PFAS Monitored Retention (PMR) and PFAS Enhanced Retention (PER)

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Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) Monitored Retention (PMR) is an approach for managing PFAS-affected sites that relies on natural retention processes that can lessen the migration and maximum concentrations of many PFAS at impacted sites. Because PFAS have not yet been shown to degrade to harmless end products by natural abiotic or biological actions, these retention processes (e.g., sorption, matrix diffusion) are expected to be the primary factors that dictate how PFAS behave and move in the environment [1] [2][3][4][5][6][7], and they form the basis for PMR. The PMR concept has been incorporated into a framework (the “PMR Framework”) that can help users determine the suitability of PMR at a specific site or identify the highest priorities among a portfolio of PFAS-affected groundwater sites. The Framework provides guidance on collecting different types of data (lines of evidence) to support a PMR evaluation. It also describes enhanced retention strategies known as PFAS Enhanced Retention (PER) that may be applicable for management of sites where natural retention is insufficient on its own to protect downgradient receptors.

Related Article(s):


Contributor(s): Dr. David Adamson, P.E., Dr. Charles Newell, P.E. and Dr. Hans Stroo


Key Resource(s):

Introduction

Groundwater sites contaminated with PFAS are difficult to investigate and remediate due to strict cleanup goals, lack of natural degradation mechanisms for some PFAS, and the high mobility and persistence of several PFAS. Several factors contribute to the urgency of developing cost-effective strategies to manage PFAS sites:

  • The number of groundwater sites with PFAS that will require some type of management could be in the tens of thousands (Newell et al., 2020; 2022). Recent estimates suggest that there are 50,000 to 60,000 potential PFAS sites in the U.S. (EBJ, 2022; Salvatore et al., 2022).
  • No technologies currently exist that are capable of effectively and efficiently destroying PFAS in situ. Furthermore, the remediation industry is unlikely to be able to practically manage the large number of PFAS groundwater plumes using the only two technologies that are currently viable, groundwater pump and treat and in situ sorbents (Newell et al., 2022).
  • Despite a lack of natural processes that permanently sequester PFAS, there are natural processes can potentially retain PFAS in the subsurface for extended periods, limiting contaminant migration.
  • Consequently, retention could be an important factor in reducing the near-term risk associated with PFAS groundwater plumes at some sites and open up approaches for managing PFAS plumes based on retention (Newell et al., 2021a, b; 2022).
  • Even after active remediation, it is likely that low residual concentrations of PFAS will remain in soils and groundwater, and these may also be effectively controlled by retention.

As a result, understanding and potentially relying on processes that reduce PFAS migration rates and mass discharge rates is of considerable interest to site managers. This includes a variety of chemical and geochemical retention processes that have been incorporated into the PFAS Monitored Retention (PMR) approach that site owners, their consultants, and environmental regulators can use to prioritize and manage their portfolios of PFAS sites.

PMR versus MNA

Figure 1. Development timeline and key attenuation processes associated with various monitored natural attenuation approaches. Years are approximately when key publications first appeared for each MNA approach.

PMR is a similar concept to monitored natural attenuation (MNA), and the term has recently been adopted in place of MNA to avoid potential confusion with destructive and/or permanent attenuation processes that are part of the MNA strategies for other constituents of concern (COCs).  However, many of the processes remain the same, and they are expected to reduce PFAS concentrations and mass discharge during transport from source areas.

The use of management based remedial approaches like MNA to control PFAS plumes poses several challenges, including that traditional MNA applications typically deal with petroleum hydrocarbons and chlorinated solvents, which can naturally break down to harmless end products at many sites. However, MNA as a remedy or site management strategy has been approved by regulators for some non-degrading metals, metalloids, and radionuclides (e.g., chromium, arsenic, and uranium) if the local geochemical conditions can sequester or immobilize these contaminants. This implies that even if some PFAS do not naturally degrade, passive management strategies may be feasible for PFAS plumes in groundwater because PFAS retention processes can help reduce their concentrations. Figure 1 shows how PMR fits into the development timeline for MNA-type approaches for different contaminant classes.

A key concept of PMR is that retention processes can provide a credible scientific basis for attenuation of PFAS concentrations or mass discharge over time (or distance) at some PFAS sites.  The concept of using these processes in a manner similar to MNA to manage PFAS sites was first proposed by Newell et al.[2][3], which identified several retention processes, including various sorption mechanisms, geologic matrix diffusion, geochemical conditions that limit the biotransformation of precursors, and dispersion of migrating PFAS plumes. A subsequent paper[8] distinguished between sequestration/immobilization and retention, noting that while permanent sequestration of PFAS compounds has not been confirmed, significant long term retention processes are likely present. These papers discussed how retention processes vary between different types of specific PFAS and PFAS classes, as well as the critical differences between the terms “retention” and “sequestration” in the context of managing groundwater plumes[8]. These two concepts may be summarized as:

  • Sequestration (or immobilization): The permanent trapping and isolation of a chemical in the environment in a natural or artificial storage compartment, such that the chemical does not impact potential receptors.
  • Retention: The storage of a chemical in the environment so that the chemical is isolated from potential receptors for a certain amount of time.

This distinction is important because permanent sequestration processes for individual PFAS in the subsurface have yet to be established. Conversely, the MNA approach designed for metals and radionuclides uses the term immobilization as a key process for implementing MNA[9]. In the case of PMR, the focus is on retention processes that reduce the mobility and risk associated with PFAS in the subsurface.

