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PFAS Leaching Characterization with the Leaching Environmental Assessment Framework (LEAF)

Aqueous film-forming foams (AFFFs) are a major source of per- and poly-fluoroalkyl substances (PFAS) impacts in soil and groundwater. Standardized tools are needed to rapidly assess the potential for retention, leaching, and transport of PFAS from the source zone to downgradient regions, so that this information can be applied towards critical facets of site management such as prioritizing PFAS-impacted sites for further investigation and remediation. Existing standard leaching methods were developed prior to concerns regarding PFAS. Therefore, studies are needed to ensure that leaching methods are compatible for use with PFAS and that resulting data are representative of the risk of PFAS leaching at impacted sites.

Related Article(s):

• Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)
• PFAS Sources
• PFAS Transport and Fate

Contributors: Dr. Jennifer L. Guelfo, Dr. David Kosson, Dr. Andy Garrabrants, Ms. Fangfei Liu, Mr. Darlington Yawson, Dr. Md. Isreq Real

Key Resource: Leaching Environmental Assessment Framework (LEAF) Methods and Guidance (EPA website)

Introduction to LEAF

The U.S. Environmental Protection Agency (EPA) Leaching Environmental Assessment Framework (LEAF) is a suite of standardized test methods for evaluating contaminant release from solids under environmentally relevant conditions (Table 1). The four leaching methods within LEAF were originally validated for inorganic constituents and included as standard leaching methods EPA 1313 – 1316 within Update V of SW-846. To address the need for standardized tests to evaluate PFAS leaching and mobility, LEAF methods have been optimized and demonstrated for use with PFAS (Methods 1313A-1316A). This article will focus on Methods 1313A, 1314A, and 1316A. Demonstration of Method 1315A for compacted granular materials (including concrete and asphalt) is ongoing.

Table 1. EPA SW-846 methods that comprise the LEAF framework

Method	Description
1313	Liquid-solid partitioning as a function of extract pH using a parallel batch extraction (i.e., equilibrium) procedure (Figure 1)
1314	Liquid-solid partitioning as a function of liquid-solid ratio (L/S) for constituents in solid materials using an up-flow percolation column procedure (Figure 3)
1315	Mass transfer rates of constituents in monolithic or compacted granular materials using a semi-dynamic tank leaching procedure
1316	Liquid-solid partitioning as a function of L/S using a parallel batch extraction (i.e., equilibrium) procedure (Figure 2)
Note: Text shown in bold indicates primary condition evaluated in each method.

Recommended Approach

Figure 1: First-order rate constants for abiotic reductive dechlorination of TCE under anaerobic conditions. Circles are data from Schaefer et al., 2021, filled squares from Schaefer et al., 2018, and Schaefer et al., 2017, and open squares from Schaefer et al., 2025.

Figure 2: Flowchart diagram of field screening procedures

The recommended approach builds upon the methodology and findings of a recent study, emphasizing field-based and analytical techniques to quantify abiotic first-order reductive dechlorination rate constants for PCE and TCE in clayey soils under anoxic conditions. Key components of this evaluation are listed below:

1. Zone Identification: The focus of the investigation should be to delineate clayey zones adjacent to hydraulically conductive zones.
2. Ferrous Mineral Quantification: Assess ferrous mineral context in clay via 1% HCl extraction at ambient temperature over a 10-minute interval.
3. Mineralogical Characterization: Conduct XRD analysis with the specific intent of identifying the presence of pyrite and biotite.
4. Reduced Gas Analysis: Measurement of reduced gases such as acetylene, ethene, and ethane concentrations in clay samples. Gas-tight sampling devices (e.g., En Core® soil samplers by En Novative Technologies, Inc.) should be used to ensure sample integrity during collection and transport.

Clay samples should be collected within a few centimeters of the high-permeability interface, with optional additional sampling further inward. For mineralogical analysis, a defined interval may be collected and subsequently subsampled. To preserve sample integrity, exposure to air should be minimized during collection, transport, and handling. Homogenization should occur within an anaerobic chamber, and if subsamples are required for external analysis, they must be shipped in gas-tight, anaerobic containers.

Estimation of the abiotic reductive first-order rate constant for PCE and TCE is based on the “reactive” ferrous content in the clay. Reactive ferrous content (Fe(II)_r) is estimated as shown in Equation 1:

Equation 1:       Fe(II)_r = DA + XRD_pyr - XRD_biotite

where DA is the ferrous content from the dilute acid (1% HCl) extraction, XRD_pyr is the pyrite content from XRD analysis, and XRD_biotite is the biotite content from XRD analysis.

Abiotic dechlorination is unlikely to contribute to mitigating contaminant back-diffusion when reactive ferrous iron (Fe(II)_r) concentrations are below 100 mg/kg (Figure 1). For Fe(II)_r above 100 mg/kg, the first-order rate constant for PCE and TCE reductive dechlorination can be estimated using the correlation shown in Figure 1. The rate constant exhibits a strong positive correlation with the logarithm of reactive Fe(II) content (Pearson’s r = 0.82), with a slope of 4.7 × 10⁻⁸ L g⁻¹ d⁻¹ (log mg kg⁻¹)⁻¹.

Figure 2 presents a decision flowchart designed to evaluate the significance and extent of abiotic reductive dechlorination. By applying Equation 1 to the dilute acid extractable Fe(II) plus measured mineral species data from clay samples, the reactive ferrous iron content (Fe(II)_r) can be quantified, enabling a streamlined assessment of the extent to which abiotic processes are contributing to the mitigation of contaminant back-diffusion.

If Fe(II)r is ≥ 100 mg/kg, a first-order dechlorination rate constant can be estimated and subsequently used within a contaminant fate and transport model. However, if acetylene is detected in the clay, even with Fe(II)r less than 100 mg/kg, then bench-scale testing using methods similar to those described in a recent study is recommended, as such results would likely be inconsistent with those shown in Figure 1, suggesting some other mechanism might be involved, or that the system mineralogy might be more complex than anticipated. Even if Fe(II)r ≥ 100 mg/kg, confirmatory bench-scale testing may be conducted for additional verification and to refine estimation of the abiotic dechlorination rate constant.

Summary and Recommendations

The approach outlined above is intended to serve as a generalized guide for practitioners and site managers to cost-effectively determine the extent to which beneficial abiotic reductive dechlorination reactions are likely occurring in low permeability (e.g., clayey) zones. This approach may be contraindicated if co-contaminants are present. It is currently unclear whether other classes of potentially reactive chemicals, such as trinitrotoluene (TNT) or chlorinated ethanes, could interact competitively with PCE and TCE.

In addition, it remains unclear how other classes of compounds such as per- and polyfluoroalkyl substances (PFAS) may interact or sorb with ferrous minerals and potentially inhibit abiotic dechlorination reactions. Coupling these recommended activities with conventional site investigation tasks would provide an opportunity to perform many of the up-front screening activities with minimal additional project costs. It is important to note that the guidance proposed herein pertains to particularly low permeability media. Sites with complex or varying lithology, where the mineralogy and/or redox conditions may vary, might require evaluation of multiple samples to provide appropriate site-wide information.

See Also

• ESTCP Project ER20-5031 – In Situ Verification and Quantification of Naturally Occurring Dechlorination Rates in Clays: Demonstrating Processes that Mitigate Back-Diffusion and Plume Persistence