Editor's perspective – Per‐ and polyfluorinated substances pose substantial challenges to remediation practitioners

This issue of Remediation is dedicated to one of the most pressing topics in the remediation industry – per- and polyfluorinated alkylated substances (PFASs). The articles comprising the issue address critical topics in the rapidly advancing area of PFAS investigation, assessment, and remediation. The articles were curated and coordinated by Dora Chiang, an AECOM Vice President, and Claudia Walecka-Hutchison, an EH&S Remediation Manager at The Dow Chemical Company. Each article was peer reviewed by at least two environmental experts specializing in the subject matter to improve the technical quality of the articles in this unique issue of Remediation. The above issues often have compounding factors on each other, further complicating the overall PFAS issue. For example, if a chemical is somewhat difficult to treat, but must be remediated to a low concentration, then the treatment complexity issue becomes magnified as remediation professionals strive to meet cleanup goals. Similarly, if polyfluorinated precursors exist that are migrating away from a source and, over time, chemically transforming into recalcitrant, and toxic, perfluorinated species, this complicates the ability to investigate the site. This is because PFAS migration and transformation can occur simultaneously; thereby, making delineation a moving target. The authors of these articles are leaders in the research and consulting fields who have dedicated substantial efforts to understanding, defining, developing, and refining the science and engineering underlying PFAS investigation, assessment, and remediation techniques. There are over 3,000 PFASs and, not only are the nomenclature and structures complex, but there are even inconsistencies within the remediation industry on how to refer to PFASs as a group. Until relatively recently, PFASs were commonly referred to as “PFCs”; however, PFCs is also an acronym for perfluorocarbons, such as those used as air conditioner refrigerants, which presented a source of confusion. When the industry, including the U.S. Environmental Protection Agency (USEPA), recognized this confusing issue, it began more routinely referring to the class of chemicals as PFASs and less so as “PFCs” (this is changing and most industry professionals now refer to the chemical class as PFASs, although not always). The common link within the PFAS class of chemicals is that each PFAS contains some version of the perfluoroalkyl moiety, CnF2n+1. PFASs can be in many different forms depending on the functional groups, similar to how the benzene-based class of mono-aromatic chemicals changes depending on the type and location of functional groups around an aromatic ring (e.g., ethylbenzene and toluene are both mono-aromatics with different alkane functional groups). However, because PFASs have up to 13 carbon-length chains and numerous different types of functional groups, there are a plethora of derivatives of PFASs. As a start, the two most common types of PFASs referred to in the literature are carboxylic acids and sulfonic acids. However, based on the various functional groups either embedded within the PFAS molecule or located at the terminal end of the molecule, the variations of PFASs are extensive. For example, there are alcohol-based fluorotelomers, fluorotelomer sulfonic acids, polyfluorinated alkyl phosphates, perfluorooctane sulfonamides, and others. The point is not to provide an in-depth, thorough explanation of PFAS chemistry, but impress that the complexity of PFAS chemistry extends beyond the 14 perfluorinated chemicals that comprise common PFAS analyte lists (the standard PFAS list reported under USEPA Method 537 contains 14 perfluorinated chemicals). Ross et al.’s article in this Remediation issue includes a brief overview of PFAS chemistry. The Interstate Technology and Regulatory Council (ITRC) recently developed a fact sheet with PFAS naming conventions (ITRC, 2017a) and Buck et al. (2011) provide additional details regarding PFAS nomenclature. Commercially available analytical techniques are only capable of quantitatively detecting a handful of PFAS precursors. Identifying additional precursors, whether qualitatively or quantitatively, requires a university-level research laboratory and highly qualified analytical chemists. Furthermore, the costs for various PFAS analyses are higher than more conventional contaminants. The cost to analyze a groundwater sample for volatile organic compounds (VOCs), which provides quantitative data on the parent and degradation byproduct concentrations, is approximately $70 to $90 and the laboratory turnaround time is one to two weeks. For PFASs, even a basic analysis may take three to six weeks and cost $300 to $400. Another issue with PFAS chemistry is that because the chemicals are used in so many consumer products, care must be taken when sampling to avoid potential cross contamination. Bartlett and Davis’ article discusses PFAS cross contamination issues and the need for conservative precautions in the field when sampling for PFASs. Generally speaking, most contaminants that exhibit toxicity to the degree of