Naturally occurring surface water foams as a PFAS sampling matrix

Surface Water Foam (SWF) and underlying Surface Water (SW) were collected from six distinct sites across Michigan using a novel sampling device. PFAS were detected in all SWF samples (Σ41 compounds, n = 14; Avg total PFAS=54,895 ng/L). SWF samples were enriched in high and intermediate molecular volume PFAS relative to SW samples (n = 10) which were dominated by low molecular volume, short-chain PFAS (Avg total PFAS=21 ng/L). Ultra-long-chain and rarely detected PFAS were quantified in SWF that were not detected in SW. Wet and dry SWFs were distinguished by appearance, liquid content, and PFAS composition. Dry SWFs had higher total PFAS concentrations than wet SWFs (Avg total PFAS difference =158,330 ng/L). Intermediate molecular volume PFAS constituted a greater percentage of total PFAS concentrations in wet SWFs, whereas dry SWFs were dominated by high molecular volume PFAS. Principal component and cluster analyses show distinct compositional differences between SW, wet SWF, and dry SWF. A conceptual model is proposed to describe changes in PFAS composition during the evolution and aging of SWFs. Bubbles created from turbulence in surface waters initially accumulate to form wet SWFs. Liquid drains as wet SWFs evolve towards dry SWFs and lower molecular weight PFAS with lower air-water interface (AWI) adsorption drain with the liquid. This enriches dry SWFs with higher molecular volume PFAS that have higher AWI adsorption (up to five orders of magnitude). This study demonstrates the value of SWFs as a complementary sampling matrix for quantifying high and intermediate volume PFAS in natural surface water systems.