Formatted Title
PFAS in Stormwater at PFAS Sites: Characteristics, Tools, and Potential Management Strategies
Background/Objectives
The increased focus on PFAS impacts to groundwater and soil has led to closer scrutiny of potential impacts from historic PFAS-containing usage on stormwater quality. For example, in October 2021 the U.S. Dept. of Defense’s SERDP program issued a research Statement of Need for “Improved Management of Stormwater Impacts at Dept. of Defense Facilities” that stated “...stormwater events can include discharge(s) that contain high volumes with low PFAS concentrations. Commonly used technologies for stormwater treatment are often not effective for PFAS removal.”
Approach/Activities
The objective of SERDP Project ER23-3714 is to better understand the nature of the PFAS mass loadings both temporally and spatially, as well as how to treat PFAS-impacted stormwater (when needed). The experimental design is modeled after the successful protocols used in the nonpoint source pollution field, where a portfolio of intra- and inter-storm stormwater sampling data, selected on-site conditions affecting stormwater discharge data, site-specific hydrologic calculations to establish anticipated design storm flows, and simple runoff loading models have enabled the current design and use of stormwater treatment systems at certain sites. However, three initial questions are:
1) What is the relative magnitude of PFAS loads in stormwater vs. groundwater at impacted sites?
2) What are the key factors that control the mass discharge of PFAS in stormwater from impacted sites?
3) What are the potential ways that PFAS in stormwater could be regulated and/or managed?
These three questions are now being evaluated using simple planning level models of stormwater runoff , erosion rates, and leaching from construction materials.
Results/Lessons Learned
1) A key controlling factor that will determine the overall magnitude of the PFAS in stormwater issue is what environmental criteria will be applied at various sites. The USEPA’s draft Maximum Concentration Levels (MCLs) may be used in situations where surface water is being used for drinking water after consideration of mixing and PFAS removal in water treatment processes; for PFOS and PFOA the proposed MCLs are 4 ng/L. Draft chronic and aquatic ambient water quality criteria have been proposed by USEPA to protect freshwater organisms; for PFOS and PFOA the proposed chronic exposure criteria are 8,400 ng/L and 94,000 ng/L, respectively. To date, there have only been a few jurisdictions that have evaluated potential water quality criteria for fish consumption, but due to potential bioaccumulation of PFAS in fish tissue, these criteria may be relatively low concentrations.
2) To directly determine PFAS stormwater loadings from sites with known PFAS impacts, both the flow and the concentration of PFAS in stormwater are needed. Temporal sampling during a storm event is needed to determine if PFAS loadings exhibit a strong first flush effect where PFAS concentrations are much higher during the initial part of the hydrograph rising limb. With sufficient data from enough sites, event mean concentrations (EMCs ) may prove useful to manage PFAS sites as they have for managing suspended solids, nutrients, and other conventional nonpoint source pollutants. Simple flow models and EMC data can identify the driving factors that control PFAS mass discharge out of a wide list of factors such as climate (precipitation patterns), storm event intensity, soil type, slope, ground cover (soil vs. grass vs. concrete/asphalt); erodibility of soils; drainage infrastructure; type of PFAS source, etc.
3) There are a wide variety of potential approaches to managing PFAS in stormwater: concentration-based limits for stormwater discharges; concentration-based limits in receiving streams after mixing calculations; establishing “best available technology” for certain PFAS source categories; and mass discharge limits via total maximum daily 4load (TMDL) approaches that are applied to entire watersheds.