USING PFAS FORENSICS, STRUCTURAL GEOLOGY, AND RECHARGE AGES TO REFINE THE CONCEPTUAL SITE MODEL FOR GROUNDWATER CONTAMINATION AT THE RUTLAND SOUTHERN VERMONT REGIONAL AIRPORT (RSVRA)

In 2018, the Vermont Dept. of Environmental Conservation discovered that many drinking water wells completed in the fractured rock aquifer (FRA) surrounding the RSVRA were contaminated with elevated levels of Per and Poly Fluoro Alkyl Substances (PFAS). These PFAS were derived from the application of Aqueous Film-Forming Foam (AFFF) to the ground surface during annual fire equipment testing and at aircraft crash sites. From 2018-2023, the Vermont Geological Survey and academic partners collaborated to characterize the FRA using physical (field mapping, spatial analysis of wells in GIS, and geophysical logging) and chemical (major and trace elements, anions, stable isotopes, PFAS, and recharge ages) methods to construct a preliminary conceptual site model (CSM).

In 2024, we focused extensively on PFAS Forensics to develop a detailed understanding of the spatiotemporal behaviors of these toxic compounds in groundwater, including the transport, sorption and transformation of PFAS compounds. Using the PFAS data from wells tested from 2018-2023, we delineated well groups based on these criteria: 1) Ratio of (PFOA + PFOS) / (PFHxS + PFPeA + PFHnA), 2) Presence/ absence of Fluoro Telomer Sulfonate (FTS) precursors, and 3) Radar plots. Group 1 wells had FTS precursors and the lowest proportion of C8 compounds, whereas Groups 2 and 3 had no FTS precursors and intermediate to high C8 proportions, respectively.

The point sources for AFFF application include 3 aircraft crash sites and the annual AFFF testing site located just NE of the runway intersection, which was used from ~1980- 2017. The crash sites establish a Time 0 for AFFF introduction and the distance to nearby wells, when supplemented with structural data, give approximate subsurface travel pathways in the FRA and the maximum travel times from source to receptor. PFAS transformation sequences and times can also be estimated along these flow paths. Legacy AFFFs, with manufacturing dates ranging from 1976-2004, which were collected from fire departments near the RSVRA and analyzed for 29 PFAS compounds, were used as proxies for parent AFFF at the point sources. We integrated structural and groundwater flow data from geophysical logging with PFAS transformations, and recharge ages, to produce a series of new and detailed cross sections that refined our CSM.