Interpretation of Characteristic-based Leaching Methods for Soils Containing Per- and Polyfluoroalkyl Substances (PFAS)

Abstract

The toxicity and persistence of per- and polyfluoroalkyl substances (PFAS) in the environment have raised concerns regarding the potential impacts of PFAS release into the environment. The strong C-F bonds and amphiphilic nature of these compounds require adaptation of characteristic-based tools to evaluate the leaching behavior of PFAS from contaminated soils in order to support PFAS site investigations and source zone management. The U.S. EPA Leaching Environmental Assessment Framework (LEAF) was used to determine fundamental leaching behaviors governing the release of PFAS field-aged soils. Testing included pH-dependent (Draft Method 1313A) and liquid-to-solid ratio (L/S)-dependent (Draft Method 1316A) batch leaching tests to characterize leaching patterns over a range of chemical conditions. Leaching testing was carried out on four soils from field sites that were contaminated with aqueous film-forming foams (AFFF). Results identified distinct desorption mechanisms: (i) matrix co-dissolution in organic-rich soils, (ii) electrostatic repulsion from clayey soils, and (iii) sequestration within insoluble organic domains in coarse-textured soils. Eluates from saturated columns and unsaturated, intermittently wetted columns were compared to quantify the potential bias introduced by PFAS sorption to air-water interfaces. In reactive soils, the collapse of the air-water interface due to wetting pulses released high eluate concentrations; however, the overall impact remained generally within an order-of-magnitude of saturated column results, suggesting that saturated methods can provide a sufficient baseline for most risk assessments. Column leached soil was oxidized by total oxidizable precursor assay to degrade PFAS precursors that can form perfluoroalkyl acid (PFAAs). Subsequent leaching of oxidized soil confirmed that precursor transformation generates a significant secondary source of mobile PFAAs capable of sustaining long-term release. This research demonstrates the integration of multi-method data into a PFAS leaching source term for soils. Accurate assessment requires defining the upper release limit using the greater of total or available content and accounting for L/S-dependent partitioning coefficients that drive sustained mass discharge. Collectively, the integration of LEAF for PFAS can provide a scientifically robust framework for moving beyond single-point leaching tests toward characteristic-based strategies that account for the evolving chemistry and sustained release kinetics inherent for PFAS source zone characterization.