The first Department of Defense (DoD) sites impacted with per- and polyfluoroalkyl substances (PFAS) have entered the remedial investigation (RI) phase, with many more sites to be characterized. Soon thereafter, many more PFAS-related feasibility studies (FS) will be conducted. The overall objective of this project is to develop and demonstrate standardized and easy-to-use field methods so that DoD Remedial Project Managers (RPMs) and their consultants can answer these three site management questions:

• Question 1 (RI): Is the key concern/risk driver from PFAS source materials in the vadose zone, saturated zone, or both?
• Question 2 (RI): Is back diffusion from low-permeability (low-k) geologic material (e.g., clays) controlling PFAS mass discharge within the aqueous film-forming foam (AFFF) source area, and/or is matrix diffusion significantly reducing the downgradient mass flux to the dissolved plume?
• Question 3 (FS): Is removal, isolation, or destruction of PFAS source zones needed to control risk? If yes, where should treatment be targeted (vadose zone, saturated zone, low-k geologic media like clays) for maximum reduction in PFAS mass discharge? Can enhanced attenuation be used to manage PFAS sources?

The project team aims to provide answers to these critical site management questions by showing how targeted, high-quality data can be used to refine conceptual site models (CSMs) and develop and screen remedial alternatives.

Answers to the questions above are highly reliant on collecting targeted, high-quality data that can be used to refine CSMs and develop and screen remedial alternatives. Recent SERDP and ESTCP research has provided new insights into and approaches for addressing these questions, including techniques for high-resolution characterization of PFAS mass discharge and matrix diffusion effects. These insights can be leveraged into a focused, data-driven demonstration that quantifies key metrics for PFAS mass distribution and discharge in the subsurface. The following figure shows the key data and knowledge required to resolve the three site management questions above. The project team will test and refine the data collection methods at two AFFF-impacted sites and compile data from three additional sites to verify and enhance the overall methodologies and then transfer this knowledge to DoD RPMs, their consultants, and regulators. Success criteria include reasonable uncertainty, ease of use, and positive mass balances when reconciling source mass discharge.

With answers to the three questions above, DoD RPMs and consultants will be informed on how to best collect vital PFAS mass discharge data from their PFAS sites. When these methods are applied in the field, the DoD will receive more cost-effective and robust PFAS RIs. These improved RIs will then lead to more reliable remediation decision-making during the FS phase and therefore help the DoD manage their PFAS sites in a more cost-effective manner. (Anticipated Project Completion - 2025)