Per- and Polyfluoroalkyl Substances Enrichment in the Surface Microlayer of a Freshwater System Impacted by Aqueous Film-Forming Foams

Accumulation of per- and polyfluoroalkyl substances (PFAS) in the surface microlayer (SML) of open water bodies has received little attention. Various SML sampling methods exist, yet none have been systematically evaluated for PFAS. Three glass-based techniques were deployed for SML sampling at 10 sites on a freshwater system impacted by aqueous film-forming foam (AFFF). Samples of SML and underlying bulk water (3 and 30 cm) were analyzed for 50 target PFAS by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QToF). Single dipped microscope slides yielded field measurements of SML thicknesses and enrichment factors for eight PFAS and used to compute field-measured air–water adsorption coefficients. Large glass plates collected a greater volume of SML, gave lower variability, and lower limits of detection and quantification. Up to 19 PFAS (concentrations up to 13,000 ng/L) were associated with the SML, while only 15 PFAS were in underlying bulk water. Enrichment factors (EFs) ranged from 1 to 20 and increased with chromatographic retention time, a proxy for relative hydrophobicity. Field-measured air–water adsorption coefficients were consistent with monolayer adsorption as described by the Gibbs adsorption equation, which indicates that only adsorption to the air–water interface is needed to explain the observed PFAS enrichment in the SML.