many PFASs do not readily migrate in soil and groundwater systems (e.g., dioxins and polychlorinated biphenyl Aroclor mixtures). However, PFASs tend to migrate in the environment in a similar manner as common VOCs. As a comparative example, perfluorooctanoic acid (PFOA) has a Koc of 17 to 230 liters per kilogram (L/kg; Agency for Toxic Substances and Disease Registry [ATSDR], 2015), which is similar to that of trichloroethene (TCE; Koc of 61 L/kg), and benzene (Koc of 150 L/kg); whereas, 2,3,7,8-tetrachlorodibenzodioxin and Aroclor 1260 have Kocs of 250,000 L/kg and 350,000 L/kg, respectively. PFOA will adsorb to soil and aquifer materials in a manner relatively similar to TCE and benzene, but has a common groundwater cleanup objective of 70 nanograms per liter (ng/L; as described later) more in line with 2,3,7,8-tetrachlorodibenzodioxin (maximum contaminant level [MCL] of 30 ng/L) and Aroclor 1260 (MCL of 500 ng/L). While there are a few regulated contaminants with cleanup objectives at the sub-part per billion (ng/L) level, such as dioxins, the remediation industry has not experienced a contaminant class with mobility like PFASs that also has such low cleanup objectives. These low cleanup objectives combined with the propensity of PFASs, both those with short and long chains, to readily migrate in soil and groundwater systems due to their low adsorptivity and recalcitrance, enhances the potential for PFAS releases to expand into large diffuse plumes with little retardation. In many cases, these plumes result from what would traditionally be considered relatively small releases. However, with a routine groundwater cleanup objective of no more than 70 ng/L, which is over 70 times lower than the 5 micrograms per liter (equal to 5,000 ng/L) cleanup objective commonly established for TCE and benzene and a similar Koc, the PFAS plume dimensions can be considerably larger than those of more common contaminants. Most of the regulated PFASs are considered recalcitrant and do not readily degrade in the environment. Further complicating the fate and transport landscape of PFASs is that many of the nonregulated PFASs that do tend to degrade, transform into regulated, nondegrading PFASs (see Casson & Chiang and Hatton et al.). Chemicals that tend to transform into regulated compounds are called precursors. The remediation industry is generally familiar with the concept of precursors through our experience with chlorinated ethenes, tetrachloroethene (PCE) and TCE, degrading to daughter byproducts, such as cis-1,2-dichloroethene and vinyl chloride. Similarly, many polyfluorinated species of PFASs transform, generally through oxidation, to form perfluorinated species, including PFOA and PFOS. However, PFAS precursor chemistry is substantially more complicated than the chlorinated ethene degradation chain for many reasons, including: (1) there are far more parent compounds capable of degrading to terminal perfluorinated species than the two common chlorinated ethenes (PCE and TCE); (2) the PFAS degradation pathways are not nearly as well defined as those for chlorinated ethenes; and, (3) commercially available analytical methods are not capable of detecting many PFAS precursors, as compared to EPA Method 8260 used for chlorinated ethenes that, while not capable of detecting every compound in the degradation chain, provides sufficient data to evaluate chlorinated ethene degradation at sites. The industry developed a partial solution to Reason #3 by analyzing for oxidizable precursors using the total oxidizable precursor assay (TOPA) – an analytical method that involves exposing samples to a strong oxidant and then measuring the perfluorinated species formed from the precursors. The TOPA analysis cost $500 to $800 per sample and the timeframe is dependent on laboratory capacity. Casson and Chiang provide an excellent description of the TOPA methodology and a case study for TOPA applications. The precursor issue also substantially complicates both the investigation and remediation phases of a cleanup project. Over time, as precursors migrate away from source areas, they can transform into regulated perfluorinated compounds. The application of oxidative treatment, such as in situ chemical oxidation, can exacerbate this transformation (see Dombrowski et al. for a chemistry-based explanation of this phenomenon). Lack of precursor distribution data and transformation rates complicate the understanding of the true footprint of a PFAS source or plume. The absence of or minimal precursor data creates uncertainty with respect to the spatial extent of remediation warranted at sites. Not addressing areas with precursors, which can migrate, may result in regulated PFASs showing up in areas not addressed by remedial efforts. The metaphor of keep your friends close and your enemies closer comes to mind as, in this case, it is difficult to really know your enemy unless the investigation involves elements of a TOPA combined with standard PFAS analysis. Even the combination of these two analyses only provides a general understanding of the precursor issue, as the TOPA is not all encompassing for every precursor and, thus, is somewhat qualitative (see Casson & Chiang for additional