This project will demonstrate that retention of per- and polyfluoroalkyl substances (PFAS) in the subsurface can be substantial at some impacted sites to the extent that monitored natural attenuation (MNA) can be utilized. This retention is not in the form of any permanent sequestration, but rather retention due to several processes that—when combined—can: 

• Retain a significant fraction of the PFAS in low-mobility compartments; 
• Prevent some or most of the PFAS mass from being transported down-gradient by groundwater advection for long time scales (e.g., decades); 
• Significantly reduce the apparent migration rate of the PFAS plumes (e.g., retard the migration of PFAS plumes); 
• Attenuate what is an unacceptable high mass discharge rate leaving a PFAS source zone to a longer term but much lower mass discharge rate at a down-gradient receptor point. 

The goal of this project is to demonstrate this retention/attenuation potential exists and develop tools that can be used to determine if a particular site is amenable to being managed by MNA. An extensive outreach program with an Expert Panel and other measures to facilitate adoption of retention-based MNA for PFAS plumes will be included as part of this project.

Although MNA is often thought of requiring destructive processes, it can be based on non-destructive processes such as immobilization as is done with metals and radionuclides. For PFAS, which are not mineralized under natural conditions, understanding the mechanisms and effectiveness of non-destructive processes that can attenuate PFAS plumes is critical. 

For this project, a detailed framework of all key processes that can attenuate PFAS plumes in groundwater will be compiled and summarized in a single PFAS MNA framework. The required field information, using both existing and new PFAS-specific characterization techniques, will be incorporated into the framework. 

This project has the objectives of beginning to establish MNA as a viable remediation approach for some PFAS sites, helping users screen sites where MNA may be applicable, and providing guidance on characterizing sites and analyzing field data to support MNA determinations. If successful, it can be a key catalyst for reducing the number of expensive, energy intensive, and extremely inefficient pump and treat systems that are now likely the only way to manage PFAS at hundreds of PFAS sites.  (Anticipated project completion - 2024)