Challenges in Modeling PFAS Fate and Transport at NAPL-impacted Sites Volume 10, Issue 6 | October 2022

Applied NAPL Science Review

Challenges in Modeling PFAS Fate and Transport at NAPL-impacted Sites

Editor: Lisa Reyenga, PE

ANSR Scientific Advisory Board
J. Michael Hawthorne, PG, Board Chairman, GEI Consultants, Inc.
Andrew J. Kirkman, PE, BP Corporation North America
Robert Frank, RG, Jacobs
Paul Cho, PG, CA Regional Water Quality Control Board-LA
Randy St. Germain, Dakota Technologies, Inc.
Dr. Terrence Johnson, USEPA
Brent Stafford, Shell Oil Co.
Douglas Blue, Ph.D., Imperial Oil Environmental & Property Solutions (Retired)
Natasha Sihota, Ph.D., Chevron
Kyle Waldron, Marathon Petroleum
Danny D. Reible, Professor at Texas Tech University
Reeti Doshi, National Grid
Mahsa, Shayan, Ph.D., PE, AECOM Technical Services

Applied NAPL Science Review (ANSR) is a scientific ejournal that provides insight into the science behind the characterization and remediation of Non-Aqueous Phase Liquids (NAPLs) using plain English. We welcome feedback, suggestions for future topics, questions, and recommended links to NAPL resources. All submittals should be sent to the editor.

DISCLAIMER: This article was prepared by the author(s) in their personal capacity. The opinions expressed in this article are the author’s own and do not necessarily reflect the views of Applied NAPL Science Review (ANSR) or of the ANSR Review Board members.

Challenges in Modeling PFAS Fate and Transport at NAPL-impacted Sites

Mahsa Shayan, AECOM

Numerical modeling of fate and transport of per and polyfluoroalkyl substances (PFAS) are already being used in the industry to assess PFAS transport and distribution in the subsurface. This article reviews the challenges and uncertainties that need to be considered for developing proper conceptual site models (CSM) and limitations of the existing numerical modeling tools.

Introduction

Reactive transport models have selectively been used at non-aqueous phase liquid (NAPL)-impacted sites to simulate, quantify and apportion mass removal processes (e.g., advective/dispersive/diffusive transport, sorption, and abiotic/biotic reactions); predict contaminant concentration and distribution, determine degradation rates, facilitate remedial performance assessment, and support remediation system designs. The numerical modeling tools used at NAPL sites can be applied to the understanding of PFAS behavior in the presence of NAPL. Understanding PFAS fate and transport (F&T) in the subsurface is critical for developing enhanced conceptual site models that lead to better risk assessment and remedial plans. Numerical modeling can be a supporting tool for understanding the current contaminant distribution and predicting future PFAS fate and transport.

A key challenge for developing numerical models to simulate PFAS reactive transport in the subsurface is the significant variation of physical properties within this family of compounds. The PFAS family is comprised of thousands of compounds with different and unique fate and transport properties, many of which are not fully understood. Due to their surfactant nature, PFAS behave differently from most contaminants, such as hydrocarbons associated with NAPL dissolution.

This article discusses the uncertainties and data gaps that present challenges when developing a proper conceptual site model (CSM) and numerical models. To find our path through this fog of uncertainties, it is crucial to start with the knowledge we gained based on modeling other contaminants, track the evolving science around PFAS fate and transport properties, and apply sensitivity analysis to assess the importance of the unknown parameters and processes on the output of the PFAS fate and transport models. It is important to keep in mind that models are supporting tools that are only useful when their limitations and simplifying assumptions are considered.

PFAS Fate and Transport Properties

As with other contaminants, the fate and transport properties of PFAS depend on the chemical structure. PFAS comprises a Hydrophobic carbon-fluorine (C-F) “tail” of different lengths and a hydrophilic head. The hydrophilic head of PFAS compounds includes functional groups with different ionic charges that lend variable fate and transport characteristics such as solubility, volatility, and sorption properties specific to each compound (ITRC, 2022).

The table below shows a comparison of fate and transport properties for two of the most studied and regulated PFAS compounds, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), in comparison to more conventional contaminants, such as benzene. PFAS compounds have higher solubility, lower volatilization, lower sorption, and lower susceptibility to biotic and abiotic degradation in the subsurface due to their strong C-F bonds. Therefore, they have the potential to form longer and wider GW plumes which in turn increase the likelihood of groundwater to surface water discharges and overall increase the potential exposure for human and ecological receptors. It should, also be noted that the  lower toxicity thresholds for some PFAS compounds also led to the establishment of very low health advisory guidelines (ITRC, 2022) for these compounds, in turn influence the length of the plumes for these compounds at many sites.

