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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 Resources:

• Development of Leaching Tests for Materials Containing SVOCs and PFAS, EPA 600/R-23/382

• 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.

Method Development for PFAS

Complete details of the development of LEAF Methods 1313A, 1314A, and 1316A for use with PFAS are available in Garrabrants et al., 2024. Representative method modifications include:

• Materials of construction for experimental apparatus: containers used for leaching vessels (Methods 1313A, 1316A) and column construction materials (Method 1314A) evaluated for background PFAS and PFAS uptake.
• Reagents and eluant composition: eluant composition was optimized to use 1 mM CaCl2 to reduce formation of colloidal matter; Method 1313A pH adjustment now conducted with nonoxidizing HCl.
• Experimental conditions: Longer equilibration times (e.g., Method 1313, 1316) may be required due to slow desorption kinetics of certain PFAS from soil and organic matrices, implementation of settling to facilitate separation of solids from eluates.
• Eluate processing (all methods): Use of centrifugation in lieu of eluate filtering, sonication of bottle prior to eluate subsampling.

Batch Test Demonstration Studies

PFAS-specific adaptations were tested in batch test demonstration studies, which included triplicate implementation of Methods 1313A and 1316A in four AFFF-impacted site soils.

Figure 1: Figure 1. a) Overview of LEAF Method 1313A and b) PFHxS leaching as a function of pH. Definitions: lower limit of quantification (LLOQ) and method detection limit (MDL)

Draft Method 1313A was used to evaluate pH-dependent leaching in PFAS-contaminated soils in parallel batch extractions where each set of batch reactors is prepared and equilibrated at different pH (Figure 1). Short-chain PFAS (≤6 fluorinated carbons) generally showed little to no variation in leaching across the tested pH range of 2-13 (e.g., PFHxS, Figure 1), whereas long-chain PFAS exhibited increased leaching at higher pH. This trend is consistent with previous findings showing that soil-water partitioning coefficients (K_d) decrease as pH increases (e.g., Higgins and Luthy 2006). The most pronounced pH effects were observed for perfluoroalkyl sulfonamides (FASAs) such as perfluorooctane sulfonamide (FOSA), which transition from neutral to anionic forms within the circumneutral pH range (~pH 6). The anionic form has a lower K_d and results in higher leaching concentrations. For many site management scenarios where pH is circumneutral, variations in anionic PFAS leaching are expected to be small over the relevant pH range. In such cases, when testing time and costs are primary considerations, Method 1313A may be a lower priority relative to evaluating leaching as a function of L/S (Method 1316A, Method 1314A). Different considerations may be needed where FASAs or PFAS with multiple, ionizable functional groups (i.e., zwitterions) are of concern.

Figure 2: a) Overview of LEAF Method 1316A and b) PFHxS leaching as a function of L/S ratio evaluated in parallel batch leaching vessels

Draft Method 1316A was used to evaluate L/S-dependent leaching of PFAS in impacted soils using parallel batch extractions where each set of batch reactors is prepared and equilibrated at a different L/S. (Figure 2). Methods 1314A and 1316A are similar in intent as they both evaluate leaching as a function of L/S; however, the experimental approach differs. Method 1314A uses a flow-through column configuration (Figure 3; discussed further below). Method 1314A may better simulate field conditions, but Method 1316A is simpler and less costly to implement. Trends in Method 1314A and 1316A are expected to be qualitatively similar but leaching concentrations are expected to exhibit differences. Despite this, leaching studies comparing Methods 1314A and 1316A for inorganics showed that cumulative release results were within one order of magnitude.

Method 1316A and Method 1314A may also provide different insights into transport mechanisms. Because Method 1316A is performed using equilibrated batch reactors at varying L/S, results can be used to develop equilibrium desorption isotherms and calculate desorption coefficients (e.g., K_d,desorption). Studies have shown that K_d,desorption values for PFAS may be greater than K_d, an effect often attributed to desorption hysteresis. Consequently Method 1316A provides a straightforward method to estimate site-specific desorption parameters. Although sorption parameters can also be inferred from column (Method 1314A) data, interpretation is often complicated by nonequilibrium processes. Conversely, the column data can be valuable for quantifying those additional mechanisms providing transport parameters that can describe rate-limited transport (e.g., fraction of non-equilibrium sorption sites and sorption rates) and other dynamic behavior.

As anticipated, trends in PFAS leaching obtained during the Method 1316A and Method 1314A demonstrations were qualitatively similar. These are further discussed below.

Column Test Demonstration Studies

Figure 3. a) Overview of LEAF Method 1314A and b) PFHxS leaching as a function of ∑(L/S) evaluated in saturated up-flow column tests. Note that acrylic here simply refers to the column material of construction used in this round of Method 1314 testing.

PFAS-specific adaptations were tested in column test demonstration studies, which included triplicate implementation of Method 1314A in three AFFF-impacted site soils.

