Beyond Legacy PFAS: Dominant Role of Ultrashort-Chain and Emerging PFAS in the Elbe River-Sea Continuum

Per- and polyfluoroalkyl substances (PFAS) are highly persistent contaminants. Current regulatory monitoring frameworks cover only a narrow set of regulated long- and short-chain PFAS, while ultrashort-chain compounds such as trifluoroacetic acid (TFA) remain largely unaddressed despite their widespread occurrence. Here, we present regional sources and tributary inputs of diverse PFAS (n = 83), including ultrashort-, short-, and long-chain PFAS as well as PFAS precursors along the 1,200 km Elbe River-North Sea continuum. Based on 129 samples from the Elbe, its tributaries, and wastewater treatment plants (WWTPs), we show that short- and long-chain PFAS concentrations in the Elbe were generally below 30 ng/L, but 1,653 ± 377 ng/L when including TFA. Major tributaries such as the Saale, Vltava, and Mulde delivered PFAS mass loads of several thousand g/day (2,340; 6,080; and 1,700 g/day, respectively, when including TFA) and together accounted for a total discharge of 160 m³ s^-1, corresponding to approximately 67% of the Elbe's water flux at Geesthacht (240 m³/s), which represents the downstream reference point for mass balance calculations prior to tidal influence. This suggests source-enriched inputs rather than simple dilution by tributary inflows, given the disproportionate increase in PFAS mass loads relative to discharge. WWTP effluents from Dresden and Pardubice added up to 185 g/day, mainly shaped by short-chain PFAS. The detection of Capstone B (CDPOS) and Bis-(trifluoromethylsulfonyl)-imide (NTf2), non-routinely monitored industrial PFAS, further highlights overlooked sources in the area of the Saale-Elbe confluence. Inclusion of TFA increased the median PFAS load in the Elbe River by nearly 50-fold (up to 39,958 g/day), demonstrating the dominant role of ultrashort-chain PFAS currently absent from regulatory monitoring. By linking source apportionment with river-to-sea transport, our study highlights critical gaps in existing monitoring frameworks and provides a transferable methodology for more comprehensive PFAS assessment and regulation in large river systems.