Per- and polyfluoroalkyl substances in Baltic sediments: Role of sediment-pore water partitioning on their distribution and fate

Understanding the accumulation and behavior of per- and polyfluoroalkyl substances (PFASs) in subsurface environments is crucial for effective environmental management. This study investigates the distribution and sorption dynamics of PFASs in Swedish offshore sediments, with a particular focus on the Bothnian Gulf in the northern Baltic Sea, where elevated PFAS concentrations were observed, reaching up to 33 μg kg⁻¹ in the Bothnian Sea and 27 μg kg⁻¹ in the Bothnian Bay for ∑PFASs. Through sediment sampling, sorption batch experiments, and partitioning analyses, the role of sediment properties from different regions in PFAS fate and transport was examined. Sediment-pore water partitioning coefficients (K_d) were particularly high in some regions, particularly for long-chain PFASs in the Baltic Proper. However, contrary to initial expectations, sediments in the Bothnian Gulf did not exhibit consistently higher sorption capacity despite their elevated PFAS levels, suggesting distinct, unidentified PFAS sources in this region. K_d values varied across locations, with the highest values observed in the southern Baltic Sea, specifically the Baltic Proper and the Southern Baltic, where the organic carbon content was also highest, ranging from 2.2% in the North Sea to 12.7% in the Baltic Proper. While organic carbon strongly influenced PFAS sorption, no consistent trends fully explained the disparities between the northern and southern Baltic regions. The elevated PFAS concentrations in the northern Baltic Sea, despite the estimated limited sorption capacity, highlight the need for further comprehensive source identification and monitoring, particularly of tributary inputs and transboundary influences, to address regional contamination. This study underscores the criticality of integrating source characterization with fate and transport studies, to effectively manage and tackle PFASs in soil-groundwater systems.