of fluorite as accessory mineral apart from occurrence of F^- in mineral lattice of biotite, hornblende, muscovite, and pyroxene were the potential sources of F^- to the percolating pore waters. Though F^- was abundantly available in solid solution the receptive hydrochemical character of solute was governing the F^- adsorption which could be one of the strong reasons for uneven distribution of F^- with in similar petrological setup.

Key words: Water-rock interaction, fluoride, groundwater, Chimakurthy, gabbro, hydrogeochemistry.