National collaborative study on the incidence and mobility of PFAS following land application of biosolids

This study evaluated the incidence and mobility of 25 per- and polyfluoroalkyl substances (PFAS) in soils following land application of municipal biosolids. A total of 440 soil samples from 21 sites in 16 states of the United States (U.S.) were evaluated, resulting in the largest U.S. database of PFAS concentrations derived from land-applied soils. The duration of land application varied, with some sites first receiving biosolids in the 1980s. Similarly, the number of annual applications and biosolid loading rates also varied, with lifetime loading rates ranging from 2.5 to 280 dry tons per acre. Findings generally indicated low incidence of PFAS across all sites and all loading rates, with median concentrations of all analytes in surface soils (0-30 cm) ranging from non-detect to 0.52 ng/g (ppb). Median concentrations of PFOA and PFOS, the two EPA-regulated analytes in drinking water, were 0.46 and 0.52 ng/g respectively in surface soils receiving higher rates of biosolids (High Biosolids plots). Mean soil concentrations were higher than median values due to the influence of occasional elevated values. Soils from High Biosolids plots showed greater soil PFAS concentrations relative to Control and Low Biosolids plots. PFAS concentrations decreased significantly with increased soil depth (i.e. attenuation). Attenuation also generally increased with chain length; longer-chain PFAS (greater than 10 fluorinated carbons) exhibited greater attenuation than intermediate (7-10 carbons) or shorter chain (6 or less carbons) PFAS. Significant attenuation of all PFAS occurred across study sites with an overall mean attenuation rate of 69 % at a depth of 180 cm. Attenuation of PFOA, averaged over all sites, was also 69 % at a soil depth of 180 cm. The corresponding attenuation of PFOS was 92 %. Overall, the observed comparatively low soil PFAS concentrations would be anticipated to result in lower potential impacts to groundwater quality.