Unsaturated Zone Transport and Mobility of PFAS: Experimental Study for New Insights, Conceptualization, and Modeling

Because of their environmental recalcitrance, high mobility, and toxicity, poly- and perfluoroalkyl substances (PFASs) are a growing and significant threat to groundwater throughout the world.  There has been progress in the detection and treatment of PFASs, however their transport behavior in the subsurface are still not well understood.  Despite the relative mobility in the subsurface, PFASs have been found in source-zone soils at significant levels even decades after their application ceased. This observation has largely been attributed to both the presence of highly sorptive polyfluorinated precursors to perfluoroalkyl acids (PFAAs) as well as the retention of PFASs at the air-water (A-W) interface.

This paper discusses the general behavior of PFASs in the unsaturated zone and identifies several shortcomings related traditional contaminant transport experimentation and modeling methods when applied to PFASs. To address these issues, significant gaps in understanding related to the conceptualization, testing, and modeling of PFAS behavior in unsaturated zone have to be investigated.  Our preliminary work confirms that PFASs are highly retarded and retained at the A-W interface.  However, when PFAS-based aqueous film forming foams (AFFFs) are used, the mixture of PFASs introduced to the environment is significantly more complex: AFFF formulations can contain hundreds of PFASs in varying concentrations.  Three key findings of our research with respect to PFAS transport in the unsaturated zone are:

1. Current methods of breakthrough curve (BTC) analysis make simplifying assumptions which are likely insufficient for quantifying PFAS retention at the A-W interface. Both the residual air trapping and dead volume impacts are assumed to be negligible in traditional BTC methods. These assumptions are likely ill-suited for PFAS analysis. A more sophisticated understanding and analysis of equilibrium BTCs is proposed for PFASs.
2. Most BTC analysis in past studies is often conducted using short (less than 100 cm) columns, on one compound at a time with gravimetric calculations of the degree of saturation of the whole soil sample. End effects create non-uniform saturation along the length of columns. Furthermore, conducting BTC analysis one compound at a time is time consuming. By combining non-targeted chemical analysis with high resolution mass spectrometry (HRMS), BTC analysis can be performed on field-relevant complex mixtures with high accuracy saturation measurements.
3. Equilibrium partitioning is often assumed to describe retention of multi-component compounds. Because unsaturated transport is controlled by capillarity that depends on interfacial tension at A-W interfaces and PFASs are surfactants with unusual transport behavior, it is unlikely that equilibrium transport models will sufficiently describe PFASs transport.  Conceptual models of physical and chemical non-equilibrium transport have been developed and are undergoing field and laboratory verification.