Formatted Title
The Hunt for PFAS: Modeling the Shenanigans of Groundwater-Surface Water Interactions
Background/Objectives
Several thousand per- and poly-fluorinated alkyl substances (PFAS), including PFOS and PFOA, are listed in the USEPA’s CompTox database. Recently, USEPA issued lifetime drinking water health advisory levels (HALs) of 4 and 20 picograms per liter, or parts per quadrillion (ppq), for PFOA and PFOS. The behavior of PFAS in the environment is very complex; for example, most PFAS compounds are resistant to biological degradation processes, sorb to sediment and microplastics, exhibit self-assembly behavior , partition into non-aqueous phase liquid, and concentrate at air-water interfaces. At many sites, PFAS impacts occurred through the uncontrolled release of aqueous film-forming foam (AFFF) at fire training areas. Over time, these compounds leach to groundwater which may, in turn, discharge to surface water at a loading rate in excess of surface water quality standards. The persistence of these compounds, decades of unregulated use, and a global anthropogenic background result in large and complex contaminant distributions at sites, with transport across multiple media types (e.g., soil, groundwater, surface water, sediment), and more opportunities for completed exposure pathways to potential receptors. As a result, there are several sites where contaminant assessment and management of PFAS at the groundwater-surface water interface (GSI) is necessary. At these sites, managers and stakeholders need a quantitative framework for combining field measurements and hydrogeologic information. More importantly, they require decision support tools to answer questions about the future state of the Site in response to proposed remedies.
Approach/Activities
A numerical modeling approach using the MODFLOWUSG-Transport code was developed and tested at multiple confidential sites where unregulated use of AFFF resulted in significant groundwater and surfacewater plumes that migrated large distances because of interactions between groundwater and surface water. In addition to simulating advection, diffusion, and adsorption to the soil matrix, MODFLOWUSG-Transport represents vadose zone transport and adsorption of PFAS at the air-water interface. Additionally, the code has sophisticated capabilities to represent complex groundwater and surface water interactions The numerical modeling approach conceptualized and abstracted information from the conceptual site model, and represented the dynamics of groundwater flow and transport, surface water flow and transport, and the complex interactions between groundwater and surface water.
Results/Lessons Learned
The proposed approach has been used at sites with PFAS contamination arising from AFFF application at fire training areas and migrating sitewide through groundwater and via complex interactions with surface water. We demonstrate this approach at a synthetic site that has many similarities with real-world locations – decades of source release, transient exchange fluxes between groundwater and surface water, flow direction reversals (streams/lakes/wetlands switching from losing to gaining and vice versa), groundwater “day-lighting” temporarily into surface water and re-entering the groundwater domain at a different location, contaminant mixing zones, and attenuation along the flow path. The ability to concurrently simulate the unique properties of PFAS while also representing their fate and transport through groundwater and surfacewater creates opportunities to develop meaningful remedies.