PFAS Retention Processes

Key retention processes for PFAS in the subsurface (shown in Figure 2) include:

Sorption of PFAS to solid phases like soil particles has been well documented.  It depends on mechanisms like hydrophobic sorption and electrostatic interactions which are a function of the physical-chemical properties of the PFAS and the environmental matrices[2]. In addition, air-water interfacial sorption of PFAS can be an important retention process in the unsaturated zone due to the presence of porewater and air-filled pore space in that soil compartment. In general, these can be considered “geologic retention” processes because they directly relate to interactions between PFAS and the surrounding porous media.

Matrix Diffusion refers to the diffusion of chemicals into and out of low-permeability soils (e.g., clays, silts) that are present in heterogeneous geologic settings. Previous studies with non-PFAS contaminants have shown that the mass retained in low-permeability units can serve as a persistent source of contaminants in groundwater[10].  Recent research has shown that matrix diffusion can also play a significant role in attenuating the expansion of groundwater plumes for non-degrading or highly recalcitrant compounds like perfluorooctanesulfonic acid (PFOS) (Kulkarni et al., 2022).

Chemical Retention refers to the retention of polyfluoroalkyl substances in their “precursor” state. The mobility of these polyfluorinated precursors may be more limited because the head group increases the size of the molecule and may include positive charges that increase sorption to particles. Precursors can be transformed to perfluoroalkyl acids (PFAAs), which are generally more mobile in groundwater but not subject to further degradation under natural conditions. Therefore, precursors can be seen as "chemically retained" PFAS mass. This distinct characteristic means that biotransformation acts as a sink for solvents and hydrocarbons but as a source of PFAAs at PFAS sites. These PFAS precursor transformations appear to occur more rapidly through aerobic biological processes (or after introducing chemical oxidants to remediate other constituents) (Choi et al., 2022), such that chemical retention is more likely under anaerobic conditions.



References

  1. ^ Interstate Technology and Regulatory Council (ITRC) PFAS Team, 2023. Technical/Regulatory Guidance: Per- and Polyfluoroalkyl Substances. ITRC PFAS Home Page Report pdf
  2. ^ 2.0 2.1 2.2 2.3 Newell, C.J., Adamson, D.T., Kulkarni, P.R., Nzeribe, B.N., Connor, J.A., Popovic, J., and Stroo, H.F., 2021a. Monitored Natural Attenuation to Manage PFAS Impacts to Groundwater: Scientific Basis. Groundwater Monitoring and Remediation, 41(4), pp.76–89. doi: 10.1111/gwmr.12486 Article pdf
  3. ^ 3.0 3.1 3.2 Newell, C.J., Adamson, D.T., Kulkarni, P.R., Nzeribe, B.N., Connor, J.A., Popovic, J., and Stroo, H.F., 2021b. Monitored Natural Attenuation to Manage PFAS Impacts to Groundwater: Potential Guidelines. Remediation Journal, 31(4), pp. 7–17. doi: 10.1002/rem.21697 Article pdf
  4. ^ Adamson, D.T., Kulkarni, P.R., Nickerson, A., Higgins, C.P., Field, J., Schwichtenberg, T., Newell, C., and Kornuc, J.J., 2022. Characterization of relevant site-specific PFAS fate and transport processes at multiple AFFF sites. Environmental Advances, 7, 100167. doi: 10.1016/j.envadv.2022.100167 Article pdf
  5. ^ Brusseau, M.L., 2018. Assessing the Potential Contributions of Additional Retention Processes to PFAS Retardation in the Subsurface. Science of The Total Environment, 613–614, pp. 176–185. doi:10.1016/j.scitotenv.2017.09.065. Article pdf
  6. ^ Guelfo, J.L., Korzeniowski, S., Mills, M.A., Anderson, J., Anderson, R.H., Arblaster, J.A., Conder, J.M., Cousins, I.T., Dasu, K., Henry, B.J., Lee, L.S., Liu, J., McKenzie, E.R., and Willey, J., 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, 40(12), pp. 3234-3260. doi: 10.1002/etc.5182 Article pdf
  7. ^ Guo, B., Zeng, J., and Brusseau, M.L., 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. doi:10.1029/2019WR026667 Article pdf
  8. ^ 8.0 8.1 8.2 Newell, C.J., Javed, H., Li, Y., Johnson, N.W., Richardson, S.D., Connor, J.A., and Adamson, D.T., 2022. Enhanced Attenuation (EA) to Manage PFAS Plumes in Groundwater. Remediation Journal 32(4), pp. 239–257. doi: 10.1002/rem.21731Article pdf
  9. ^ U.S. Environmental Protection Agency, 2007. Monitored natural attenuation of inorganic contaminants in groundwater, Volume 2 Assessment for Non-Radionuclides Including Arsenic, Cadmium, Chromium, Copper, Lead, Nickel, Nitrate, Perchlorate, and Selenium, Edited by R.G. Ford, R.T. Wilkin, and R.W. Puls. EPA/600/R-07/140. Report pdf
  10. ^ Chapman, S.W., and Parker, B.L., 2005. Plume persistence due to aquitard back diffusion following dense nonaqueous phase liquid source removal or isolation. Water Resources Research, 41(12), W12411. doi: 10.1029/2005WR004224 Article pdf
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