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When perfluoroalkyl and polyfluoroalkyl substance (PFAS)-contaminated groundwater is a source—or potential source—of drinking water, remediation strategies are commonly selected with a goal of protecting the health of human populations who may use the contaminated water for extended periods. Long-term human exposure to certain PFAS at environmentally relevant levels has been associated with increased serum cholesterol, decreased vaccine response, and an increased risk of cancer (Interstate Technology and Regulatory Council [ITRC], 2023; US Environmental Protection Agency [US EPA], 2023a, 2023b). Recently, the International Agency for Research on Cancer (IARC) classified perfluorooctanoic acid (PFOA) as “carcinogenic to humans” (IARC Group 1) and perfluorooctane sulfonic acid (PFOS) as “possibly carcinogenic to humans” (IARC Group 2B) (Zahm et al., 2023). Remediation strategies are chosen that can decrease PFAS levels in water to applicable health-based criteria and thereby limit exposure of local populations to PFAS through ingestion of drinking water. However, this approach does not consider the potential for human exposure throughout the life cycle of the remediation technology, in which spent media may need to be disposed of, regenerated, or destroyed over the many years the technology is likely to be in place. In this commentary, we consider four PFAS remediation technologies and identify those places in the life cycle that have the potential for environmental releases from the handling, transport, disposal, regeneration, and/or destruction of remediation wastes. Importantly, we also identify where those releases have the potential to result in human exposure to PFAS, focusing on the long-chain perfluoroalkyl acids (PFAAs) and using PFOA and PFOS as examples. Technologies that have been demonstrated at multiple sites, under diverse conditions, by multiple practitioners, are commercially available, and are well documented in practice or peer-reviewed literature. Field-implemented technologies have been demonstrated to meet site-specific PFAS treatment objectives, at the intended final application scale, and are widely accepted in the regulatory and scientific community. Four technologies are listed within this category for treatment of PFAS-impacted liquids that have relevance to groundwater/extracted groundwater. The technologies are (1) granular activated carbon (GAC), (2) ion-exchange resin (IX), (3) foam fractionation (FF), and (4) in situ remediation with colloidal activated carbon (CAC). It is these technologies that the present commentary considers. It is noted that a category of “high-pressure membranes” is listed within the ITRC “field-implemented” technology category for liquid PFAS treatment and as a proposed treatment technology by US EPA (2023c). However, this technology is omitted from the present commentary, as its application is principally in point-of-use drinking water purification systems—often as part of a treatment train—rather than in groundwater remediation. Analogous life cycle considerations for wastes generated by this technology would nevertheless also apply. Importantly, three of the considered remediation technologies are variations of “pump and treat” and produce waste over the entire life of the project. That waste must first be managed in the field and then transported for disposal, regeneration, or destruction. The management of these remediation wastes has the potential to release PFAS into air, soil, surface water, and/or groundwater, with the attendant potential for human exposure at many points in the waste cycle. In contrast, passive in situ remediation with CAC—the fourth technology—generates no waste. Pump and treat remediates a contaminated aquifer by extracting the contaminated groundwater; this water is then treated before being returned to the aquifer or discharged to surface water. Pump and treat has been used extensively for groundwater remediation for over 40 years (National Research Council [NRC], 2013). Most of this experience has been for chlorinated solvents such as trichloroethene (TCE) (NRC, 2013). Pump and treat is not particularly effective as a remedy for chlorinated solvents at many sites because the solvents tend to sorb to aquifer materials or diffuse into nontransmissive portions of the aquifer where they are less available to be extracted (Chapman & Parker, 2005; Guo et al., 2019; Mackay & Cherry, 1989; NRC, 1994). The same situation applies to PFAS compounds. As discussed below, pump and treat should be even less effective for long-chain PFAS compounds such as PFOA or PFOS. PFOA or PFOS sorb more strongly to aquifer solids than, for example, TCE. The potential for sorption in aquifers is typically attributed to organic carbon in the aquifer solids. The effects of sorption can be predicted from the partition coefficient between aquifer water and organic material in aquifer solids [Koc (L/kg)]. The Koc for PFOA is approximately 448 L/kg (geometric mean of 37 values reported in tab. 4-1 of ITRC, 2023). The Koc of PFOS is approximately 2380 L/kg (geometric mean of 43 values reported in tab. 4-1 of ITRC, 2023). In contrast, the Koc of TCE is approximately 94 L/kg (geometric mean of 21 values reported in US EPA, 1996). Therefore, based on a comparison of Kocs, PFOS sorbs to aquifer solids 20 times more