Per- and polyfluoroalkyl substances (PFAS) transport from biosolids-amended soils: An experimental and numerical approach.

Per- and polyfluoroalkyl substance (PFAS) do not mineralize in conventional wastewater treatment processes and accumulate in effluents and biosolids. Land-application of biosolids improves soil health; however, PFAS may leach from applied biosolids posing a threat to underlying groundwater systems. In general, transport mechanisms controlling PFAS leaching from biosolids are not fully understood. In this study, two municipal biosolids were mixed with soil at agricultural loading rates and packed in saturated flow-through columns. Biosolids, biosolids-amended soils (pre- and post-leaching) and column eluants were analyzed via targeted analysis, total oxidizable precursor assay (TOP), and suspect screening. Saturated column results were modeled using HYDRUS 1-D to obtain transport parameters. Resulting parameters were used to simulate long-term leaching of two PFAS under field-relevant conditions. Mass balances in column systems show that the majority of precursors (>65 %) in biosolid-amended soils remained sorbed after flow-through experiments. TOP assay results for column eluants suggested that the small fraction of unknown precursors mobilized in column experiments were short-chain precursors (≤C7). Transport modeling in HYDRUS 1-D demonstrated that PFAS desorption from biosolid-amended soils was rate-limited under saturated conditions. Long-term modeling of perfluorooctanoic acid and perfluorooctane sulfonate transport using conditions representative of the upper midwestern United States found aqueous concentrations of 107 and <1 ng/L, respectively, reaching the saturated zone after 40 yrs of annual biosolids applications. This suggests that at some land application sites, best management practices focused on PFAS in municipal biosolids should include multi-pathway exposure analysis, considering the soil to groundwater pathway for more mobile PFAS and shallow soils and porewater, along with processes such as plant uptake, runoff, and tile drainage for less mobile PFAS.