ORIGINAL ARTICLE Hydrogeochemical processes of fluoride enrichment in Chimakurthy pluton, Prakasam District, Andhra Pradesh, India A. G. S. Reddy 1 • D. V. Reddy 2 • M. Sudheer Kumar 3 Received: 2 August 2015 / Accepted: 18 February 2016 / Published online: 11 April 2016 Ó Springer-Verlag Berlin Heidelberg 2016 Abstract Thirty-six groundwater samples were collected from Chimakurthy area, Prakasam District, Andhra Pra- desh. 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 con- taminated with NO 3 - 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 HCO 3 - were progressively increasing along with F - from Group A to C, whereas Ca 2? and Mg 2? 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 Ca 2? ? Mg 2? :Tz ? ratios were \ 1 in all samples indicating that Na ? was released from silicate weathering process. It also confirms that F - was con- tributed from lithogenic sources of aquifer material. Mean molecular ratios of \ 1 for Ca 2? :Mg 2? in Group B samples and 1–1.2 in Groups A and C support that the Ca 2? and Mg 2? 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 - con- centration 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 ground- water 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. Keywords Groundwater  Fluoride  Na ?  Bicarbonate  Gabbro  Silicate weathering  Water types Introduction Fluoride contamination in groundwater is one of the most studied topics in hydrogeochemistry worldwide. Though fluoride in small concentrations is essential for human metabolism, if the same element exceeds a certain limit it proves to be a potential health hazard. F - is well dis- tributed in lithosphere and is available in many items consumed by human beings. Nature has provided F - to the extent required for sustenance of life; however, due to certain geogenic and occasionally anthropogenic actions, it gets accumulated in some natural resources, particularly in groundwater. Fluoride is absorbed by the human body due to ingestion from various sources apart from water; however, groundwater often accounts for many of the F - - related health issues over the years. Though a lot of research is carried out and many remedial measures were put forth in many parts of the world, still ambiguity per- sists on dynamics of fluoride enrichment in groundwater. & A. G. S. Reddy sugriveadla@gmail.com 1 RGNGWT&RI, Raipur, CG, India 2 CSIR-NGRI, Hyderabad, India 3 CGWB, SR, Hyderabad, TS, India 123 Environ Earth Sci (2016) 75:663 DOI 10.1007/s12665-016-5478-8