strongly than TCE, and PFOA sorbs five times more strongly. A much smaller proportion of the PFOS or PFOA in the aquifer will therefore be in the groundwater and available for extraction by pump and treat. Consequently, more pumping will be required to remediate the aquifer. The significance of the strength of aquifer absorption in field–implementation, is a slowing of the removal rate of PFAS contamination and the consequent extension of the remedial timeline. This is driven by the sorbed mass effectively acting as a “reservoir” from which groundwater is recontaminated through desorption to establish a new equilibrium as remediation progresses. This process will continue, slowing the decline in groundwater concentration, until the reservoir itself is depleted. Taking PFOS as an example and using a soil density of 1.7 g/cm3, a porosity of 0.23, an organic carbon fraction of 0.2%, and the above Koc value, the sorbed mass reservoir would be 35 times the aqueous mass at equilibrium—a “reservoir factor” of 35. This contrasts with an equivalent reservoir factor of 1.4 for TCE under the same aquifer conditions. The chlorinated solvents are also removed from groundwater through destructive natural attenuation processes such as biodegradation and abiotic degradation (Lebrón et al., 2015; Wiedemeier et al., 1998). These degradation processes contribute to remediation and reduce the time needed to attain a remedy. However, organisms that can metabolize perfluorinated organic compounds are rare (Wackett, 2021). Studies compiled by the PFAS Team of the ITRC show that natural degradation in groundwater can transform precursors to form PFOA and PFOS, but the team does not cite any studies that claim significant degradation of PFOA or PFOS in aquifer materials (ITRC, 2023). It is unlikely that natural degradation processes will contribute to pump and treat remedies for recalcitrant perfluoroalkyl substances, such as PFOA or PFOS. Considering the greater propensity of PFOA and PFOS to partition onto aquifer materials compared to chlorinated solvents and the unlikely contribution of degradation mechanisms to removal of these contaminants, it follows that pump and treat will require significantly longer periods of time to achieve a desired reduction in concentrations of PFOA and PFOS compared to chlorinated solvents. Given that pump and treat clean up of chlorinated solvents at many sites may require many decades (Mackay & Cherry, 1989; NRC, 1994, 2013; Travis & Doty, 1990), it is unrealistic to consider the approach as a means of cleaning an aquifer. Rather, pump and treat provides an effective strategy for ongoing plume capture and containment. PFAS contamination must be removed from groundwater extracted by a pump and treat system before the water may be returned to the aquifer or discharged. Contemporary treatment commonly employs sorption media for this purpose (GAC, IX). PFAS mass is transferred from the water to the sorption media, which then requires treatment or disposal. PFAS may also be separated from extracted water using FF. All these processes yield concentrated PFAS wastes. The handling, transport, and eventual disposal or destruction of the accumulated PFAS mass required in each case are components of the remediation life cycle. These approaches for extracted water treatment—GAC, IX, and FF—represent three of the field-implemented PFAS treatment technologies (ITRC, 2023) that are considered in this commentary. These technologies and their associated waste life cycles, plus the fourth technology of in situ remediation with CAC, are discussed in the following sections. GAC is widely applied as a sorption medium for separating PFAS from water (ITRC, 2023). Extracted groundwater is moved through packed beds of activated carbon; PFASs adsorb to the surface of the carbon and are removed from the water without degradation (ITRC, 2023). GAC removes PFOA, PFOS, PFHxS, and other PFAS (McCleaf et al., 2017), with perfluorosulfonic acids (PFSAs) generally removed more readily than perfluorocarboxylic acids (PFCAs) and long-chain PFAAs removed more efficiently than short-chain PFAAs (Appleman et al., 2014; McCleaf et al., 2017). When the surface area of the GAC can no longer effectively adsorb additional PFAS, the spent GAC can either be treated by incineration, disposed of in a landfill, or thermally regenerated for reuse (US EPA, 2020; Wang et al., 2022; Watanabe et al., 2016; Xiao et al., 2020). Depending on the equipment and operating conditions, both regeneration and incineration may release gas-phase PFOA, PFOS (Watanabe et al., 2016), and other PFAS to air if temperatures less than 1000°C/1800°F are used (US EPA, 2020; Wang et al., 2022; Watanabe et al., 2016). Both thermal treatment and incineration may also produce ash, which requires landfill disposal, an activity that holds the potential for release of PFAS to air and other environmental media (US EPA, 2020; see also Figure 1 and Table 1) should the incineration or regeneration not be complete (DiStefano et al., 2022). If the spent GAC or ash is placed in a lined landfill, PFAS can desorb into the landfill leachate (US EPA, 2020). PFASs were present in ash from a municipal solid waste incinerator (Liu et al., 2021) and in landfill leachate from incinerator ash monofills (Liu et al., 2022; Solo-Gabiele et al., 2020). PFAAs have been regularly detected in landfill