19. Groundwater Geochemistry and Identification of Hydrogeochemical Processes of Fluoride Enrichment in the Consolidated Aquifer System in a Rain Shadow Area of South India

The present study on fluoride contamination in surface water and groundwater in and around Attappady tribal area of Palghat district, Kerala State, India reveals that fluoride dispersal is controlled by country rocks and their structures along with hydrogeomorphology and drainage network. The leaching of F⁻ in groundwater is also controlled by semi-arid climate, which is the result of very less rainfall (936 mm) in the area. Total 42 samples including both surface water and groundwater have been collected during both pre-monsoon and post-monsoon periods and have been analyzed for fluoride apart from other chemical parameters. High fluoride (>1.50 mg/L) in groundwater has been observed in major parts of the Attappady area. The surface water contain fluoride within the range of 1.22–2.86, whereas groundwater in phreatic aquifers have the range of 2.18–2.56 mg/l and deeper fractured aquifers have the fluoride range of 3.20–4.20 mg/l during pre-monsoon and post-monsoon periods, respectively. Maximum fluoride concentration (4.20 mg/L) is recorded in the east-central part of Attappady. Low rainfall and high rate of evaporation promote the dissolution of fluorine-bearing minerals and help in increasing the F⁻ content in groundwater. Fluoride contamination is geogenic in nature and major contribution is made by the hornblende-gneiss formation (1.67 mg/l).