Identifying the hydrochemical features, driving factors, and associated human health risks of high-fluoride groundwater in a typical Yellow River floodplain, North China

Fluoride enrichment (> 1.5 mg/L) in groundwater has become a global threat, particularly given the hazards to human health. This study collected 58 unconfined groundwater samples from Fengpei Plain in June 2022 for hydrochemical and stable isotope analyses combined with multiple methods to explore sources, influencing factors, and potential health hazards of groundwater F^-. The results showed that groundwater F^- concentration ranged from 0.08 to 8.14 mg/L, with an average of 1.91 mg/L; over 41.4% of them exceeded the acceptable level of 1.5 mg/L prescribed by the World Health Organization (WHO). The dominant hydrochemical facies changed from Ca·Mg-HCO₃ and Ca·Mg-SO₄·Cl type in low-F^- groundwater to Na-HCO₃ and Na-SO₄·Cl water types in high-F^- groundwater. The Self-Organizing Map (SOM) and ionic correlation analysis indicated that F^- is positively correlated to pH, EC, Na⁺, K⁺, SO₄^2-, and TDS, but negatively to Ca²⁺ and δ¹⁸O. Groundwater F^- accumulation was primarily driven by F^--bearing minerals dissolution such as fluorite. Simultaneously, the carbonates precipitation, positive cation exchange processes, and salt effect were conducive to groundwater F^- enrichment. However, competitive adsorption between OH^-/HCO₃^- and F^-, evaporation, and anthropogenic activities only had a weak effect on the F^- enrichment in groundwater. The hazard quotient (HQ) assessment results show that 67.2% of groundwater samples pose a non-carcinogenic risk (HQ > 1) for infants, followed by 53.4% for children, 32.8% for females, and 25.9% for males. The Monte Carlo simulation results agreed with those of the deterministic model that minors are more susceptible than adults. These findings are vital to providing insights into the geochemical behavior, driving factors, and drinking water safety of high-F^- groundwater worldwide.