US Department of Defense–Funded Fate and Transport Research on Per- and Polyfluoroalkyl Substances at Aqueous Film–Forming Foam–Impacted Sites

Abstract

The US Department of Defense (DoD) has made significant investment in understanding the fate and transport of per- and polyfluoroalkyl substances (PFAS) in the subsurface, primarily through the Strategic Environmental Research and Development Program (SERDP) and the Environmental Security Technology Certification Program (ESTCP). The SERDP is the DoD's environmental science and technology program and invests across a broad spectrum of basic and applied research, whereas ESTCP is the DoD's environmental technology demonstration and validation program and is intended to collect cost and performance data to overcome implementation barriers. Through these 2 programs, the DoD has funded a variety of projects to date related to the occurrence, transformation, and retention of PFAS in both the unsaturated and saturated subsurface, which have proven critical to the development of accurate conceptual site models (CSMs) and optimal remedial strategies. The chronology of SERDP/ESTCP projects related to PFAS fate and transport can be found online at https://www.serdp-estcp.org/ (see “Featured Initiatives” tab located on the website), and is highlighted in Figure 1. This Focus article is intended to summarize this work. The need for PFAS fate and transport research has become more pressing in recent years as the DoD has recently committed to investigating the occurrence and environmental distribution of PFAS at thousands of potentially contaminated sites where aqueous film–forming foam (AFFF) was used. These AFFF-impacted sites are characterized by a variety of operational scenarios involving AFFF since 1970 (see Interstate Technology Regulatory Council 2018 for additional detail). A current summary of groundwater concentrations for those PFAS routinely reported by DoD-accredited commercial laboratories observed within representative source zones is provided in Figure 2; perfluorohexane sulfonic acid and perfluorooctane sulfonic acid are among those PFAS observed with the greatest concentrations and detection frequencies (>90%), suggesting that electrochemical fluorination–based AFFF was historically used at most sites (Interstate Technology Regulatory Council 2018 provides additional detail). Although the DoD is currently focused primarily on confirming PFAS contamination at all AFFF-impacted sites and mitigating any resulting potential for human exposure, program-wide efforts to delineate the full extent of all PFAS contamination, to include source strength evaluations and human and ecological risk assessments, are forthcoming. The SERDP/ESTCP-funded research on PFAS fate and transport began in 2011 (Figure 1) when it became obvious that AFFF formulations were complex and that little progress could be made toward site remediation without understanding their composition and behavior in the subsurface. Although most of these projects are still in progress, the earliest projects have been completed. Early pioneering projects were led by Jennifer Field from Oregon State University and Chris Higgins from the Colorado School of Mines and have generated a foundation of knowledge for the research that has followed. Recent projects have been guided by a 2017 workshop on environmental PFAS research needs (Strategic Environmental Research and Development Program 2017), the results of which are summarized herein as they relate to fate and transport (Textbox 1). The following review of SERDP/ESTCP-funded research in this area is not intended as a thorough summary of the entire literature but rather provides insight into the unique problems posed by AFFF-impacted sites and the current status of the DoD efforts to solve those problems. Fundamental fate and transport questions critical for AFFF-impacted site management that have guided SERDP/ESTCP investments include the following. 1) What is the composition of AFFF formulations? Understanding AFFF composition (both the PFAS and non-PFAS constituents) is important to focus research on the most important perfluoroalkyl acids (PFAAs) present at AFFF-impacted sites, as well as other PFAS and non-PFAS constituents that are or may eventually be of regulatory concern. 2) Can we measure and predict the magnitude of PFAS fate and transport processes? The cumulative (and potentially interactive) effects of all applicable processes may mitigate or in some cases exacerbate transport. Accurate models, even at the screening level, would help managers faced with prioritizing sites and designing remediation strategies. 3) What is the fate of PFAA precursors in various AFFF formulations under different conditions? Precursors can represent a significant proportion of the total PFAS mass in some AFFFs and may represent an ongoing environmental source of PFAAs. 4) How do non-PFAS AFFF constituents affect PFAS fate and transport? Non-PFAS AFFF constituents can potentially sequester PFAS, may facilitate transport as a cosolvent, or may compete for sorption/retention sites. If biodegradable, they also may consume electron acceptors, preventing or delaying precursor biotransformation. 5) What measurements and monitoring methods should the DoD use to adequately characterize AFFF-impacted sites? Given the DoD's aggressive plans for investigating thousands of sites and the current lack of accurate models, there is an urgent need for characterization guidance to foster efficient delineation efforts and inform management decisions. The first SERDP statement of need (SON) relevant to PFAS fate and transport following the 2017 workshop focused on AFFF source zones. The overall goal was to improve predictions and measurements of vadose zone processes over time. Six projects were funded under this SON, each comprising different strategies to interrogate the various processes and develop predictive models useful for different purposes. General objectives were to investigate 1) the fate of precursor PFAS, and 2) the extent to which residual PFAAs retained throughout vadose zone soils contribute to groundwater contamination. These projects are briefly summarized individually in Table 1. All 6 projects are ongoing. Two subsequent SERDP SONs have resulted in funded projects specific to fate and transport (Figure 1). The first SON aimed to develop improved forensic methods and tools for PFAS source tracking and allocation. General objectives included 1) evaluation of conventional or novel analytical techniques or methodologies to differentiate PFAS from AFFF versus non-AFFF sources; 2) development of spectral libraries of PFAS to include both AFFF-derived PFAS as well as PFAS derived from other sources; and 3) development of improved analytical methods and/or validated models to predict changes to AFFF