details). Given the number of PFASs (over 3,000) and the lack of toxicological data for the vast majority of them, the human health risks posed by PFAS contamination is difficult to ascertain. For example, ATSDR's toxicological profile for perfluoroalkyls is limited and only addresses the potentially toxic effects of perfluorinated chemicals (ATSDR, 2015). The currently accepted theory is that longer chain PFASs are more toxic than shorter chain PFASs and that perfluorinated species are more toxic than polyfluorinated species. However, polyfluorinated species can transform to perfluorinated compounds not only in the environment, as described earlier, but also in the human body. ATSDR (2015) states “[E]xposure to 8-2 fluorotelomer alcohol may result in the formation of PFOA as a metabolite within the body.” (ATSDR, 2015, p. 432). Therefore, it would seem prudent to regulate PFAS precursors, yet this is generally not the case. The fact is that regulators are faced with a very difficult situation and are forced to focus on the perfluorinated species that are perceived to pose the greatest risk. However, this does not address the entire risk profile potentially posed at PFAS sites. To date, the EPA has health advisories for two PFASs, PFOA and PFOS, and although some states have established guidance values for other PFASs; these are few and far between. This further complicates the PFAS issue because many industry experts anticipate substantial challenges as we can expect regulatory requirements to shift as more toxicological studies are completed. The ITRC has developed a fact sheet summarizing the regulatory levels for water and soil for the various U.S. states and countries that developed guidance or regulatory values as of November 2017 (ITRC, 2017b) – a very useful resource. Another significant issue is the relatively high toxicity of many PFASs. The EPA's lifetime health advisory for PFOA and PFOS is 70 ng/L for the two chemicals combined (USEPA, 2016a, 2016b). In addition, the New Jersey Department of Environmental Protection (NJDEP) recently established an MCL for PFOA of 14 ng/L (NJDEP, 2017). As described previously, these levels are extremely low, demonstrating the perceived toxicity of these compounds. It is important to recognize that as more is learned about PFASs and further toxicological studies are conducted on additional PFASs, the universe of regulated PFASs is almost certain to expand beyond PFOA, PFOS, and the other sparsely state-regulated perfluorinated chemicals in the PFAS class. There are no federal cleanup standards for PFASs in any environmental media, although USEPA issued the aforementioned health advisory levels for PFOA and PFOS. In addition, USEPA Region 4 issued health advisories for soil exposure in 2009 for PFOA (16 milligrams per kilogram [mg/kg]) and PFOS (6 mg/kg) based on the provisional health advisories in place at the time (USEPA, 2009). Furthermore, PFASs are not included on USEPA's list of hazardous substances established pursuant to the Comprehensive Environmental Response, Compensation and Liability Act (CERLCA) nor are PFASs included in Appendix IX of the Resource Conservation and Recovery Act. This complicates cost recovery under CERLCA and, to date, USEPA has enacted limited enforcement at PFAS sites (only one site with PFAS contamination is included on the Agency's National Priorities List, the Saint Gobain site in Hoosick Falls, New York [USEPA, 2017]). Fortunately, from a drinking water perspective, as part of USEPA's Unregulated Contaminant Monitoring Requirement, Stage 3 (UCMR3; https://water.epa.gov/lawsregs/rulesregs/sdwa/ucmr/), the Agency required community water suppliers to analyze water samples for six PFASs, including PFOS and PFOA. Vedagiri et al. summarize the UCMR3 results as does an article by Hu et al. (2016). Hu et al. reported that 66 public water supply systems providing water to six million people contained at least one sample at or above USEPA's combined health advisory for PFOS and PFOA of 70 ng/L. Under the UCMR3 program, water suppliers detected PFOA and PFOS concentrations up to 349 ng/L and 1,800 ng/L, respectively. Examples of water systems with PFASs detected at concentrations above the EPA's drinking water advisory levels include: Suffolk County Water Authority (New York), Security Colorado Water System (Colorado), Sanford Water District (Maine), United Water Pennsylvania (Pennsylvania), Issaquah Water System (Washington), and Warminster Municipal Water Authority (Pennsylvania). The regulatory focus to date has primarily been contaminated drinking water supplies as a result of the UCMR3 program data, resulting in considerable community concerns regarding health effects from drinking PFAS-contaminated water in areas with PFAS-impacted water supplies. In addition, the various branches of the U.S. Department of Defense have launched a widespread PFAS investigation due to the use of aqueous film forming foams (AFFFs) for fire suppression that involves testing at over 400 bases (McDaniel & Crystal, 2017). As of August 2017, approximately 36 bases had some form of water contamination (McDaniel & Crystal, 2017) and, as the testing progresses, additional contamination detections are almost a