Geochemical and spatial assessment of fluoride contamination in hard rock aquifers of the Karaipottanar sub-basin of Southern India

Fluoride contamination in groundwater poses significant health risks globally, necessitating comprehensive research to understand its sources and transport mechanism. The present study explores the geochemical factors influencing the fluoride levels and their spatial distribution in the hard rock aquifer system of the Karaipottanar sub-basin, which covers an area of about 1,116 km² and is influenced by both geological and human activities. Groundwater sampling was carried out at 44 locations representing various geological formations. Fluoride in groundwater shows that, 43% complied with BIS (2012) standards, while 25% exceeded the permissible limit (1.5 mg/l). Elevated fluoride levels were particularly associated with the alluvial formation (2.9 mg/l). Hydrogeochemical plots (Gibbs and Piper diagram) clearly indicate that groundwater chemistry is predominantly controlled by rock–water interaction processes as a governing factor contributing to fluoride contamination of groundwater, along with anthropogenic activities. The ratio of ionic strength vs. log pCO 2 , and the scatter plot of pH, ORP, and EC with fluoride, indicates the fluoride enrichment in groundwater results from both ion-exchange process and agricultural input. The speciation and saturation index study identifies the possible sources of fluoride dissolution influenced by magnesium-bearing minerals and calcite-rich formations. Statistical analysis further reveals that the sources of fluoride in the groundwater are influenced by both geology and human activities. The present study highlights the combined influence of geological, hydrogeochemical, and human-induced factors on fluoride concentrations in groundwater of the Karaipottanar sub-basin, offering critical insights for developing effective groundwater management and remediation strategies.