Figure 1. PFAS F&T Properties in Comparison to a Few Conventional Contaminant Compounds. (Reference:  Woodward et al., 2016)

The fate and transport properties of PFAS are affected by both the length of the C-F chain and the charge state of the ions. Anionic (negatively charged) compounds are known to be more mobile and transport further distance from the source compared to cationic (positively charged) and zwitterionic (both positively and negatively charged compounds). The readers are referred to the ITRC PFAS guidance document (ITRC, 2022) to learn more about the chemical properties and classification of more PFAS compounds (i.e., short and long chain compound with various charge state).

The chart below extracted from the NGWA PFAS document is color coded from green to red as to where different PFAS are more likely to reside in vapor, aqueous or solid phase. The color coding from green to red shows an increased likelihood of presence in those phases. This chart is a useful guide when developing conceptual or numerical site models for PFAS-impacted sites. However, it should be noted that like many other contaminants, PFAS presence and distribution in the subsurface are highly site-specific and depends on other hydrogeology and geochemical properties of the sites (ITRC, 2022; Weber et al., 2017).

Figure 2. Likelihood of Presence of Various PFAS Compounds in Vapor, Aqueous or Solid Phases. (Reference:  NGWR, 2021)

Key Factors Complicating PFAS Mass Balance and Fate and Transport Modeling

Because PFAS compounds were historically not considered an environmental threat, at many sites the history of PFAS use and mass release is not fully known (ITRC, 2022; NGWR, 2021; Guelfo, 2021). Another challenging aspect of PFAS fate transport that is directly applicable to NAPL impacted sites is understanding and simulating complex retention mechanism of these compounds in the subsurface. Due to their surfactant nature, PFAS tend to reside at various interfaces including solid-phase sorption, and partitioning to the air-water interface, NAPL-water partitioning, and NAPL-air interfaces.

When simulating PFAS transport in the vadose zone, unlike many conventional contaminants, PFAS adsorption and retention at the air-water interface should be considered. Similarly, additional adsorption interfaces and corresponding partitioning coefficients should be considered when a NAPL source co-exists with a PFAS source in the subsurface. Depending on the type and concentration of the corresponding NAPL and PFAS compounds, the PFAS compounds are expected to be retained in the source area where NAPL is present. PFAS retention mechanisms are not only more complex than previous contaminants we previously modeled, but also for individual PFAS compounds these properties are different and yet not fully studied and evaluated for many compounds, which add more complexity for setting up comprehensive models. Further details on this topic can be found in the peer-reviewed literature (e.g., Brusseau et al., 2018-2022; Silva et al., 2019-2021; Guo et al., 2020;
McKenzie et al., 2016).

Table below provides a summary of additional factors complicating migration and mass balance estimation for PFAS. All these parameters should be taken into account when conducting numerical fate and transport modeling:

   Table 1. Summary of Factors Complicating PFAS F&T Modeling and Mass Balance Analysis

The readers are referred to the ITRC PFAS guidance document (ITRC, 2022) to learn more about how each of the above factors can affect the migration of PFAS through the surface and subsurface in various media (soil, sediment, surface water, groundwater, and air).

State of Science

To address the knowledge gap described above additional research and enhancements to the current modeling platforms are required. Filling the current knowledge gap will pave the way to building numerical models capable of predicting PFAS fate and transport in the subsurface and could better support risk assessment and design of remedial systems based on reliable modeling results. Many researchers are currently working on a range of relevant topics, and some examples are presented in the table below:

Table 2. Examples of Relevant Literature Available on These Topics While More Projects are Still Underway

The readers are referred to the ITRC PFAS guidance document (ITRC, 2022) and the interactive map of PFAS projects on the SERDP/ESTCP PFAS website (SERDP, 2022) to learn more about the ongoing and PFAS projects, and those focused on understanding and modeling PFAS fate and transport, in particular.

A Word of Caution

PFAS fate and transport models are already being used in the industry to assess PFAS transport and distribution in the subsurface. However, it should be noted that current modeling efforts are predominantly based on simplifying assumptions regarding the source parameters (i.e., exact location, PFAS mixture, precursors transformation and co-contaminant presence) while overlooking PFAS unique sorption properties or without taking into account the effect of historical or current remedial activities on source strength and effect of precursors biotransformation on plume evolution and persistence.

