Hydrogeochemical processes of fluoride enrichment in Chimakurthy pluton, Prakasam District, Andhra Pradesh, India

Thirty-six groundwater samples were collected from Chimakurthy area, Prakasam District, Andhra Pradesh. On hydrogeochemical evaluation, it was found to be contaminated. Based on fluoride concentration, the samples were segregated into Group A, Group B, and Group C. Fluoride concentration varies from 0.23 to 9.84 mg/l, with the mean value of 2.01 mg/l; 44 % of the tested samples had F⁻ > 1.50 mg/l. Apart from F⁻, the water was contaminated with NO₃ ⁻ and Cl⁻ implying both geogenic and anthropogenic pollution of the aquifer. Ionic dominance pattern was unique in samples with low, moderate to high F⁻ concentrations. Na⁺ and HCO₃ ⁻ were progressively increasing along with F⁻ from Group A to C, whereas Ca²⁺ and Mg²⁺ were depleting. Groundwater facies were unique among different group samples especially in end members with low and high F⁻ concentration suggesting that F⁻ dissolution was strongly influenced by the chemical nature of solution. Na⁺/Cl⁻ molar ratio was >1, whereas Na⁺ + K⁺:Tz⁺ and Ca²⁺ + Mg²⁺:Tz⁺ ratios were <1 in all samples indicating that Na⁺ was released from silicate weathering process. It also confirms that F⁻ was contributed from lithogenic sources of aquifer material. Mean molecular ratios of <1 for Ca²⁺:Mg²⁺ in Group B samples and 1–1.2 in Groups A and C support that the Ca²⁺ and Mg²⁺ were added into aquatic medium by calcite and dolomite dissolution. Contradictory distribution of F⁻ in north (low) and south (high) indicates that availability of F⁻ minerals in aquifer matrix influenced the rate of F⁻ absorption in accompanying water. Variation in F⁻ concentration in groundwater of the core area (south) supports the hypothesis of aquatic chemistry acting as catalyst in F⁻ dissolution into formation waters even though the aquifer was enriched with F⁻ minerals. Very high F⁻ in groundwater in the contact zone of country rock and plutonic intrusive indicates that F⁻ mineralization occurred under geothermal conditions. The fracture network in disturbed and fragile contact zone has facilitated development of potential aquifers and enhancement in F⁻ concentration.