Fluoride enrichment and hydrogeochemical characteristics in coastal multi-aquifers: Implications for public health and nutrition

Coastal freshwater is essential for drinking water needs, but its scarcity is increasing due to saltwater intrusion and human inputs in many tropical anthropogenically modified coasts around the worldwide. However, in some areas, including Bangladesh, groundwater quality has not been extensively studied concerning fluoride contamination, its hydrogeochemical properties, human health impacts, and nutrient dynamics. The present study aimed to assess the fluoride enrichment and broad hydrogeochemical properties of multiple aquifers along the modified tropical coast of Cox's Bazar, Bangladesh. A total of 91 groundwater samples from shallow aquifers (SA), middle aquifers (MA), and deep aquifers (DA) were analyzed for hydrogeochemical elements. The results revealed that fluoride concentrations exceeded the permissible limit (1.50 mg/L) in 29 %, 25 %, and 27 % of SA, MA, and DA samples, with a maximum concentration of 13.5 mg/L. The saturation index indicated that evaporation intensified fluoride enrichment in over 40 % of groundwater samples, coinciding with increased total dissolved solids (TDS). The fluoride enrichment in the SA system was attributed to evaporite dissolution, carbonate weathering, desorption, competitive adsorption, cation exchange, and mineral dissolution-precipitation processes. Stable isotope analysis suggested that MA water was unaffected by evaporation during recharge, while recent recharge events influenced the isotopic composition of SA and DA waters, primarily through rainfall infiltration. The entropy water quality index classified 87.4 %, 91.8 %, and 82.9 % of respective SA, MA, and DA groundwater samples as very poor in quality. Health risk assessment revealed that SA posed the highest non-carcinogenic risk (83.02 %), followed by MA (12.03 %) and DA (9.75 %). Adults using SA water faced five times higher health risks than children, with the northern region identified as a hotspot for nutritional risks. Daily water intake exacerbated the health risks, particularly among adults. The findings emphasize the urgent need for sustainable groundwater management strategies to ensure safe drinking water supplies in tropical, anthropogenically altered coastal regions.