The objective of this project was to improve the understanding of the nature and extent of per- and polyfluoroalkyl substances (PFAS) within representative Department of Defense (DoD) sites by performing detailed, systematic investigations at three sites. The purpose was to develop improved general conceptual site models that site managers can use to inform cost-effective management decisions at PFAS-impacted sites. As part of this objective, this project also aimed to provide verification of PFAS fate and transport processes that have been increasingly documented in controlled laboratory settings but have yet to be comprehensively investigated at the field scale.

High-spatial resolution sampling of groundwater and soil was conducted at three DoD fire training areas. Samples were analyzed for PFAS using state-of-the-art analytical methods such as enhanced soil extraction, high-resolution mass spectrometry, and semi-quantiation of PFAS without analytical standards. A soil extraction method was developed under this project to enhance the extraction of anionic, cationic, and zwitterionic PFAS from AFFF-impacted soils. Furthermore, particule-induced gamma ray emission (PIGE) was evaluated as a total fluorine screening tool for groundwater and soil samples.

The three sites chosen in this project were i) Naval Air Station (NAS) Jacksonville, FL; ii) Naval Base Ventura County (NBVC) Pt. Mugu, CA; and iii) Naval Auxiliary Landing Field (NALF) Fentress, VA. A conceptual site model was developed for each site based on the extensive data collected; relevant transport processes were identified and the distributions of various fractions of PFAS were calculated. Potential transformation pathways were also identified based on the extensive list of PFAS analyzed by high-resolution mass spectrometry.

The soil extraction method that was developed during this project showed it was capable of sufficiently recovering a wide variety of PFAS from both spiked and field-collected AFFF-impacted soils. This extraction method was used on all the soil samples from the three sites and samples were screened for a list of ~1400 PFAS. To determine the relative importance of PFAS lacking commercially available analytical standards, their concentrations were estimated by a novel semi-quantitative approach. Total PFAS concentrations determined by semi-quantitation were compared with concentrations determined by the total oxidizable precursor assay.

The conceptual site model at NAS Jacksonville, based on the vast groundwater and soil datasets, showed that zwitterionic and cationic PFAS made up a majority of the total PFAS mass (up to 97%) in firefighter training area soil. The percentage of branched perfluorooctane sulfonate (PFOS) increased with depth and distance from the source zone, consistent with differential isomeric transport; on the other hand, linear perfluorooctanoic acid (PFOA) was enriched throughout the site suggesting fluorotelomer precursor transformation had occurred. Perfluorohexane sulfonamide, a potential transformation product of other sulfonamide-based PFAS precursors, was present at high concentrations (maximum 448 ng/g in soil, 3.4 mg/L in groundwater). These results suggest that precursor compounds may create long-term sources of PFAS to groundwater, although the particular pathways remain largely unknown. A mass balance of PFAS across the site was performed, showing that PFAS precursors were being converted to perfluoroalkyl acids at an annual rate of 2% or less, and that the relative contribution from precursors on a mass basis was decreasing with distance from the source.

Mechanisms such as retardation due to the partitioning of PFAS to solid-phase surfaces from hydrophobic and electrostatic interactions were consistently observed at all three sites, along with slow advection and diffusion into shallow lower-permeability soils. Field K_d values were calculated for PFAS at all three sites. At NAS Jacksonville, retardation was particularly evident for zwitterionic/cationic precursors, which were strongly retained within source area soils with low permeability. At NBVC Pt. Mugu, retention within the source area could also be attributed to enhanced electrostatic interactions due to the presence of high salinity groundwater. At NALF Fentress, PFAS concentrations were generally highest in shallow unsaturated soils within the source area and then decreased with depth and distance downgradient. This retention could be partially attributed to the enhanced partitioning of PFAS at the air-water interface within the vadose zone. In particular, retention within the vadose zone due to air-water interfacial partitioning was evident based on vertical profiling that showed that PFAS concentrations decreased significantly between shallow unsaturated soils and the top of the saturated zone. Collectively, these processes contribute to the attenuation of PFAS concentrations moving away from the source, with decreases in concentration of several orders of magnitude observed between the source area and the farthest downgradient locations. 

A distinguishing characteristic at NALF Fentress was the presence of relatively high PFAS concentrations, including zwitterionic/cationic compounds, in shallow soils at mid-plume locations. These results suggest that surface releases and/or runoff of AFFF at downgradient locations may be serving as a secondary source of PFAS to groundwater.

