Biomonitoring Reveals the Transition from Legacy to Emerging PFAS in Subtropical Estuaries: Dual Roles of Usage Shift and Environmental Parameters

Following global restrictions on legacy per- and polyfluoroalkyl substances (PFAS), emerging alternatives have been widely introduced, yet their environmental trajectories remain poorly understood. Using oysters as biomonitors, this study revealed the spatiotemporal dynamics of both legacy and emerging PFAS in the Pearl River Estuary from 2015 to 2020. Among frequently detected PFAS, total median concentrations decreased significantly from 48.4 to 5.56 ng/g dry weight, largely due to declines in legacy compounds. Conversely, emerging PFAS, such as hexafluoropropylene oxide dimer acid (HFPO-DA) and 6:2 fluorotelomer sulfonic acid (6:2 FTS), persisted at elevated levels, reflecting ongoing industrial use and their potential environmental accumulation. PFAS composition shifted from legacy-dominated in 2015-2016 to emerging-compound-dominated by 2020. Seasonal trends were pronounced for legacy PFAS, with higher concentrations in wet seasons, whereas emerging compounds exhibited substance-specific seasonal patterns. Spatial distribution of HFPO-DA correlated strongly with industrialized and urbanized zones, highlighting the role of regional socioeconomic activity. Environmental factors (pH, salinity, and temperature) modulated PFAS partitioning and bioaccumulation. Overall, regulatory actions and industrial substitution drive compositional shifts in estuarine PFAS contamination, while hydroclimatic conditions influence their bioavailability. Our study underscores the need for monitoring emerging PFAS in estuaries to assess ecological risks under evolving regulatory pressures.