Monitored Natural Attenuation to Manage PFAS Impacts to Groundwater: Scientific Basis

Abstract

This column reviews the general features of PHT3D Version 2, a reactive multicomponent transport model that couples the geochemical modeling software PHREEQC-2 (Parkhurst and Appelo 1999) with three-dimensional groundwater flow and transport simulators MODFLOW-2000 and MT3DMS (Zheng and Wang 1999). The original version of PHT3D was developed by Henning Prommer and Version 2 by Henning Prommer and Vincent Post (Prommer and Post 2010). More detailed information about PHT3D is available at the website http://www.pht3d.org.The review was conducted separately by two reviewers. This column is presented in two parts.

Enhanced attenuation (EA) to manage PFAS plumes in groundwater

“…There are a number of retention processes that are relevant for PFAS in the subsurface, some of which are similar to those for other groundwater contaminants (e.g., TCE, benzene) while others are distinctly different. Newell et al (2021a) summarized PFAS retention processes that provide a scientific basis for PFAS MNA (Figure 2). A key concept is that all of the processes below are retention processes rather than permanent sequestration processes, based upon the following definitions:…”

“…There are a number of retention processes that are relevant for PFAS in the subsurface, some of which are similar to those for other groundwater contaminants (e.g., TCE, benzene) while others are distinctly different. Newell et al (2021a) summarized PFAS retention processes that provide a scientific basis for PFAS MNA (Figure 2). A key concept is that all of the processes below are retention processes rather than permanent sequestration processes, based upon the following definitions: Sequestration : 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; and Retention : The storage of a chemical in the environment so that the chemical is isolated from potential receptors for a certain time period.…”

“…For PFAS, the key retention processes that have the potential to impact fate and transport in the subsurface include air/water interfacial sorption, hydrophobic interactions, NAPL/water sorption, electrostatic interactions, “salting out,” and matrix diffusion (Costanza et al, 2019; Higgins & Luthy, 2006; Guelfo et al, 2021; Newell et al, 2021a; Sharifan et al, 2021; Van Glubt & Brusseau, 2021). These concepts were used to develop some potential guidelines on how retention based MNA could be applied at actual PFAS field sites (Newell et al, 2021b) and will be further developed as part of a Department of Defense‐sponsored (DoD) project on PFAS MNA (ESTCP, 2021a).…”

“…PFAS precursors can transform to PFAAs that do not degrade any further in groundwater under natural conditions. Maintaining precursors in the untransformed state is described as “chemical retention” by Newell et al (2021a). If PFAS are kept in the precursor form, the environmental risk profile may be lower, because the greatest potential health risks are currently focused on PFAAs.…”

“…Newell et al (2021a) observed that the salting-out effects of PFAS may be an important natural attenuation process in environments…”