Vapor-phase transport of per and polyfluoroalkyl substances: Processes, modeling, and implications

An increasing number of studies have demonstrated the presence of per and polyfluoroalkyl substances (PFAS) in the vapor phase. It is therefore important to consider the potential for vapor-phase transport of PFAS in soil and the vadose zone and to investigate the processes impacting the retention and transport of volatile PFAS in soil. It is also critically important to evaluate existing models and develop new models as needed for their application to PFAS vapor-phase transport. The objectives of the present work were to provide an overview of vapor-phase transport processes and modeling, with a specific focus on their relevance for PFAS, and to discuss implications for mass discharge to groundwater, vapor intrusion, and soil vapor extraction. Decades of research have been devoted to the retention and transport of legacy volatile organic contaminants in the vadose zone. This work provides an abundant source of information concerning the many factors and processes of relevance, and insights into the development and application of mathematical modeling. However, given the unique properties of PFAS, there is a need to conduct research to investigate vapor-phase transport of PFAS and to develop PFAS-specific models. We highlight with illustrative examples that vapor-phase transport can be significantly more rapid than aqueous-phase advective transport, which can result in enhanced mass discharge to groundwater.