Formatted Title
Impacts of Bioventing and Soil Cover Remedies on Commingled PFAS and Petroleum Hydrocarbon Plume
Background/Objectives
Parsons is conducting a Remedial Investigation (RI) of PFAS at a former Fire Protection Training Area (FPTA) at a site in the interior of Alaska adjacent to the Yukon River. Fire training exercises, including use of aqueous film-forming foam (AFFF), were conducted from the 1950s until 1991. Remedies including bioventing and a soil cover were implemented at the site in 2018 to address petroleum hydrocarbons (PHCs) and resulted in a rapid decrease in PHC concentrations in source area groundwater. Bioventing was terminated in April 2023. The FPTA is in a local topographic low point that concentrates snow melt in the spring. Historically, the ponded water infiltrated through the most contaminated soil once the ground thawed. The soil cover redirected snowmelt away from the former FPTA circle reducing infiltration through the most contaminated soil. The remedies changed the redox conditions in unsaturated soil and the infiltration pattern of surface water. This presentation evaluates the impacts of the PHC remedies on total PFAS concentrations and the composition of PFAS.
Approach/Activities
During previous investigations and prior to implementing the remedies in 2018, 10 surface soil, 18 subsurface soil, and 20 groundwater samples were analyzed for PFAS. RI sampling in 2022 included groundwater samples from nested monitoring wells (14 wells in the spring and 18 wells in the fall), 20 surface soil samples, and 57 subsurface soil samples from 10 boreholes. RI samples were analyzed for 25 PFAS using QSM 5.3 Table B-15. Total oxidizable precursor (TOP) analysis was also performed on two surface soil, two subsurface soil, and two groundwater samples. Additional samples were collected during the 2023 field season and analyzed using EPA Method 1633. Historical and post-remedy PFAS results were compared using graphical techniques to evaluate the effects of the PHC remedies on PFAS composition.
Results/Lessons Learned
After implementation of the remedies the following were observed:
- TOP analysis documented the presence of oxidizable precursors in surface and subsurface soil within the treatment area (concentrations gradually decreasing with depth).
- After installation of the soil cover the quantity of surface water infiltration immediately upgradient of the former FPTA circle increased. In an upgradient well, located within the area of ponding, total molar concentrations of PFAS increased over an order of magnitude after the remedies were implemented; however, the relative composition of the PFAS (predominantly PFAAs with six or fewer carbon atoms) appears unchanged. PFAS in this well prior to and after remedy implementation are thought to have resulted from oxidation of precursors present in surface soil and subsurface soil.
- RI data indicate a higher fraction of short-chain PFAAs in a well located within the former FPTA than downgradient wells, possibly the result of precursor oxidation by the bioventing remedy. There are no pre-remedy data for the source area well to confirm.
- In a well located approximately 80 feet downgradient of the former FPTA circle, total molar concentrations of PFAS appear to be decreasing and to fluctuate slightly with water level. There are no clear impacts to PFAS composition from the remedies. Similar observations were made for wells located further downgradient.
- PFAS samples will be collected in spring 2024 and evaluated to determine if the PFAS composition change (i.e., concentrations of short-chain PFAA decrease) is the result of termination of bioventing and return of reducing conditions to soil within the former FPTA.
-
Lesson learned – perform more robust PFAS sampling early (including pre-remedy implementation) to evaluate impacts of remedies for non-PFAS contaminants. The presence of PFAS precursors should be considered when implementing aerobic (oxidative remedies).