Given the current limitations, sensitivity analysis should be conducted to quantify the significance of uncertainties stemming from unknown parameters and processes that control PFAS fate and transport. It should be noted that in general,   models are supporting tools that provide one additional line of evidence that must be considered within the context of the full site conceptual model (CSM) and our current understanding of the dominant PFAS fate and transport properties.

References 

Anderson, R. Hunter, Dave T. Adamson, and Hans F. Stroo. 2019. “Partitioning of poly- and perfluoroalkyl substances from soil to groundwater within aqueous film-forming foam source zones.” Journal of Contaminant Hydrology 220:59-65.

Anderson, R.H., Thompson, T., Stroo, H.F. and Leeson, A. 2021, US Department of Defense–Funded Fate and Transport Research on Per- and Polyfluoroalkyl Substances at Aqueous Film–Forming Foam–Impacted Sites. Environ Toxicol Chem, 40: 37-43.

Barton, C. A., M. A. Kaiser, and M. H. Russell. 2010. ” A site-specific screening comparison of modeled and monitored air dispersion and deposition for perfluorooctanoate.” Journal of the Air and Waste Management Association 60.

Brusseau, Mark L. 2018. “Assessing the potential contributions of additional retention processes to PFAS retardation in the subsurface.” Science of The Total Environment 613-614:176-185.

Brusseau, Mark L. 2019. “The influence of molecular structure on the adsorption of PFAS to fluid-fluid interfaces: Using QSPR to predict interfacial adsorption coefficients.” Water Research 152:148-158.

Brusseau, Mark L. 2019. “Estimating the relative magnitudes of adsorption to solid-water and air/oil-water interfaces for per- and poly-fluoroalkyl substances.” Environmental Pollution 254:113102.

Brusseau, Mark L. 2020. “Simulating PFAS transport influenced by rate-limited multi-process retention.” Water Research 168:115179.

Brusseau, M. L., and B. Guo. 2022. “PFAS concentrations in soil versus soil porewater: Mass distributions and the impact of adsorption at air-water interfaces.”  Chemosphere 302:134938.

Costanza, Jed, Linda M. Abriola, and Kurt D. Pennell. 2020. “Aqueous Film-Forming Foams Exhibit Greater Interfacial Activity than PFOA, PFOS, or FOSA.” Environmental Science & Technology 54 (21):13590-13597.

Costanza, Jed, Masoud Arshadi, Linda M. Abriola, and Kurt D. Pennell. 2019. “Accumulation of PFOA and PFOS at the Air–Water Interface.” Environmental Science & Technology Letters 6 (8):487-491.

Field, J., D. Sedlak, and L. Alvarez-Cohen. 2017. Final Report – Characterization of the Fate and Biotransformation of Fluorochemicals in AFFF-Contaminated Groundwater at Fire/Crash Testing Military Sites. SERDP Project ER-2128.

Field, Jennifer A., and Jimmy Seow. 2017. “Properties, occurrence, and fate of fluorotelomer sulfonates.” Critical Reviews in Environmental Science and Technology 47 (8):643-691.

Galloway, Jason E., Anjelica V. P. Moreno, Andrew B. Lindstrom, Mark J. Strynar, Seth Newton, Andrew A. May, and Linda K. Weavers. 2020. “Evidence of Air Dispersion: HFPO–DA and PFOA in Ohio and West Virginia Surface Water and Soil near a Fluoropolymer Production Facility.” Environmental Science & Technology 54 (12):7175-7184S. and Brusseau, M. L., Contribution of nonaqueous-phase liquids to the retention and transport of per and polyfluoroalkyl substances (PFAS) in porous media. Environ. Sci. Technol. 2021, 55, (6), 3706-k.

Gefell, Michael J., Hai Huang, Dan Opdyke, Kyle Gustafson, Dimitri Vlassopoulos, John E. McCray,
Sam Best, and Minna Carey. 2022. “Modeling PFAS Fate and Transport in Groundwater, with and Without Precursor Transformation.” Groundwater 60 (1):6-14.

Guelfo, Jennifer L., Stephen Korzeniowski, Marc A. Mills, Janet Anderson, Richard H. Anderson, Jennifer A. Arblaster, Jason M. Conder, Ian T. Cousins, Kavitha Dasu, Barbara J. Henry, Linda S. Lee, Jinxia Liu, Erica R. McKenzie, and Janice Willey. 2021. “Environmental Sources, Chemistry, Fate and Transport of Per- and Polyfluoroalkyl Substances: State of the Science, Key Knowledge Gaps, and Recommendations Presented at the August 2019 SETAC Focus Topic Meeting.” Environmental Toxicology and Chemistry.

