PFAS retention and distribution in the vadose zone of three soil types impacted by biosolids application

Beneficial reuse of biosolids as organic fertilizers can introduce per- and polyfluoroalkyl substances (PFAS) into agricultural soils. This study evaluates the impact of soil properties on PFAS retention in the vadose zone of an agricultural site with a long history of biosolids applications until a hiatus in 2017, followed by a new application in 2023. The site is dominated by three soil series: deep and slowly permeable Acredale silt loam, deep and rapidly permeable Bojac sandy loam and moderately permeable Tomotley loam. Soil cores were collected pre- and post the 2023 biosolids application event at 0-60 and 0-240 cm, respectively, and analyzed for 54 PFAS. Total PFAS between 24.3 and 133.4 ng/g were detected within the upper vadose zone pre-2023 biosolids application demonstrating PFAS persistence. PFAS concentrations and distribution in pre- and post-2023 biosolids application soil cores collected from the same soil series were similar and dominated by long-chain perfluoroalkylsulfonates which accounted for 70 % of quantifiable PFAS. Long-chain PFAS concentrations generally decreased with depth, except in the Tomotley loam which had relatively higher concentrations in the deeper soil layers. Single and multiple regression analysis showed that PFAS retention in the vadose zone was best predicted by organic carbon content and clay + silt. However, organic carbon was mostly significant for long-chain compounds at soil depths beyond 60 cm. Due to local high precipitation conditions of the site and substantial attenuation in clay-rich soils, textural-driven hydraulic processes likely exert dominance on PFAS retention at the site.