HIGH RESOLUTION FIELD DATA FOR UNDERSTANDING PFAS TRANSPORT IN THE CAPILLARY FRINGE

While it is understood that per- and polyfluoroalkyl substances (PFAS) have an affinity for accumulating at air-water interfaces, it remains unknown how water table variations within the capillary fringe may impact PFAS concentrations in groundwater both in high concentration (>10 mg/L) zones near release areas and down-gradient lower concentration (1 to 1,000 ng/L) zones. It is anticipated that because PFAS accumulate at air-water interfaces, changes in groundwater levels and vadose zone moisture content may result in a PFAS “storage” zone in the capillary fringe that may cause temporal changes in groundwater concentrations and influence long-term PFAS mass flux in the saturated zone. Completed, ongoing, and planned phases of work are used to develop a high-resolution empirical field data set to characterize temporal PFAS distribution and mass flux as well as dynamics of the hydraulics in the vadose zone and capillary fringe. Field data consist of high-resolution PFAS and water level/content data collected from various methods including soil moisture probes and tensiometers, lysimeters, pressure transducers, continuous multichannel tubing (CMT) multi-port wells, nuclear magnetic resonance (NMR) geophysical imaging, and Vertebrae™ multi-port horizontal wells. Analysis of these data includes interpretation in the context of existing conceptual models for PFAS behaviors.

The primary focus of this project is examination of spatiotemporal relationships between moisture content changes above the water table, water table elevation, PFAS concentrations in groundwater, and mass contributed to groundwater plumes. An additional focus is the collection of a benchmark field dataset for understanding PFAS transport in the capillary fringe, which is critical to understanding PFAS distribution and behavior. Site investigation is driven by the following key hypotheses:

• PFAS sequestration and release in the capillary fringe is analogous to the “smear zone” phenomena observed for LNAPL; however, the mechanisms are different which will result in empirically observable behavior.
• Changes in moisture content and air-water interfacial area in the capillary fringe are related to changes in PFAS concentrations and mass flux/discharge contribution to groundwater plumes.