The objective of this Statement of Need (SON) was to seek innovative research to further explore the fate and transport of per- and polyfluoroalkyl substances (PFAS) in the subsurface, in both the vadose zone as well as the saturated zone. Proposed efforts should have focused on one or more of the following objectives:

• Determine physical-chemical properties of PFAS by measurement and predictive modeling to support assessments of PFAS fate, transport, bioavailability, and remediation including identification of “indicator PFAS” as diagnostic compounds.
• Assess processes impacting migration and fluxes of PFAS derived from source zones.
• Understand the impact of AFFF composition on fate and transport of PFAS.
• Evaluate rate-limited processes with respect to PFAS release and migration in saturated and unsaturated soils.
• Evaluate the relevance of PFAS vapor transport and assess the applicability of current vadose zone models for quantitative prediction of PFAS migration.
• Interrogate PFAS transport and fate at the capillary fringe.

Proposals addressed one or more of the objectives listed above. Proposers were directed to review the document Summary Report: Strategic Workshop on Management of PFAS in the Environment for additional information on these research objectives. This document provides a summary of the March 2022 strategic workshop on PFAS in which research and demonstration needs were identified so as to improve the management and treatment of PFAS in the environment, ultimately reducing risk and site management costs.

Researchers had to provide the rationale for selected PFAS of study; at a minimum, measurement of the 40 PFAS that can currently be measured by U.S. EPA Method 1633 should be prioritized as possible. Fate and transport of PFAS at environmentally relevant concentrations was of particular concern, and proposed efforts should have reflected this concern or provide the rationale if different concentrations were proposed.

Research and development activities at laboratory-, bench-, and field-scale were be considered, although work did not necessarily have to culminate in a field-scale effort.

Research should lead to improved management of PFAS sites by facilitating the establishment of more cost-effective and efficient remedial action plans that are protective of human health and the environment. An improved understanding of fate and transport of PFAS in the environment will ultimately lead to more effective management and will expedite the cleanup and closure of Department of Defense (DoD) impacted sites.

Aqueous film-forming foams (AFFFs) have been used since the 1970s to suppress liquid fuel fires; there are thousands of sites impacted by PFAS. Although the DoD’s legacy use of AFFF included various fluorotelomer-based formulations, the vast majority of DoD’s environmental liability likely results from the use of perfluorooctanesulfonic acid (PFOS)-based AFFF.

Due to their chemical structure, PFAS are very stable in the environment and are relatively resistant to biodegradation, photo-oxidation, direct photolysis, and hydrolysis. Any quantitative approach to analyzing the behavior of PFAS in treatment systems or in the environment requires knowledge of the distribution of the PFAS of interest in the multiple phases that comprise the system being considered. For example, a successful treatment system should be designed such that the PFAS of interest is primarily associated with the treatment phase and not sequestered in other phases in the system such as soil or other matrix materials. This requires that the speciation the PFAS be considered. All of this information is required to build a reliable model of PFAS fate and transport within any media. If PFAS is primarily associated with stationary phases, its mobility is much less than if it is in the vapor or aqueous phases. Evaluation of the bioaccumulation and toxicity of PFAS also require consideration of its phase distribution and speciation. PFAS bound to phases not exposed to the organism or distributed among multiple chemical species will accumulate differently in organisms depending on the bioavailability of each species and their toxic potential. This is not a new requirement for understanding the transport, transformations, and toxicity of environmental contaminants, but the challenges and unknowns are extreme for PFAS.

Investigations of DoD sites impacted by AFFF have indicated significant retention of PFAS in the vadose zone, particularly in unsaturated soil and deeper horizons. The longevity of PFAS in source zones is a critical component of assessing management and remediation approaches for AFFF-impacted sites; significant uncertainties remain with respect to the key processes impacting the distribution and fluxes of AFFF-derived PFAS during vertical migration (and lateral dispersion) to groundwater. Given the diversity of hydrogeologic and climatic conditions throughout AFFF-impacted sites, it is likely that migration and flux of water, and associated PFAS transport, are highly variable. If quantitative models of source zone longevity are to be developed, there is a need to assess and validate the fundamental processes and parameters impacting both water and PFAS fluxes through the vadose zone. To date, field-scale modeling of PFAS fate and transport in the vadose zone has been limited, in particular for multi-dimensional systems that are likely to be important due to the potential effects of PFAS and AFFF constituents on water flow and retention. There is also a lack of model simulations that directly link PFAS mass discharge from the vadose zone with groundwater flow and transport, making it difficult to simulate the effects of fluctuating water tables and potential impacts on downgradient receptors (e.g., residential wells, surface water bodies). Further, most of the model simulations conducted to date focus on single PFAS, and the effects of potential competitive or synergistic interactions between multiple PFAS are not considered.

Several recent studies have shown that PFAS release from soils is a rate-limited process, and equilibrium models may be inappropriate for describing PFAS transport under many relevant field conditions. These rate limitations can be due to mechanisms at the grain scale, and/or due to mass transfer between advective and non-advective domains. While these rate-limited processes are recognized, there are several fundamental issues that remain unresolved.

The cost and time to meet the requirements of this SON were at the discretion of the proposer. Proposers submitting a Standard Proposal had to provide the rationale for this scale. The two options were as follows:

Standard Proposals: These proposals describe a complete research effort. The proposer should incorporate the appropriate time, schedule, and cost requirements to accomplish the scope of work proposed. SERDP projects normally run from two to five years in length and vary considerably in cost consistent with the scope of the effort. It is expected that most proposals will fall into this category.

Limited Scope Proposals: Proposers with innovative approaches to the SON that entail high technical risk or have minimal supporting data may submit a Limited Scope Proposal for funding up to $250,000 and approximately one year in duration. Such proposals may be eligible for follow-on funding if they result in a successful initial project. The objective of these proposals should be to acquire the data necessary to demonstrate proof-of-concept or reduction of risk that will lead to development of a future Standard Proposal. Proposers should submit Limited Scope Proposals in accordance with the SERDP Core Solicitation instructions and deadlines.