Comparison of surface freshwater PFAS sampling methods to evaluate potential for bias due to PFAS enrichment in the surface microlayer.

This article focuses on evaluating whether inclusion of the surface microlayer (SML) in bulk surface water sampling biases measured concentrations of per- and polyfluoroalkyl substances (PFAS). Through a pilot study at two sites and a full field study at 11 sites, three common bulk water sampling methods were compared: a peristaltic pump excluding the SML, a fully submerged bottle excluding the SML, and a partially submerged bottle including the SML; the SML itself was sampled using the glass plate method. Results showed no evidence that including the SML via the partially submerged bottle caused a high bias in PFAS concentrations; unexpectedly, this method often yielded slightly lower PFAS levels, especially for less hydrophobic PFAS, compared to methods excluding the SML. Additionally, PFAS enrichment factors in the SML generally increased with chromatographic retention time, but this pattern was not consistent across all sites or PFAS. Overall, differences among sampling methods were small and unlikely to cause meaningful bias in PFAS monitoring programs.