Guelfo, Jennifer L., Assaf Wunsch, John McCray, John F. Stults, and Christopher P. Higgins. 2020. “Subsurface transport potential of perfluoroalkyl acids (PFAAs): Column experiments and modeling.” Journal of Contaminant Hydrology 233:103661.

Guo, Bo, Jicai Zeng, and Mark L. Brusseau. 2020. “A Mathematical Model for the Release, Transport, and Retention of Per- and Polyfluoroalkyl Substances (PFAS) in the Vadose Zone.” Water Resources Research 56 (2):e2019WR026667.

ITRC 2022, PFAS Technical and Regulatory Guidance Document, Updated June 2022, https://pfas-1.itrcweb.org/

McKenzie, Erica, Robert L. Siegrist, John McCray, and Christopher Higgins. 2016. “The influence of a non-aqueous phase liquid (NAPL) and chemical oxidant application on perfluoroalkyl acid (PFAA) fate and transport.” Water Research 92:199-207.

NGWR 2021, PFAS Fate and Transport 2021, white paper updates section 4 of the NGWA document, Groundwater and PFAS: State of Knowledge and Practice, published in 2017. 2021 / 17 pages, Catalog #D1123.

Nickerson, A., A. Maizel, C. Olivares, C. E. Schaefer, and C. P. Higgins. 2021. Simulating Impacts of Biosparging on Release and Transformation of PFASs from AFFF-Impacted Soil. Environmental Science and Technology, 55(23):15744-15753.

Schaefer, C. E., D. M. Drennan, D. N. Tran, R. Garcia, E. Christie, C. P. Higgins, and J. A. Field. 2019. Measurement of Aqueous Diffusivities for Perfluoroalkyl Acids Journal of Environmental Engineering, 145(11):06019006.

SERDP/ESTCP PFAS Webpage, Accessed 2022, https://www.serdp-estcp.org/focusareas/e18ec5da-d0de-47da-99f9-a07328558149

Shin, Hyeong-Moo, Verónica M. Vieira, P. Barry Ryan, Russell Detwiler, Brett Sanders, Kyle Steenland,
and Scott M. Bartell. 2011. “Environmental Fate and Transport Modeling for Perfluorooctanoic Acid Emitted from the Washington Works Facility in West Virginia.” Environmental Science & Technology 45 (4):1435-1442.

Silva, Jeff A., Jiří Šimůnek, and John E. McCray. 2020. “A Modified HYDRUS Model for Simulating PFAS Transport in the Vadose Zone.” Water 12 (10).

Silva, Jeff A. K., William A. Martin, and John E. McCray. 2021. “Air-water interfacial adsorption coefficients for PFAS when present as a multi-component mixture.” Journal of Contaminant Hydrology 236:103731.

Silva, Jeff A. K., William A. Martin, Jared L. Johnson, and John E. McCray. 2019. “Evaluating air-water and NAPL-water interfacial adsorption and retention of Perfluorocarboxylic acids within the Vadose zone.” Journal of Contaminant Hydrology 223:103472.

Tokranov, Andrea K., Denis R. LeBlanc, Heidi M. Pickard, Bridger J. Ruyle, Larry B. Barber, Robert B. Hull, Elsie M. Sunderland, and Chad D. Vecitis. 2021. “Surface-water/groundwater boundaries affect seasonal PFAS concentrations and PFAA precursor transformations.” Environmental Science: Processes & Impacts.

Weber, Andrea K., Larry B. Barber, Denis R. LeBlanc, Elsie M. Sunderland, and Chad D. Vecitis. 2017. “Geochemical and Hydrologic Factors Controlling Subsurface Transport of Poly- and Perfluoroalkyl Substances, Cape Cod, Massachusetts.” Environmental Science & Technology 51 (8):4269-4279.

