The significant sorption of per- and polyfluoroalkyl substances (PFAS) onto soils opens the door to the possibility that two existing geophysical technologies, nuclear magnetic resonance (NMR) and complex resistivity (CR), might show some measurable response to high PFAS concentrations in aqueous film-forming foam (AFFF) source zones. This proof-of-concept project investigated the hypothesis that “sorption of PFAS onto soil-fluid interfaces will result in a detectable CR and/or NMR response.” Specific objectives of the project focused on evaluating the potential for using these technologies as rapid screening tools for evaluation of PFAS in soils and sediments.
In the first phase, 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. These soils were saturated with a synthetic groundwater based on the composition of groundwater obtained from JBMDL. In the second phase, three natural soil samples from JBMDL were monitored for one month during continuous flushing of the samples with the synthetic groundwater to promote desorption of PFAS compounds from the impacted soil. In a related effort, the CR response of PFAS-impacted soils from JBMDL before and after methanol treatment to remove strongly sorbed PFAS constituents was investigated. The second phase also exploited a unique opportunity to perform a series of preliminary field-scale CR measurements on a transect crossing the source zone at JBMDL.
The study conclusively showed that the low-field NMR geophysical method does not have adequate sensitivity to detect PFAS in soils. No detectable signals were observed in the laboratory, so the technology was not pursued in the subsequent preliminary field-scale investigations. In contrast, laboratory measurements provided evidence that sorption of PFAS constituents onto artificial and natural soils may result in a detectable CR signature. CR signals on artificial soils saturated with synthetic PFAS-impacted groundwater captured the temporal evolution of a polarization attributed to PFAS sorption. However, laboratory measurements on natural soils acquired from JBMDL did not conclusively establish a link between soil PFAS concentrations and CR signal. Although no detectable CR response was recorded from flushing a natural PFAS-impacted soil to promote desorption over a one-month period, a significant decrease in imaginary conductivity was recorded following a methanol wash procedure to promote PFAS desorption. Field-scale CR measurements at JBMDL revealed an intriguing complex conductivity pattern, with imaginary conductivity increasing towards the expected source of the impact.
Any further geophysical investigations of impacted soils from PFAS source zones should focus on the CR method only. Future experiments should determine, to the extent possible, the sensitivity limits of CR to PFAS impact. Additional experiments on natural soils and/or field-scale measurements could be performed at other well-characterized PFAS source zone sites where much higher source zone PFAS concentrations are documented. Finally, given the growing evidence that PFAS constituents sorb to the air-water interface in addition to the mineral-water interface, CR measurements on PFAS-impacted unsaturated soils are warranted. (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.