Fluoride Sources and Enrichment Mechanisms in a Coastal Plain Under the Influence of Seawater Intrusion

Fluoride (F^-) enrichment in coastal groundwater poses a persistent threat to drinking water safety, yet its controlling mechanisms under seawater intrusion remain incompletely understood, particularly in northern coastal China. This study integrates hydrogeochemical analysis, Bayesian-optimized random forest modeling, and positive matrix factorization (PMF) to elucidate the sources and enrichment processes of F^- in shallow groundwater of the Qinhuangdao coastal plain. Results demonstrate that elevated F^- concentrations are not directly driven by seawater mixing. Instead, maximum F^- levels occur under mild seawater intrusion, while severely intruded zones exhibit lower and less variable F^- concentrations. PMF results further reveal that F^- enrichment is dominated by geogenic sources related to fluorite dissolution, accompanied by contributions from silicate weathering and evaporation-leaching processes. Geochemical evidence indicates that seawater intrusion indirectly promotes F^- release by increasing groundwater alkalinity, enhancing ionic strength, and reducing Ca²⁺ activity through carbonate precipitation, thereby facilitating fluorite dissolution. However, end-member mixing analysis confirms that the direct contribution of seawater-derived F^- is limited. These findings clarify the nonlinear relationship between seawater intrusion intensity and groundwater F^- enrichment and emphasize that seawater intrusion acts primarily as an amplifying factor rather than a primary source of fluoride. This study provides a process-based understanding of fluoride enrichment in coastal aquifers and offers scientific guidance for groundwater management and drinking water risk mitigation in seawater-intruded coastal regions.