Surface-water PFOA/PFOS in Northern Vietnam: spatiotemporal patterns and a control-prioritization framework

Perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS) are persistent PFAS of growing regulatory concern, yet robust spatiotemporal evidence in Vietnam remains limited. Here, we developed a 12-month surface-water monitoring dataset with triplicate sampling across an ≈45-site lake-river-estuary-coastal network in Northern Vietnam, quantified PFOA/PFOS by LC-MS/MS, and translated the observations into a control-prioritization framework. Across pooled station-month observations (n = 552), PFOA showed a median of 7.56 ng/L (p95 = 30.95), while PFOS was higher with a median of 16.55 ng/L (p95 = 107.77). Concentrations consistently increased along the basin transport-retention continuum, with stable hotspots in downstream and estuarine nodes (e.g., Ba Lat, Cua Day, Do Son, Sam Son, Tra Co) and the water-body hierarchy estuaries > river (downstream) > coastal > lakes. Clear seasonality was observed, with PFOS peaking in January/December and PFOA peaking in February/December. Mechanistic interpretation combined field covariates with water-particle-DOC partitioning concepts, adsorption-desorption tests using reference sediment, and multivariable modeling, consistently identifying suspended solids and dissolved organic carbon as dominant drivers, especially for PFOS. Ecological screening using risk quotients (RQ) indicated PFOS-driven concern at priority downstream/estuarine nodes. In parallel, integrated exposure scenarios (drinking water and fish consumption) suggested seafood intake can dominate PFOS exposure, yielding tail risks for children and high-seafood consumers even when mixture-level indices remain generally acceptable under baseline scenarios.