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Alternative Methodologies for Assessing PFAS Soil Leaching
Nathan Hagelin, Jim Feild, Bethany Flynn
Wood Technical Consulting Solution
The objective of this presentation is to present two case-study examples of developing site-specific soil leaching criteria.  
Abstract

As our attention turns from site characterization to remediation for the emerging contaminant class, per- and polyfluoroalkyl substances (PFAS), we are struck pondering not only the treatment challenges these recalcitrant compounds present, but the scale of the problem and lack of consensus on how clean is clean. With guidelines and standards for aqueous media established at low parts per trillion, the handful of default standards for soil leaching are low. Further, standard leaching procedures such as synthetic precipitation leaching procedure (SPLP) yield detectable PFAS from soils with very low PFAS concentrations, single digit parts per billion. But are these technically justifiable approaches to determining remediation liability, or are they overly conservative methods that would result in bankrupting responsible parties if enforced?

PFAS release areas vary widely in mechanism and source. Many are aged and may be at a point of steady state with regards to leaching to groundwater. Many site-specific factors influence the complex behavior of PFAS in release areas. PFAS retention is controlled by PFAS properties, compound chain length, functional group, co-contaminants, stratigraphy/geology, depth to water, and soil properties including grain size, cation/anion exchange capacity, organic carbon content and pH. The analysis of soil physiochemical properties is used to evaluate how clay content, presence of organic matter, cation and anion exchange capacities, and pH affects PFAS sorption to soil. Calculation of site-specific soil screening levels (SSLs) using the US Environmental Protection Agency’s Regional Screening Level calculator can result in very low SLLs, below laboratory detection limits; any detection of PFAS may require mitigation to protect groundwater.
Direct measurement of soil leaching by lysimetry may provide the most accurate, site-specific measure of soil leaching. Porewater data from lysimeters can provide a definitive measurement of the spatially integrated in-situ mass discharge of PFAS to groundwater, and account for site-specific soil retention processes and rates. Relevant retention processes for PFAS include hydrophobic interactions with soil organic matter, electrostatic interactions with soil mineral phases, and air-water interfacial partitioning. These processes are non-linear, irreversible to some extent, and depend on the composition of the soil solution (both the cation/anion composition and total dissolved solids).

This presentation will present two case-study examples of developing site-specific soil leaching criteria. The first is a recently completed assessment at a fire training area (FTA) where site-specific data was used to calculate SSLs. This example suggests that well-drained soils at a decommissioned FTA may no longer be contributing to groundwater impacts. The second case study presents a methodology for using lysimetry as a tool to understand the site-specific mass discharge of PFAS to groundwater at two aqueous film forming foam source areas. The sites will be instrumented with shallow/deep lysimeter pairs and porewater will be collected over four quarters for analysis of 16 PFAS compounds. As part of the investigation, the vertical soil profile will be characterized through use of a hydraulic profiling tool to determine the location of transmissive zones and for visual soil logging prior to installation. Soil samples will be collected at the same depth as the lysimeter is installed and analyzed for 16 PFAS compounds. Soil samples from each area will also be for analyzed for total organic carbon, grainsize analysis, permeability, pH, and anion and cation exchange capacity.

The goal of these studies is to advance technically robust assessment methods for soil leaching that will focus remediation efforts on soils that actually present an ongoing risk to groundwater.

Nathan Hagelin, Principal and Global Technical Leader, Environmental Remediation, Wood
Nathan Hagelin is a Principal and a Global Technical Leader for Environmental Remediation at Wood. He is the remediation technology leader in Wood’s Emerging Contaminants Work Group. He is a Certified Geologist, Licensed Environmental Professional, and Board Certified Environmental Scientist working for 30 years on the remediation of contaminated industrial properties and military installations. He has prior experience as a Hydrologist with the U.S. Geological Survey Water Resource Division.

Air Emissions Testing for PFAS: Methods, Procedures and Challenges
Jennifer Son1, Tara Bailey1, John Kirby2, Johnsie Lang2
1Arcadis Canada Inc.
2Arcadis U.S. Inc.
The objective of this presentation is to provide insight on air emissions and related diffuse, large-area PFAS deposition, as well as discuss stack testing for PFAS and the series of challenges that is related to uncertainty to the analyte list, low required detection limits, absence of established methodologies and cost.  
Abstract

Per- and polyfluoroalkyl substances (PFAS) are incorporated into a wide range of products and manufacturing processes, in addition to aqueous film forming foams (AFFF). Although analytical method challenges remain, PFAS testing of soil and water (groundwater and surface water) are becoming routine. Potential air emissions and related diffuse, large-area PFAS deposition is being suspected at a growing number of sites. Stack testing for PFAS presents a series of challenges, reflecting uncertainty related to the analyte list, low required detection limits, absence of established methodologies and cost. The general lack of a regulatory framework and emission standards further complicate the objectives and outcomes of stack testing programs. Ahead of these challenges it is expected that regulatory requirements for stack testing will be forthcoming in the near future. A modified US Environmental Protection Agency (EPA) Method 23 procedure is the currently accepted stack testing practice which generally follows a dioxins/furans methodology. However, this current procedure includes a 7-fraction analysis, which requires each part of the stack testing sample train to be recovered, containerized, labelled and analyzed as separate distinct samples. Developments to the current PFAS stack testing procedure may allow reduction of overall sample fractions. Research and development of the test procedure continues to result in an evolving methodology.

Isokinetic stack testing procedures are commonly used to measure PFAS emissions since PFAS emissions typically include particulate matter and/or aerosols (aka condensable particulate). Additional considerations for stack testing include the requirement for straight laminar stack flow (minimum US EPA required straight run distances of 2.5 diameters are needed). In addition, moisture from combustion sources, sample durations to meet targeted emissions levels, and safe access to stack test ports with structural supports for sample trains (which can weigh upwards of 70 pounds) are necessary concerns when planning a stack test program for PFAS. The ability to analyze the samples collected using a decreased sample fraction analysis may reduce laboratory analytical and total overall costs significantly. Ultra-low detection levels for PFAS compounds are expected, similar to the low drinking water standards established for several PFAS compounds. Robust quality assurance and quality control programs are also required to identify cross contamination and ensure accurate measurement.

As the science of source testing for PFAS develops, it will help inform stakeholders in their evaluation of potential PFAS stack emissions. Coupled with actual emission standards and soil standards, this will lead to the ability to assess risk and make informed decisions related to mitigation.

Jennifer Son, Principal Environmental Engineer, Arcadis Canada Inc.
Jennifer Son is a Principal Environmental Engineer (P.Eng ON) at Arcadis Canada Inc. with over 18 years of professional experience. She currently leads the Strategic Environmental Consulting Service Service Line and works closely with the atmospheric sciences community of practice in Canada.

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