Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7i. PFAS Fate and Transport Properties

Formatted Title:

Recent Advancements in Mechanistic Understandings of PFAS Fate and Transport in the Vadose Zone

Background/Objectives:

Per- and polyfluoroalkyl substances (PFAS) are a group of highly recalcitrant, bioaccumulative, and toxic chemicals that are frequently introduced to groundwater through land surface exposure. As a result, the vadose zone has been identified as a significant long-term source zone for PFAS leaching into groundwater. Much research has taken place over the last 5 years to understand the processes that impact PFAS fate and transport to the vadose zone. This talk summarizes critical findings regarding PFAS transport in the vadose zone and introduces several emerging topics expected to develop over the next several years.

Approach/Activities:

Early efforts in understanding PFAS fate and transport processes focused on their multi-phase retention processes. Specifically, understanding the degree of equilibrium partitioning to the solid phase and the air-water interface have been hotly studied topics. Equilibrium partitioning to solid and air-water interfaces generally increases with increasing molecular volume, with exceptions for very long chain perfluoroalkyl acids (PFAAs), cationic and zwitterionic PFASs, perfluoroalkyl ethers, and PFAS with very large headgroups. Recent research and emerging field data suggest many sites are impacted by non‑ideal, non‑equilibrium processes. Evidence of PFAA generation from precursor transformation, physically driven non-ideal transport, and chemically driven non-idealities have emerged as environmentally relevant topics. A specific list of topics for discussion is presented below:

Precursor Transformation: Despite the name “forever chemicals”, there are many PFAS known as PFAA precursors. Precursors transform into PFAAs, which are more mobile and can serve as a centurial source of PFAS contamination to underlying aquifers.
Non-Fickian Transport Mechanisms can Accelerate PFAS Leaching: Non-Fickian transport can be readily described as physically driven non-equilibrium. Examples include flow path channelization, immobile water formation, sheet flow, and reduced accessibility of air-water interfaces. It is likely that these mechanisms drive rate-limited desorption from solid-phase materials. Non-Fickian transport is more prevalent at lower saturations.
Tension-driven Flow Accelerates PFAS spreading during AFFF applications: PFAS are known surfactants, and their ability to reduce interfacial tension allows for increased lateral spreading in the vadose zone.
Multi-component solute interactions: PFAS are frequently introduced into the environment in mixtures of other surfactants and organic chemicals. The presence of these chemicals can have synergistic and antagonistic effects on equilibrium adsorption properties.
Additional Transport Processes: PFAS retention and retardation by non-aqueous phase liquids (NAPLs) in media are starting to receive more research attention. Early modelling evidence suggests vapor phase transport of PFASs can cause upward migration of PFAS.
Evidence of Self-Assembly and Chemically Driven Rate-Limited Desorption: Molecular self-assembly refers to the potential of PFAS and other surfactants to form discrete microstructures at liquid interfaces. These thermodynamically stable microstructures may contribute to the consistently elevated PFAS concentrations observed in source zones.

Results/Lessons Learned:

Many considerations are needed when assessing the fate and transport risks for PFAS at a given site. While there is early evidence that equilibrium models can predict long-term mass flux, this is likely only for locations with more idealized transport. As research continues, more readily generalizable PFAS fate and transport assessment techniques will emerge. Practitioners will need a better understanding of PFAS-specific non-idealities to better interpret field data.

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Cooper Rockwell