Woodward, D., Houtz, E., Burdick, J., Understanding PFAS Fate and Transport, Technical training for Waste Site Cleanup Professionals, Technical Presentation, 2016, accessible at https://www.newmoa.org/events/docs/239/PFAS_FateTransportWebinarNov2016.pdf

Research Corner

Per- and Polyfluoroalkyl Substances (PFAS) and Aqueous Film-Forming Foam Impacted Sites: New PFAS Discovery and Sorption of Anionic, Zwitterionic, and Cationic PFASs

Krista A. Barzen-Hanson
Doctor of Philosophy
Oregon State University

Abstract:

Public attention and concern about per- and polyfluoroalkyl substances (PFASs) are increasing due to detection of PFASs in drinking water supplies, the environment, including remote locations, and wildlife and to the lowering of the federal health advisory levels of perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) in drinking water. Aqueous film-forming foams (AFFFs), which typically contain anionic, zwitterionic, and cationic PFASs, are one route of environmental entry for PFASs. AFFFs were routinely applied since the 1960s to extinguish hydrocarbon-based fuel fires during emergencies and fire fighter training. Routine releases of AFFF into the environment have resulted in high concentrations (mg L) of PFASs in groundwater. Attention typically focuses on the well-known homologs of the perfluoroalkyl carboxylates (PFCAs) and perfluoroalkyl sulfonates (PFSAs), including PFOA and PFOS, and other anionic, zwitterionic, and cationic PFASs receive little attention. Recent data on AFFF-impacted groundwater indicates that ~ 25% of the PFASs are currently unidentified. A complete understanding of the composition of PFASs in AFFF-impacted groundwater is needed in order to investigate biodegradation pathways and to develop effective remediation techniques that capture PFASs with a wide range of water solubilities and subsurface mobilities. Zwitterionic and cationic PFASs present in groundwater, soil, and sediment have not been characterized with respect to partitioning (sorption) behavior. Sorption studies typically focus on a select number of well-known PFCAs and or PFSAs, and a limited number of studies simulate AFFF discharge field conditions. By enhancing understanding of zwitterionic and cationic PFAS sorption, transport and likely subsurface location (i.e. predominantly in groundwater or sorbed to soil) can better direct subsurface remediation efforts and mitigate off-site migration. Chapter 2 discusses a data analysis test for non-target analysis and the subsequent serendipitous discovery of two ultrashort chained PFSAs. Select 3M AFFFs and AFFF-impacted groundwater samples, each from 11 different U.S. military bases were analyzed using quadrupole time-of-flight mass spectrometry (qTOF-MS). Kendrick mass defect plots were used to identify known homologs within a homologous series. Careful inspection of the PFSA homologous series led to the serendipitous discovery of the C₂ and C₃ PFSAs in 3M AFFF and AFFF-impacted groundwater. The C₂ and C₃ PFSAs were quantified using liquid chromatograph tandem mass spectrometry. Chapter 3 uses the developed non-target data analysis strategy to attempt to close the mass balance of PFASs in AFFF-impacted groundwater. Select 3M and fluorotelomer AFFFs, commercial products, and AFFF-impacted groundwater samples from 15 different sites were used to identify the remaining PFASs. Liquid chromatography qTOF-MS was used for compound discovery. Nontarget analysis and suspect screening were conducted. For nontarget analysis, a ‘nontarget’ R script in combination with Kendrick mass defect plots aided in compound identification. Suspect screening compared detected masses against a list of previously reported PFASs. Forty novel classes of anionic, zwitterionic, and cationic PFASs were discovered, and an additional 17 classes of previously reported PFASs were observed for the first time in AFFF and or AFFF-impacted groundwater. All 57 classes received an acronym and IUPAC-like name. Overall, of the newly discovered PFASs, ~ 68% were zwitterionic or cationic PFASs. Chapter 4 selects the representative National Foam AFFF to determine the soil properties influencing the sorption of model anionic fluorotelomer sulfonates (FtSs), zwitterionic fluorotelomer sulfonamido betaines (FtSaBs), and the cationic 6:2 fluorotelomer sulfonamido amine (FtSaAm). Batch sorption experiments were conducted using the whole National Foam AFFF, with initial aqueous phase concentrations of the 6:2 FtSaB ranging from 1,000 to 138,000,000 ng L, which represent concentrations of dilute groundwater plumes up to the application of 3% AFFF used in fire fighter training and emergency responses. Six blank soils with varying organic carbon, cation exchange capacity (CEC) and anion exchange capacity as well as a select soil buffered to pH 4 and 7 were used to determine the factors predominantly impacting sorption. A new, aggressive soil extraction method was developed due to incomplete mass balance of the FtSaBs and the 6:2 FtSaAm using published extraction methods. Hydrophobic interactions drove the sorption of the anionic FtSs, while the FtSaBs were influenced primarily by CEC. The 6:2 FtSaAm was depleted from the aqueous phase in all but one soil, and therefore, sorption is likely driven by a combination of CEC and organic carbon.

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