The presence of specific perfluoroalkyl acids (PFAAs) and their polyfluorinated precursors in groundwater, notably the PFAAs at aqueous film-forming foam (AFFF)-impacted fire training areas, has raised concerns about the risk these PFAAs may pose to human health. Unfortunately, there are serious data gaps with respect to the understanding of the behavior of poly- and perfluoroalkyl substances (PFASs), which include PFAAs, in groundwater systems, particularly in the presence of co-contaminants present at AFFF-impacted sites. Moreover, very little is known about how PFAA fate and transport will be affected when sites are subject to in situ remediation aimed at attenuating the co-contaminants also present. Thus, understanding the interplay between the co-contaminants typically present at AFFF-impacted sites, the biogeochemical changes brought about by their remediation, and the transport potential of PFAAs is crucial in designing and implementing any remediation strategies aimed at PFAAs.
The overall goal of this research was to evaluate the relative importance of key physicochemical and biological parameters in determining the fate and transport of PFAAs in groundwater in the presence of co-contaminants and during remediation of co-contaminants.
Controlled experiments were conducted under laboratory conditions to carefully evaluate a range of factors that affect PFAA transport, including the presence of co-contaminants such as chlorinated solvents and fuel-derived nonaqueous phase liquids (NAPLs). In particular, the importance of co-contaminant concentration, composition, interfacial area, and extent of remediation on the retardation of PFAAs was examined. This research helped to elucidate the extent to which current in situ technologies widely used for co-contaminant remediation alters the fate and transport of PFAAs present in groundwater. Monitored natural attenuation (MNA), enhanced reductive dechlorination (ERD), and in situ chemical oxidation (ISCO) impacts on PFAA fate and transport were explored. Batch tests and flow-through experiments in 1-D columns and 2-D cells were completed to examine the complex interactions between hydrological, biogeochemical, and remedial conditions on PFAA behavior. The data from various scales were analyzed to determine the key variables impacting PFAA behavior in contaminated groundwater during ambient and remediation-induced conditions.
Prior to the start of this project, little was known about how PFAA fate and transport would be affected when AFFF-impacted sites are subject to in situ remediation aimed at attenuating co-contaminants. When microbial co-contaminant remediation technologies were employed, the interactions between PFAAs and the microbial populations were complex. For hydrocarbon-degrading populations, PFAAs did not appear to significantly affect hydrocarbon degradation, though the PFAAs did appear to induce biofilm formation which may subsequently have impacted PFAA transport. Conversely, high levels of PFAAs appeared to interfere with microbial dehalogenation reactions. The extent to which these types of interactions were relevant for polyfluorinated precursors remains unknown, though impacts were suspected. Further research, using actual AFFF-impacted soils, groundwater, and/or aquifer materials, may help evaluate the extent to which the microbially mediated remediation technologies impact PFAA fate and transport.
With respect to ISCO remediation technologies for co-contaminants, the results of this study suggested ISCO, particularly the application of persulfate and permanganate, will significantly impact PFAA fate and transport. Notably, in the case of persulfate, the effects appeared to be primarily driven by the low pH that results from persulfate application. Catalyzed hydrogen peroxide and permanganate, however, appeared to either not impact or speed up the transport of PFAAs. Of particular note was the apparent capacity of permanganate to not only mobilize PFAAs from AFFF-impacted soil, but also to facilitate significant conversion of some polyfluorinated precursors to PFAAs. Coupled with the apparent slower desorption kinetics of polyfluorinated precursors, these data collectively suggested that additional studies related to PFAAs release from source zones is warranted.
The results of this project significantly helped the environmental restoration community’s effort to develop conceptual site models of PFAAs at AFFF-impacted sites as well as move closer to modeling PFAA plumes at these sites. PFAA fate and transport under conditions relevant to AFFF-impacted sites is a complex topic; though fundamental questions have been addressed through this project, many more questions remain. With further study, the results of this project will meaningfully contribute to modeling and site assessment guidelines that can be used by engineers and contaminant hydrologists for practical implementation.
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