certainty. The results of this comprehensive U.S. Department of Defense PFAS investigation resulted in cleanup projects at U.S. military installations. Similarly, at some point, oil refineries, large petrochemical facilities, and other facilities with firefighting training locations with historical AFFF usage are likely to conduct similar investigations either voluntarily or by future regulatory mandates. To date, most of the remediation focus related to PFASs has been on contaminated groundwater affecting drinking water supplies. The biogeochemical properties of PFASs described earlier adversely affect the ability to remediate PFAS-contaminated groundwater, resulting in substantial challenges to the remediation industry, as described by Ross et al. The low adsorption affinity combined with the need to achieve extremely low cleanup objectives, decreases the effectiveness of the various types of commercially available activated carbon, although granular activated carbon (GAC) appears to remain as the most commonly applied treatment technology. Specially designed ion exchange resins are somewhat more effective than GAC from an adsorption perspective; however, the ability of resins to be cost competitive with GAC depends on site-specific circumstances. Also, the low volatility and recalcitrance of PFASs renders air stripping, air sparging, and bioremediation inadequate for PFAS treatment (however, there may be some opportunity for developments with bioremediation). The difficulty in treating PFASs using conventional methods has spurned research for innovative treatment technologies. Ross et al. provides an overview of potentially applicable PFAS treatment technologies. Two other innovative technological developments related to PFAS treatment are also discussed in detail in this issue of Remediation. One proposes that fungal pathways should be considered for the biotransformation of potential PFAS precursors, such as 6:2 and 8:2 fluorotelomer alcohols, (Merino et al.) and the other describes the potential to use electrochemical oxidation to treat PFOA and PFOS in concentrated waste streams, such as the liquid produced from regenerating ion exchange resins (Liang et al.). From an in situ perspective, the only commercially available technology for PFAS-contaminated groundwater is chemical oxidation, although, as described by Dombrowski et al., ISCO has not been demonstrated successfully on a widespread basis and the presence of precursors, particularly at AFFF sites, exacerbates the potential implementation of ISCO. However, there is some promise for ISCO at some sites if optimum activators are used under the right conditions. Other in situ technologies are still under development and most common technologies are not nearly as effective for PFASs as with TCE, benzene, and many other common contaminants. This issue of Remediation provides the remediation industry current information on a broad range of topics involving PFASs from a variety of subject matter experts. PFASs are an emerging contaminant with information developing from many perspectives, particularly the fate and transport, toxicology, and remediation technology fields. The overall landscape of PFAS remediation is going to change as more is learned about the class of chemicals and as regulatory agencies expand regulatory efforts as more PFAS investigation data develop. For example, New York has an informal policy of requesting responsible parties for inactive hazardous waste sites to include PFASs in the analytical suite for at least one groundwater monitoring event, unless a specific reason to exclude such testing can be made. Other states are expected to expand their scope of PFAS testing, which will undoubtedly identify additional sites that require investigation and remediation. I would like to thank Dora Chiang of AECOM and Claudia Walecka-Hutchison of The Dow Chemical Company who spearheaded identifying topics and authors for this special issue and assisted with the technical review of the articles. The following PFAS experts assisted in the peer review process in addition to Dr. Chiang and Ms. Walecka-Hutchinson: Jennifer Guelfo of Brown University, Charles Schaefer of CDM Smith, Jack Huang of University of Georgia, Nathan Hagelin of Wood PLC, Raymond Ball of En-Chem, Jinxia Liu of McGill University, and Scott Grieco of O'Brien & Gere. John A. Simon is the editor-in-chief of Remediation. He is also a director at Gnarus Advisors LLC (Arlington, Virginia), a consulting firm specializing in providing expertise in environmental and economic matters. He frequently consults private industry on the assessment and remediation of hazardous sites, as well as environmental liability and risk-transfer issues. Mr. Simon is the President of the U.S. Sustainable Remediation Forum and leads the ASTM Greener Cleanups Task Group. In addition, Mr. Simon recently served on the steering committee for the Battelle Symposium on Bioremediation and Sustainable Environmental Technologies. He received his BE in civil and environmental engineering from Vanderbilt University and his MS in civil engineering, environmental engineering, and science from Stanford University.

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