The significant sorption of per- and polyfluoroalkyl substances (PFAS) onto soils creates the possibility that two existing geophysical technologies, nuclear magnetic resonance (NMR) and complex resistivity (CR), might show measurable response to high PFAS concentrations in aqueous film-forming foam (AFFF) source zones. Phase I of this project was a proof-of-concept investigation of these technologies as rapid screening tools for the evaluation of PFAS in soils and sediments.

The objective of Phase II of this work is to better understand the apparent correlation between CR signals and PFAS concentrations in soils from an AFFF-impacted source zone as observed in Phase I. Specific objectives of this work are to better understand [1] detection boundaries relative to soil PFAS concentrations; [2] the relative contributions of different AFFF constituents on the CR response; and [3] the influence of the soil matrix on variations in the electrical signals in the presence of AFFF. Another objective is to confirm the sensitivity of field-deployed CR to AFFF impacts (including variations with depth) on transects across two source zones.

The premise behind using CR is its potential to detect soil PFAS and hydrocarbon surfactants (HCS) concentrations, primarily through polarization responses rather than conduction responses. Both polarization and conduction are significantly affected by soil texture variations, which could obscure the detection of PFAS/HCS. Normalizing the polarization signal by the conduction signal is expected to reduce the influence of soil texture while enhancing the sensitivity to polarization associated with elevated soil PFAS concentrations. In Phase I, CR and NMR measurements were acquired on artificial and natural soils collected from the PFAS source zone at Joint Base McGuire-Dix-Lakehurst (JBMDL). The artificial soil experiments involved measurements on sand/organic material, sand/clay material, and pure sand mixtures to evaluate the sensitivity of CR and NMR measurements to sorption of PFAS constituents in a commercial AFFF solution. Initial measurements on natural soils focused on samples acquired from impacted, suspected impacted, and assumed unimpacted locations around the source zone at JBMDL. After promising Phase I results using CR, Phase II will further pursue CR experiments. The CR method is a non-invasive electrical geophysical technology that can, in certain cases, be used to detect electrochemical alterations to the mineral-fluid interface that cannot be detected with conventional geophysical methods (e.g., the well-established electrical resistivity method). In this respect, the technique is foremost sensitive to sorption processes onto soils. Deployment of this technique is non-invasive and is very similar to the more commonly used electrical resistivity geophysical survey whereby a pair of electrodes is used to inject electric current into the ground and the resulting electric field gradient is measured between another pair. Previous studies have demonstrated the ability of CR to detect sorption of metals, nutrients, and other ionic species to soils. Consequently, CR might be sufficiently sensitive to the sorption of PFAS and/or the persistent non-fluorinated HCS associated with historically AFFF-impacted source areas. This possible dependence on HCS, in addition to PFAS, encourages investigations of the use of the CR method as a screening technology for characterization of AFFF-impacted areas.

Phase I Results

The results of Phase I of this research focused on the detectability of AFFF impacts in soils using NMR and CR methods. Phase I determined that low-field NMR geophysical methods are insufficiently sensitive to detect PFAS in soils, as no signals were found in laboratory tests, leading to the abandonment of field-scale investigations. However, laboratory experiments indicated that PFAS sorption onto soils could be identified through changes in CR signals, linked to surface charge and ionic mobility. CR signal differences were observed in artificial soils with synthetic AFFF-impacted groundwater. However, natural soils from an impacted site showed inconsistent results; only two of three samples exhibited elevated CR signals. Experiments to assess CR changes in natural soils before and after PFAS removal yielded mixed outcomes. Flushing soils with synthetic groundwater for a month did not alter CR, suggesting difficulty of removing PFAS from soil. However, a significant CR reduction was noted after treating soils with methanol, implying the removal of sorbed ionic constituents, a result supported by PFAS concentration analyses of the treated soils. Field measurements over an AFFF source zone indicated high soil polarizability, correlating with a localized area of significant PFAS contamination, as confirmed by soil sampling and analysis. This high polarizability was not related to soil texture changes but likely due to sorption of AFFF constituents.

Complex resistivity (CR), while unable to directly quantify PFAS concentrations, can be instrumental in identifying areas with potentially higher impacts. This capability could significantly reduce the number of samples needed for analysis, thus improving the accuracy of quantifying PFAS soil loads compared to random sampling. The practical application of CR in rapidly characterizing AFFF source zones hinges on establishing a significant correlation between field-measured CR data and soil PFAS concentrations through statistical evidence. However, a primary challenge with geophysical technologies, including CR, is the non-uniqueness of the results, as signals are influenced by various physical and chemical properties of the subsurface. Ultimately, the value of the CR technology lies not in estimating soil PFAS concentrations but in its ability to rapidly screen sites for informed soil sampling. This approach could lead to more effective site characterization, assessment of total soil PFAS loads, and prioritization of hot spots for sampling and analysis, thereby reducing costs compared to traditional grid sampling methods. (Anticipated Phase II Completion - 2025)

Slater, L., S. Falzone, C. Schaefer, K. Keating, C. Caro, and K. Rodriguez. 2020. Could Emerging Geophysical Technologies Characterize PFAS Contamination in Source Zones? FastTIMES, 25(2):41-47.