leachate, with PFOA being one of the most frequently detected PFCAs (Hamid et al., 2018). Lang et al. (2017) detected PFOA, PFOS, and PFHxS in more than 50% of US landfill leachate samples. Air, soil, surface water, groundwater, drinking water Inhalation, ingestion, dermal contacta Air, soil, surface water, groundwater, drinking water Inhalation, ingestion, dermal contacta Air, soil, surface water, groundwater, drinking water Inhalation, ingestion, dermal contacta Landfill leachate is often disposed of to a wastewater treatment plant (WWTP), where treated water may be discharged to holding ponds or directly to surface water. Because conventional municipal wastewater treatment does not degrade or consistently remove PFAS (Schultz et al., 2006; Sinclair & Kannan, 2006), PFASs originally present in the landfill leachate will re-enter the environment after treatment, where they may migrate to soil, surface water, or groundwater. PFAS present in wastewater may also sorb to biosolids (Arvaniti & Stasinakis, 2015); if land-applied for agriculture, biosolids may also be a source of PFAS to soil, surface water, or groundwater (Johnson, 2022), although Pepper et al. (2021) found limited migration of PFASs in biosolid-impacted soils. If GAC is disposed of in an unlined landfill (leachate is not collected), PFAS, if desorbed, may migrate to soil, surface water, or groundwater in processes analogous to those described (above) for treated wastewater (ITRC, 2023). Landfills also may release PFAS to air, with PFOA and PFOS detected in the gas phase of ambient landfill air (Ahrens et al., 2011), and ionic PFOA, PFOS, and other PFAAs detected in all samples of particulate-phase landfill air (Weinberg et al., 2011). PFASs that are released to air may be subject to long-range transport (Faust, 2023). Once contaminated with PFAS, air and other environmental media hold the potential for human exposure via inhalation (air) or by incidental ingestion or dermal contact with soil or water. The handling and transport of PFAS wastes may also result in environmental releases and human exposure over the course of the remediation life cycle. While waste handling will likely be conducted under standard operating procedures designed to protect workers and the environment, both waste handling and the transport of waste to disposal facilities hold the potential for accidental releases of PFAS. The biological significance of any PFAS exposure will depend on human contact with each environmental medium, the frequency and length of that contact, on PFAS concentrations, and on the toxicological characteristics of the PFAS (see Figure 1 and Table 1). Ex situ IX resins remove PFAS from pumped groundwater based on both adsorption and on the ionic interaction of negatively charged PFAS with positively charged sites on the IX resin (McCleaf et al., 2017; Woodward et al., 2017). PFASs are not degraded by IX treatment. Both single-use and regenerable IX resins are available for PFAS removal, with single-use IX generally more effective at removing PFSAs than PFCAs (ITRC, 2023). Spent single-use resins are disposed of either by incineration or in a landfill (ITRC, 2023). Saturated IX resins can be regenerated using either an organic solvent, inorganic salts, or both, with the selected regeneration process dependent on resin properties and PFAS functional groups, among others (Dixit et al., 2021). IX regeneration yields PFAS-rich brines whose disposal currently requires incineration (Dixit et al., 2021) or landfilling (ITRC, 2023) and associated handling and transport. Other emerging disposal approaches show promise but are less well-documented (ITRC, 2023). Examples include supercritical water oxidation (Sahle-Demessie et al., 2022) and electrochemical oxidation (US EPA, 2021). Irrespective of potential destruction efficiency, these share waste handling and transport considerations common to established treatment approaches. Thus, both single-use IX resins and IX regeneration may release PFAS to air, soil, surface water, and groundwater via the mechanisms and processes discussed for GAC. Once PFAS are released, there is the potential for human exposure via inhalation, incidental ingestion, or dermal contact (Figure 1 and Table 1). Ex situ FF is a physical and chemical process in which amphiphilic PFASs such as long-chain PFAAs are separated from pumped groundwater by introducing air bubbles that rise through a column of contaminated water. PFAS preferentially accumulate at the air–water interface and therefore effectively sorb to the surface of the bubbles, accumulating in a PFAS-enriched foam at the top of the water column (Buckley et al., 2021). PFOA, PFOS, and PFHxS were efficiently removed from water in field trials (Burns et al., 2021, 2022), landfill leachate (Wang et al., 2023), and in a laboratory-scale system (Smith et al., 2022). The concentrated foam waste that is a by-product of FF can be disposed of by incineration or by landfilling (Burns et al., 2021; Wang et al., 2023) subsequent to handling and transport of the wastes. In addition to potential releases associated with incineration or landfilling of FF waste (see GAC discussion), high levels of PFOA, PFOS, PFHxS, and other PFAAs were detected in air (gas phase) and in aerosols of a pilot FF system (Smith et al., 2022). Although such releases may be effectively mitigated through suitable control