Seasonal transport characteristics of PFASs from a typical small catchment to a lake: Evidence from water, SPM, sediment, and porewater.

Small-scale watersheds directly connected to lakes are important pathways for per- and polyfluoroalkyl substances (PFASs), yet their seasonal transport processes remain poorly understood. This study investigated the occurrence and distribution of nineteen PFASs in water, suspended particulate matter (SPM), sediment, and porewater from the Tonglong River (TLR), eastern China, during dry and wet seasons. The median total levels of PFASs in surface water decreased from 2,800 ng/L in the dry season to 790 ng/L in the wet season, whereas porewater concentrations remained much higher, ranging from 52,000 to 5,700,000 ng/L and from 3,200 to 390,000 ng/L during the dry and wet seasons, respectively. Perfluorooctanoic acid (PFOA) was among the predominant compounds across the four media during the dry season. In the wet season, perfluoroalkyl carboxylic acids (PFCAs) became increasingly prominent in surface water, accounting for 90% of total PFASs, compared with 52% during the dry season. Dry-season PFAS concentrations exhibited clearer upstream enrichment, whereas wet-season runoff promoted dilution and redistribution, thereby weakening longitudinal gradients. In contrast, porewater consistently retained a relatively stable upstream-enrichment pattern. Partial least squares path modeling revealed seasonal shifts in the relative controls on interphase partitioning, with goodness-of-fit values ranging from 0.36 to 0.60. Surface-water positive matrix factorization identified three factors in the dry season and five in the wet season, consistent with more diverse aqueous inputs under rainfall-runoff conditions. These findings highlight the value of seasonally integrated analyses of water, SPM, sediment, and porewater for elucidating PFAS transport and retention processes in small catchment-lake systems.