Laboratory batch representation of PFAS leaching from aged field soils: Intercomparison across new and standard approaches

Abstract

Relating laboratory leaching methods to partitioning and transport of per- and poly-fluoroalkyl substances (PFAS) in field soils is challenging, making estimation of fluxes to groundwater and surface water uncertain. Existing laboratory leaching methods have limitations when assessing field leaching. For 37 aged field soils from five sites historically contaminated with PFAS over decades, we assess PFAS leaching using new and existing laboratory leaching methods to provide alternative methods better reflecting PFAS risks posed by its leaching and movement. Dominant PFAS in the soils were perfluorooctane sulfonic acid, perfluorohexane sulfonic acid, and perfluorohexanoic acid and to a lesser extent perfluorooctanoic acid. Leaching from intact soil cores (Exp 1) was taken to reflect field conditions. These were compared to two new laboratory batch tests, saturate-spin (Exp 2) and saturate-tumble-spin (Exp 3), and two standard approaches; Australian Standard Leaching Procedure (ASLP, Exp 4) and the Leaching Environmental Assessment Framework (LEAF, Exp 5). The tests varied in terms of liquid:soil ratio, tumbling time and pH of the starting solution, with LEAF-1313 conducted across seven pHs (2-12). Correlations between leachate and soil concentrations were highest for Exp 4 and Exp 5 (R² = 0.72-0.98) and lowest for Exp 3 (R² = 0.53). The PFAS mass leached as a fraction of the total increased such that: soil core leaching (27 %) < saturate-spin (30 %) < saturate-tumble-spin (65 %) ≤ LEAF-1313 (65 to 88 % at pH 5-9) < ASLP (90 %). As a fraction of individual PFAS compounds in leachate compared with soil, the shorter chain PFAS (e.g., perfluorobutanoic acid) were higher in the leachate in all tests. Across all tests, the saturate-spin batch test most closely represented intact soil core leaching and therefore potentially provides a measure more analogous of in situ soil leaching at field sites. Other methods would apply to broader applications such as landfill disposal.

Keywords: Aged soils; Cores; Leaching; PFAS; Soil; Standards.