While per- and polyfluoroalkyl substances (PFAS) occurrence and transport has been extensively studied, it remains unclear whether gaseous-phase PFAS migration in the vadose zone warrants routine investigation. Several PFAS (e.g., fluorotelomer alcohols), may have sufficient vapor pressure to be designated as vapor-forming chemicals based on current federal and state vapor intrusion guidance. The overall goals of this proposal are to provide an improved understanding of the potential occurrence and transport of semi-volatile PFAS in the vadose zone by (a) investigating the occurrence and migration of PFAS in the gaseous phase and (b) assessing the applicability of mathematical models to predict the potential for vapor transport.

This project will measure the phase distribution of volatile and semi-volatile PFAS in soil, soil gas, and groundwater (including the capillary fringe) at three sites. The project team will examine the behavior of identified volatile and semi-volatile PFAS in environmental media using laboratory-scale column studies to assess the potential for gaseous-phase transport. Laboratory experiments will focus on measuring PFAS partitioning into the gaseous phase from aqueous and solid sources.

Experiments will be conducted initially on simple systems, focusing on one process, and then progress to more realistic systems consisting of multiple processes and dynamic conditions. A wide range of PFAS will be used, including all those currently covered by standard regulatory methods and non-targeted compounds and precursors with the greatest potential for volatility. Fundamental physical-chemical properties such as vapor pressure and Henry’s Law constants will be measured for PFAS to use as input parameters for mathematical modeling of vadose zone PFAS behavior. The project team will assess the applicability of existing simple one-dimensional and three-dimensional (3- D) models using data from laboratory experiments and field monitoring to develop screening approaches for assessing the gaseous-phase migration potential.

This project will evaluate the relevance of PFAS gaseous-phase transport and assess the applicability of current vadose zone models for quantitative prediction of PFAS migration. This research will determine whether gaseous-phase transport of PFAS is a relevant consideration for future site assessments. (Anticipated Project Completion - 2027)