Sensitive assessment of groundwater-associated, multi-exposure health hazards in a fluoride-enriched region of West Bengal, India

Ninety groundwater samples were collected from Khayrasole and Rajnagar blocks of Birbhum district, West Bengal, India, during pre-monsoon and post-monsoon in 2016 to assess the hazards of fluoride in groundwater. Fluoride concentration fluctuated from 0.3 to 17.6 mg/L, with 70% of samples reported beyond the modified regional optimal fluoride level (0.7 mg/L) with a statistically significant level of p < 1.7E−24. The average cation and anion concentrations exhibited a descending order of Ca²⁺ > Mg²⁺ > Na⁺ > K⁺ and HCO₃⁻ > Cl⁻ > SO₄²⁻ > NO₃⁻ > F⁻, respectively. Notably, groundwater quality in 50% of the places ranged from poor to unfit for drinking purposes in terms of water quality index. The mean total hazard index (THI) was 1.1 for adults and 1.9 for children, signifying a greater chance of non-carcinogenic threats to both age groups. In calculating the THI, ingestion and dermal pathways accounted for approximately 96% and 3% health hazards, respectively. The Monte Carlo simulation and sensitivity analysis identified that the diurnal water ingestion rate, exposure duration, and fluoride concentration were the significant sensitive variables that triggered most groundwater-associated non-carcinogenic health issues, signifying more risks among children. Further, dental health surveys (N = 746), following Dean’s norms for classification based on regional optimal fluoride level, designated the borderline grade of the community dental hazard. The subsequent hydrogeochemical characterization directed that dissolution from fluoride-bearing minerals and water–rock interaction, such as halite dissolution and calcite–dolomite precipitation, were the governing factors for F⁻ enrichment in groundwater. This study will serve as baseline data for delineating fluoride-induced dental and other health hazards through sensitivity and spatial analysis in the GIS platform for hazard zonation and effective groundwater quality management.