mixtures over time, including chemical pathways to the most toxic compounds. Six projects were funded under this SON as well and are individually summarized in Table 1. All 6 projects will be initiated in 2020. The other recent applicable SON is specific to analytical methods to assess the leaching and mobility of PFAS. Standard operating protocols for such methods would be helpful for screening soils and sediments for treatment and/or disposal. The goal was to develop and validate a standard analytical method, similar to the Synthetic Precipitation Leaching Procedure (US Environmental Protection Agency 1994) commonly used for other contaminants, that could be used to support decision-making for PFAS site investigations and source zone management and possibly to evaluate stabilization and soil-washing treatment technologies. One project (ER20-1126) was selected under this SON and will be initiated in 2020 (see Table 1 for more detail). Only 2 ESTCP projects specific to PFAS fate and transport have been funded to date. Project ER20-1633 is ongoing and intended to provide a case study of soil and groundwater delineation at an AFFF-impacted site, with a careful accounting of mass distribution between the vadose and saturated zones, including PFAAs and precursors. Site-specific factors (e.g., organic carbon content, ion exchange capacity, redox conditions) that mitigate PFAS transport will be demonstrated at field scale utilizing an array of novel characterization tools, including the first demonstration of particle-induced gamma-ray emission spectroscopy to measure total organic fluorine in environmental matrices and the total oxidizable precursor assay. The former fire-training area under investigation also is located near a former wastewater-treatment facility, which may enable fingerprinting of AFFF-based sources compared to non-AFFF sources. The results are intended to provide a protocol for efficient AFFF site investigations and quantitative CSMs to guide selection of mitigation and remediation strategies. Finally, the detailed plume characterization will provide information on transformation pathways and estimates of natural attenuation under prevailing groundwater redox conditions that can be generalized across the DoD's portfolio of AFFF-impacted sites. The other ESTCP project (ER20-5088) is a new start and aims to attain insight into the transformation and mass discharge of PFAS from AFFF sources that reside in the vadose zone and capillary fringe and to understand how these processes change over time with AFFF composition and mass. Lysimetry will be used to sample soil water to focus on the unique processes that occur in the vadose zone (e.g., varying saturation as a result of wetting and drying, aerobic/anaerobic cycling, creation of air–water interfaces, and release of natural colloids) at field sites. A key aspect of this research is to assess, describe, and quantify the relationship between PFAS mass removal and mass discharge (i.e., the source strength). Determining the processes that control the source strength and identifying methods to characterize this relationship for AFFF-impacted sites would greatly improve site characterization approaches. The research to date demonstrates that AFFF-impacted sites have unique characteristics that complicate PFAS fate and transport. Different AFFF formulations were used over time, and all of these contained many different PFAS, as well as a wide range of other constituents that can interact with these PFAS. There is still uncertainty regarding the full composition and sources of PFAS and cocontaminants among the portfolio of DoD sites, but there is consensus specifically regarding the following: 1) historic AFFF discharges exclusively occurred at the ground surface given the operational context, 2) retention of PFAS in soil is significant (Anderson et al. 2019), and 3) aerobic biological pathways dominate the transformation of precursors in soil and the subsurface (see Interstate Technology Regulatory Council 2018 for a summary of relevant literature). Therefore, vadose zone processes are critical to accurate CSMs and, consequently, have become a priority topic area of the PFAS fate and transport research funded by SERDP/ESTCP. Although much of the SERDP/ESTCP-funded research on PFAS retention is ongoing, the work to date (and other published research) has resulted in several overarching conclusions. Specifically, soil retention of PFAS is primarily affected by 1) the specific chemical and physical properties of the individual PFAS to specifically include the perfluorinated chain length and functional group (e.g., Anderson et al. 2016); 2) pH-dependent electrostatic sorption to various clay minerals (e.g., Li et al. 2019); 3) hydrophobic sorption to soil organic matter (e.g., Anderson et al. 2019), specifically including the proteinaceous components (Li et al. 2019); and 4) fluid–fluid interactions, including both air–water and nonaqueous phase liquid–water interfacial sorption (e.g., Costanza et al. 2019; Schaefer et al. 2019). Moreover, these processes are all enhanced to some extent in soil solution relative to reagent water (typically utilized in experimental studies for simplicity) because of pH, ionic strength, and cation composition effects (e.g., Costanza et al. 2019; Schaefer et al. 2019). Collectively, these conclusions demonstrate the novelty and criticality of PFAS fate and transport processes to accurate CSMs and remedial strategies. Complicating factors to field-scale predictions, however, currently are thought to include 1) lack of methods to assess the rate and extent of precursor biotransformation; 2) nonideal PFAA transport behavior attributable to complex sorption/desorption kinetics, resulting in attenuation over time (e.g., Brusseau et al. 2019); and 3) greater retardation of linear versus branched isomers of a given PFAS (e.g., Kärrman et al. 2011). Additional uncertainty highlighted in recent literature results from the extrapolation of experimental observations (typically performed at relatively high concentrations because of experimental constraints) down to environmentally relevant concentrations that are highly dependent on the selected sorption isotherm (Schaefer et al. 2019). Moreover, the various retention processes have to date been studied independently but could potentially interact in some way depending on transient site conditions (e.g., soil moisture content). Clearly, research funded by SERDP/ESTCP and others has overwhelmingly demonstrated the complexity of AFFF-derived PFAS behavior in soil and the subsurface and, thus, the critical need for additional studies, in particular studies that provide rigorous field validation of transport models in support of quantitative groundwater mass discharge estimates. Data are available from Richard Anderson ([e-mail]).