Controls on Groundwater Fluoride Contamination in Eastern Parts of India: Insights from Unsaturated Zone Fluoride Profiles and AI-Based Modeling

Abstract

Groundwater fluoride (F) occurrence and mobilization are controlled by geotectonic, climate, and anthropogenic activities, such as land use and pumping. This study delineates the occurrence and mobilization of F in groundwater in a semi-arid environment using groundwater, and an artificial intelligence model. The model predicts climate, soil type, and geotectonic as major predictors of F occurrence. We also present unsaturated zone F inventory, elemental compositions, and mineralogy from 25 boreholes in agricultural, forest, and grasslands from three different land use terrains in the study area to establish linkages with the occurrence of groundwater F. Normalized unsaturated zone F inventory was the highest in the area underlain by the granitic–gneissic complex (261 kg/ha/m), followed by residual soils (216 kg/ha/m), and Pleistocene alluvial deposits (78 kg/ha/m). The results indicate that the unsaturated zone mineralogy has greater control over F mobilization into the groundwater than unsaturated zone F inventory and land-use patterns. The presence of clay minerals, calcite, and Fe, Al hydroxides beneath the residual soils strongly retain unsaturated zone F compared with the subsurface beneath Pleistocene alluvial deposits, where the absence of these minerals results in enhanced leaching of unsaturated zone F.