Draft Method 1314A was implemented in saturated, up-flow columns to evaluate leaching of PFAS as a function of cumulative L/S (∑(L/S)); Figure 3; total volume of water that has passed through the column divided by the soil mass in the column). As noted, the intent of Method 1314a and 1316a is similar, and in both tests, similar qualitative results were observed. For example, short-chain PFAS exhibited high initial concentrations that decreased rapidly. However, in Method 1314a, these rapid drops in short-chain PFAS tended to occur by ∑(L/S) ≈ 2 (e.g., Site 1 and 3 soils, Figure 3) whereas in some cases, such as for PFHxS, Method 1316A produced slightly flatter elution curves than Method 1314A (Figures 2b and 3b). Long-chain PFAS generally displayed flatter elution profiles than short-chain PFAS across both methods. These trends are consistent with chain length dependent sorption documented in the literature. Although column modeling is beyond the scope of this article, prior studies have shown that saturated transport can be influenced by rate-limited desorption, particularly for long-chain PFAS. As noted, data from Methods 1316A and 1314A can support estimation of transport parameters representing equilibrium and nonequilibrium behavior, respectively.

LEAF Screening Evaluations

Figure 4. Example screening assessment for perfluorooctane sulfonate (PFOS) using total content and data from Methods 1313A and 1314A. Figure format adapted from Garrabrants et al. 2021.

LEAF provides a standardized, robust approach for evaluating PFAS release from impacted granular materials under a range of environmental conditions. The tests are complementary, capture a range of conditions, and vary in ease of implementation. This provides the flexibility for users to select the test or test combinations that best suit their project objectives, timeline, and budget. A common use of LEAF data is in screening level assessments. These are stepwise assessments that establish increasingly refined maximum leaching concentrations, C_leach,max (Figure 4), which can then be compared to regulatory limits such as maximum contaminant levels, when available. For example, a stepwise screening assessment might include:

1. Assume the maximum leaching concentration is represented by the total content (total initial mass of contaminant present) leaching into the first L/S.
2. Assume only the available content leaches into the first L/S where available content is the maximum mass released over pH 2-13 measured using Method 1313a. For many PFAS, total content is equal to available content meaning that all of the PFAS mass is available for leaching.
3. Assume the leaching concentration at natural pH (measured in Method 1313A at natural pH or Method 1316A at L/S of 10) is maximum leaching concentration adjusted to the first L/S.
4. Consider the maximum leaching concentration over the L/S range (Method 1314A or Method 1316A) and the upper estimate of leaching, or C_leach,max, is the concentration from either Step 3 or Step 4, whichever is greater.

Screening assessments may be sufficient to meet project goals, but when additional refinements of leaching estimates are needed, site-specific data (e.g., infiltration) can be combined with test data and computational approaches (e.g., fate and transport models) for more site-specific estimates of leaching. Example scenarios where LEAF may be used to evaluate PFAS-impacted solids include 1) estimating PFAS release from AFFF-impacted soils, 2) estimating PFAS release from biosolids-amended soils at land application sites, 3) providing transport parameters to model PFAS transport from the source zone to the saturated zone, and 4) evaluating PFAS release from treatment residuals such as soils or sediments treated by soil washing or thermal approaches.

Summary and Ongoing Research

The LEAF framework offers a reliable, replicable approach to evaluating PFAS release from solids. With recent adaptations for PFAS-specific considerations, LEAF methods provide valuable tools for regulators and practitioners in managing PFAS-contaminated materials and assessing long-term environmental risks. However, there are key areas of ongoing research, including:

• An interlab validation of Methods 1313A, 1314A, and 1316A in coordination with the EPA
• Optimization and demonstration of Method 1315A for use with PFAS-impacted solids
• Evaluation of an unsaturated Method 1314A protocol to assess the need for and ability of LEAF testing to capture air-water interfacial partitioning of PFAS
• Application of the total oxidizable precursor (TOP) assay for evaluating the maximum additional PFAS leaching that may occur as a result of polyfluoroalkyl precursor transformation
• Comparison of LEAF testing data to previously collected field-scale enhanced flushing data collected from the same site

Additionally, there are key areas for consideration in future research:

• Collection of paired field-laboratory data under ambient conditions to further validate the applicability of LEAF assessments for estimation of field-relevant PFAS leaching and mobility
• Consideration of biotransformation in modeling and interpretation of LEAF data, as the state of the science regarding biotransformation of polyfluoroalkyl substances to terminal perfluoroalkyl acids advances

Other LEAF Resources

There are numerous resources describing the development of the LEAF leaching methods for inorganics. They are not specific to PFAS, but are still valuable resources focused on LEAF implementation, applications, and management of LEAF data. They include:

• Leach XS Lite - a tool for LEAF data management and visualization; free to download after registering for a free license key
• LEAF “How-To” Guide - guidance on LEAF background, implementation, test result interpretation; includes case studies
• USEPA LEAF Methods and Guidance homepage