measures, inhalation exposure remains of potential relevance. Thus, FF may pose additional human health concerns beyond those associated with spent GAC or IX disposal, regeneration, or destruction, in that releases of PFAS in the immediate vicinity of the FF system could expose worker populations to airborne PFAS unless appropriate and functional air pollution controls (APCs) are in place. However, even if APCs are used, they represent another waste stream that requires disposal or destruction over the life of the FF system. Injectable in situ CAC offers a means of engineering the capacity of aquifer solids to sorb PFAS. Equilibrium sorption of PFAS to CAC may be quantitatively described with isotherms (Carey et al., 2022). These provide a mathematical description of the equilibrium between sorbed and dissolved phases at different concentrations of contaminant and quantities of CAC. If the PFAS species isotherm and the quantity of applied CAC are known, the equilibrium sorption concentration for a given quantity of the PFAS species can be calculated. From this, the quantity of CAC necessary to attain the regulatory goal for PFAS remaining in the groundwater can be determined. A CAC application may typically deposit 0.1%–0.5% CAC in the target zones as a proportion of soil mass. Measured field averages of 0.02%–0.76% have been reported by Carey et al. (2022). Since the sorption of PFAS to CAC compared to natural soil organic carbon is very high, the CAC emplacement is sufficient to locally increase the coefficient of distribution (Kd) of PFAS in soil by two to three orders of magnitude at most sites. Groundwater PFAS concentrations are consequently reduced by a corresponding degree. The increased sorption of PFAS slows plume migration through proportionally increasing the contaminant retardation factor. The net result is comparable by analogy to changing the plume migration rate in sand to the rate in clay. The difference would be that, in the case of CAC addition, the contaminant migration is slowed through retardation rather than through a reduction in the velocity of groundwater. Contaminant migration is therefore slowed significantly while groundwater flow remains unaffected. Remediation is consequently achieved through containing the plume and reducing/eliminating exposure to PFAS. may the form of in situ (Carey et al., These treat groundwater as it through the treatment length of CAC through a 20 will approximately of water for of example, this would to approximately of water over a life of a at with no pumping The sorption of PFAS onto CAC migration of a PFAS This may contamination for exposure to populations who on the aquifer for drinking water (Carey et al., CAC has effectively decreased aquifer concentrations of PFOA and PFOS as well as a of other PFAS as over a (Carey et al., 2022). Carey et al. the of CAC at a PFAS and that CAC could be effective for of the CAC may be by increasing the of CAC used or by additional CAC (Carey et al., of CAC to the functional life of a PFAS may be under a analogous to and and on a pump and treat system. A of 20 years would be in either case (Carey et al., 2019; of et al., Because CAC remains in the aquifer sorbed to there is no waste management disposal, regeneration, or and no human exposure from the use of this remediation technology 1 and Figure 1). decades of and and diverse have released PFOA, PFOS, and other PFAS to the environment where they have been detected in air, soil, water, and for and 2021; ITRC, 2023; US EPA, 2023a, 2023b). Remediation of PFAS in groundwater is a and the disposal or destruction of remediation wastes may not have to of PFAS in or the However, the distribution of groundwater documented by US Contaminant et al., and the ongoing drinking water of PFAS in (US EPA, PFAS remediation will likely increase in frequency and Pump and treat may and reduce PFAS mass within a plume or aquifer but will not all of the contamination in a of The associated and transport of wastes will to increased for accidental and the disposal, regeneration, or destruction of an increasing of remediation waste has the potential to contribute to human exposure to PFAS released to air, soil, surface water, and groundwater for many The of PFAAs and certain other PFAS for and 2013; Wang et al., with the potential to 2021; et al., 2021) and et al., et al., 2005; et al., 2022) the significance to of environmental of PFAS to the environment can be human exposure the use of remediation of landfill leachate, and other However, exposure to PFAS in remediation waste can be by the use of a remediation technology that does not waste in the first place. CAC is an example of a technology that is of PFAS concentrations in groundwater without to the of PFAS in the environment or to potential human exposure associated with this and for the of the which this by is an whose practice has on PFAS is in as of the and Group of the ITRC PFAS In that as an and of and of the PFAS Regulatory has 40 years of experience in human exposure to and the of environmental is the at Environmental 35 years a at US EPA, on biodegradation and natural attenuation of chlorinated and components in groundwater. is Technology at where is for new technology and the and of remediation and has 35 years of experience in environmental remediation in groundwater and aquifer is not applicable to this as no new were or in this