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Applied NAPL Science Review (ANSR) is a scientific ejournal that provides insight into the science behind the characterization and remediation of non-aqueous phase liquids (NAPLs) using plain English.
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PFAS and LNAPL/PFAS Microemulsions at NAPL Remediation Sites
Volume 10, Issue 5 | August 2022
Sid Park, Jacobs
Where historical NAPL fires were extinguished using AFFF, PFAS may be retained in the subsurface in residual NAPL mass, at the air- or NAPL-water interface, or as a viscous LNAPL/PFAS microemulsion (LPME). PFAS retention in-situ and potential presence of LPME with phase behavior that is very different from both the LNAPL and PFAS require special consideration for management or treatment. This article summarizes the current understanding PFAS partitioning to NAPL and potential LPME formation for consideration in-situ characterization and remedial design.
Aqueous film-forming foam (AFFF) contains high concentrations (grams per liter) of per- and polyfluoroalkyl substances (PFAS) and was historically used to combat fires where the fuel source was non-aqueous phase liquids (NAPLs) including light non-aqueous phase liquids (LNAPLs) such as diesel, gasoline, or waste oil and/or dense non-aqueous phase liquids (DNAPLs) such as trichloroethene (TCE) (Figure 1). These events occurred at fire-training areas, fuel spill locations, in hangars or on runways, and at bulk fuel storage areas. Conceptually, after being sprayed on the fire, AFFF foam eventually collapsed and AFFF liquid infiltrated the subsurface. PFAS dissolved in water also infiltrated the subsurface. Although several studies showed preferential association of PFAS to NAPL and at the air-water interface (AWI), evidence from one study indicated PFAS can form an immobile LNAPL/PFAS microemulsion (LPME) with milligrams per liter (mg/L) concentrations of PFAS (Figure 2) at the NAPL-water interface (NWI). PFAS retained in NAPL, at the AWI, or as LPME may represent an ongoing source of PFAS to groundwater and limit disposal options for recovered LNAPL, which was previously treated through traditional means that are likely ineffective for PFAS treatment. PFAS persistence in the subsurface and potential treatment difficulty can increase remedial durations and associated costs. Therefore, understanding the presence and behavior of PFAS at sites with NAPL or where LPME may have formed is necessary for site characterization, remediation, and revitalization.
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