The PIGE results showed a statistically significant relationship with the LC-MS/MS data for aqueous samples at NAS Jacksonville (linear regression r2=0.57), but the relationship was below the selected performance threshold. In part, this was due to non-linear distribution of the data, particularly at the lower-end PFAS concentrations where PIGE appeared to provide less resolution. These data suggest that PIGE provides a reasonable indication of the relative PFAS levels in these groundwater samples, but may be better suited as a screening tool for characterizing higher-concentration areas. The PIGE comparison results of groundwater and soil data at NBVC Pt. Mugu showed a statistically significant relationship for both matrices, but only the groundwater datasets demonstrated a correlation meeting the r² > 0.7 criteria. There were a number of samples where PIGE did not detect PFAS even though the total PFAS through the higher-resolution techniques was greater than 4000 ng/L in all samples. This was a result of the high limit of quantification for PIGE at this site (approximately 5,000 ng/L), which was influenced by the high ionic strength of the groundwater. These data provide confirmation that PIGE is likely better suited for source area characterization efforts at sites where the water is not highly saline.

The detailed datasets acquired during this project provide a much more comprehensive view of environmental PFAS than was previously available, including the transport, transformation, and retention of a wide variety of PFAS at three AFFF-impacted sites. Major findings such as the high concentrations of zwitterionic/cationic PFAS in source zone soils and the high retention of PFAS in low-permeability zones will be relevant for many other DoD sites.

This approach can be implemented with standard field equipment, but requires specialty analytical laboratory services. In this project, academic research laboratories provided innovative high-resolution analytical capabilities for both soil and groundwater. This included analysis of a large suite of PFAS (including suspect screening) that are otherwise not commercially available at this time. To evaluate this large suite of analytes at a generic site, a research laboratory would need to be identified that could provide a similar level of high-resolution data capabilities, although it is anticipated that commercial laboratories would also be able to provide this service in the future. In lieu of this, an assay measuring total organofluorine such as TOF or the total oxidizable precursor assay could provide an estimation of the total PFAS mass at a site. 

PIGE analysis has high quantitation limits in the configuration used in this project, especially in areas where the water composition indicates high TDS or salinity. This may be a shortcoming with respect to the application of PIGE with variable geochemical composition, though as PIGE sample preparation methods are refined these limitations can likely be overcome. (Project Completion - 2022)

Adamson, D.T., P.R. Kulkarni, A. Nickerson, C.P. Higgins, J. Field, T. Schwichtenberg, C. Newell, and J.J. Kornuc. 2022. Characterization of Relevant Site-Specific PFAS Fate and Transport Processes at Multiple AFFF Sites. Environmental Advances, 100167. https://doi.org/10.1016/j.envadv.2022.100167.

Adamson, D.T., A. Nickerson, P.R. Kulkarni, C.P. Higgins, J. Popovic, J. Field, A. Rodowa, C. Newell, P. DeBlanc, and J. Kornuc. 2020. Mass-Based, Field-Scale Demonstration of PFAS Retention within AFFF-Associated Source Areas. Environmental Science & Technology, 54:15768-15777.

Kulkarni, P.R., D.T. Adamson, J. Popovic, and C.J. Newell. 2022. Modeling a Well-Characterized Perfluorooctane Sulfonate (PFOS) Source and Plume Using REMChlor-MD Model to Account for Matrix Diffusion. Journal of Contaminant Hydrology, 247.

Nickerson, A., A.E. Rodowa, D.T. Adamson, J.A. Field, P.R. Kulkarni, J.J. Kornuc, and C.P. Higgins. 2020. Spatial Trends of Anionic, Zwitterionic and Cationic PFAS at an AFFF-Impacted Site. Environmental Science and Technology (ES&T), 55:313-323.

Nickerson, A., A. Maizel, P. Kulkarni, D.T. Adamson, J. Kornuc, and C. Higgins. 2020. Enhanced Extraction of AFFF-Associated PFASs from Source Zone Soils. Environmental Science and Technology (ES&T), 54:4952-4962.

Rodowa, A.E., E. Christie, J. Sedlak, G.F. Peaslee, D. Bogdan, B. DiGuiseppi, and J.A. Field. 2020. Field Sampling Materials Unlikely Source of Contamination for Perfluoroalkyl and Polyfluoroalkyl Substances in Field Samples. Environ. Sci. Technol. Lett. 7(3):156-163  https://doi.org/10.1021/acs.estlett.0c00036