Seasonal dynamics of emerging and legacy PFAS in the Beibu Gulf: Insights from a combined source apportionment and predictive modeling approach.

Emerging and legacy per- and polyfluoroalkyl substances (PFAS) have become a growing concern in coastal waters, as regulatory phase-outs and short-chain replacements reshape their occurrence patterns. Yet the extent to which seasonally distinct source regimes and environmental controls differentially influence PFAS in semi-enclosed coastal systems remains unclear. This study addresses this gap by developing a transferable analytical framework that links PFAS patterns to seasonally varying source and environmental controls, with the aim of supporting monitoring and management in coastal systems. We apply this framework to the Beibu Gulf, a semi-enclosed, monsoon-influenced coastal system, where 29 PFAS were quantified in 94 surface seawater samples collected in the Beibu Gulf, including 47 samples per season. Total PFAS concentrations ranged from 0.20 to 4.79 ng/L, with perfluorooctanoic acid (PFOA) dominating in both seasons. Short-chain emerging PFAS were significantly elevated in summer under enhanced riverine inputs, whereas long-chain legacy PFAS increased in winter under stronger offshore transport and precursor-derived sources. PMF (Positive Matrix Factorization) consistently linked short-chain PFAS to Guangxi riverine inputs, while attributing PFOA mainly to ocean-transported and transformation-related factors with marked seasonal differences. The eXtreme Gradient Boosting (XGBoost) model achieved the highest predictive accuracy for short-chain PFAS, and Shapley Additive exPlanations (SHAP) analysis identified salinity and temperature as the major contributors, confirming the stability of their terrestrial signatures. In contrast, the weaker predictive performance for legacy PFAS, together with their sensitivity to temperature and dissolved oxygen, suggests that winter biogeochemical controls govern their behavior. Taken together, this work advances a seasonal, mechanism-based framework to guide differentiated monitoring, source-oriented assessment and management of emerging and legacy PFAS in dynamic coastal environments.