The overriding objective of this proof-of-concept project is to evaluate the performance of absorbent wicking-patch samplers as a method of collecting tension-held porewater samples for characterizing per- and polyfluoroalkyl substances (PFAS) within vadose zone source areas. Operationally, the wicking-patch sampling approach is suggested to collect tension-held soil moisture, and dissolved chemicals, including PFAS, from vadose zone soils via capillary action. When deployed within a borehole, a “snapshot” of the distribution of chemicals within the porewater is obtained. If the samplers are redeployed over time, the approach could be used to monitor the mass flux/discharge of PFAS and co-occurring chemicals from source areas. Such an approach could provide a cost-effective means of monitoring PFAS source zone dynamics; however, a rigorous performance evaluation of this approach for sampling PFAS in the vadose zone environment has not yet been performed.
This evaluation will utilize physical experiments at the bench- and intermediate scale, and numerical simulation. Two silica sands (30-mesh and 100-mesh sands), a silt loam, and a clay loam field soil will serve as the test media in this work. Geotechnical methods will be used to characterize these test media for porosity, bulk density, and saturated hydraulic conductivity. Water characteristic curves (i.e., capillary pressure-saturation relationships) will also be developed for each test media and used to derive hydraulic parameters to support numerical simulation. Likewise, the wicking patch media will be characterized for these same parameters.
The HYDRUS unsaturated flow and transport simulator will be used in this work. Bench-scale experiments will be conducted to evaluate wicking patch sampler performance in terms of the mass/volume of pore water collected and the analytical accuracy of the PFAS collected. Wicking-patch sample performance will be compared to that provided using standard suction lysimeters. Properly conditioned to the results of the physical experiments, the simulator will be used to expand the performance evaluation across a larger range of soils, moisture conditions, and wicking patch hydraulic characteristics than could be achieved experimentally within the timeline of this project. The simulator will also be used to initially assess the utility of the wicking patch sampling approach to monitor PFAS mass flux/discharge during an infiltration event. The simulated results will be compared with experimental mass flux/discharge estimates performed at the intermediate scale, where the wicking patch samplers positioned on FLUTeTM liners will be emplaced to replicate deployment via direct-push technology-based methods.
The completion of this project will benefit the Department of Defense and the scientific community in that the project (1) could provide a more cost-effective and rapidly deployed method of characterizing the nature and extent of PFAS in vadose zone source areas, and PFAS mass flux/discharge from these source zones, to improve risk evaluation and prioritize cleanup, (2) could be used to evaluate the effectiveness of source remediation activities, and (3) could support quantifying the PFAS source term for use in numerical modeling efforts to assess impacts to down-gradient receptors. The wicking patch media, and sample preparatory methods developed from this project could additionally be used as an alternative means of extracting and retaining porewater samples from vadose zone soil samples collected via traditional soil coring operations.