National‑scale classification and quantitative contribution of fluoride enrichment mechanisms in China's groundwater

High-fluoride groundwater has become a global public environmental and health issue due to its potential threat to human health and ecology. The spatial distribution and controlling factors for F⁻ in groundwater has been widely studied. However, the classification of F-containing groundwater and the quantification of the contribution from different enrichment mechanisms for the F⁻ in groundwater across China remains an unresolved issue. Thus, a total of 10,288 F-containing groundwater samples were collected, complied from 354 articles published between 2004 and 2024. The statistical results showed that the average F⁻ concentration in the collected groundwater samples reached 1.17 mg/L, and 47.24% of samples exceeded the Chinese drinking water standard (1 mg/L). Based on significant differences in F⁻ concentration and hydrochemical characteristics, the collected groundwater samples were categorized into four groups through the self-organized mapping. The four identified hydrogeochemical groups exhibited distinct evolution processes and average F⁻ concentrations: Group I (rapid runoff with insufficient water-rock interaction; 0.56 mg/L); Group II (evaporation-affected; 3.05 mg/L); Group III (dissolution-controlled; 0.89 mg/L); and Group IV (cation exchange-affected; 3.55 mg/L). Finally, the relative contribution of different hydrochemical processes for the F⁻ concentration has been quantified. Fluorite dissolution constituted the primary source of F⁻, accounting for 62.08%-86.18% of its enrichment. Evaporation and cation exchange notably contributed for the elevated F⁻, with the maximum values reaching 4.42% and 13.07%, respectively. Overall, this nationwide classification and quantification framework provides a robust foundation for predicting fluoride hotspots and designing targeted management strategies of water quality.