Research Project Full Title: Quantifying the Distribution of Biotic & Abiotic Transformation Rate Constants in Low Permeability Clay Zones for Improved Assessment of TCE Impacts to Groundwater at DoD Field Sites
Principal Investigator(s): Charles Werth
Sponsor(s): Strategic Environmental Research and Development Program (SERDP).
Back diffusion of trichloroethylene (TCE) and daughter products from low permeability zones (LPZs) such as clay aquitards is one of the biggest challenges facing attainment of groundwater cleanup goals at DoD sites. These pollutants are slowly released from LPZs to more permeable stratigraphic units at mass discharge rates that often result in extracted groundwater exceeding MCLs. At issue is whether these discharge rates will significantly decrease over time due to natural attenuation, or whether some interim remedial measure is needed to more quickly bring these rates down to acceptable levels. Chief among factors confounding the prediction of future discharge rates is poor knowledge of long-term transformation rates of TCE and associated reaction products. Our approach is to collect cryogenic LPZ cores at a TCE contaminated site, and then section, thaw, and analyze them for vertical concentrations of solutes and mineralogy. Similar LPZ cores from an adjacent “clean” site will be collected, placed into laboratory columns, and subject to varying boundary conditions in the laboratory to promote selected TCE reaction pathways. They will be frozen after reaction pathway development is apparent, and then sectioned, thawed, and analyzed. Complementary batch experiments will be performed to explore a larger suite of conditions that promotes reactive mineral formation near the LPZ-HPZ interface. A mechanistically-based reactive transport model will be coupled with a geochemical speciation model to simulate vertical profiles of solutes and minerals measured in the aforementioned cores. This project will demonstrate whether cryogenic methods can be used to preserve vertical concentration profiles of TCE associated volatiles at field sites, how these and other (e.g., mineralogy) vertical concentration profiles can be simulated to obtain distributions of reaction rate constants for biotic and abiotic TCE transformation pathways, and how these distributions can be used to select the appropriate conceptual model for TCE reaction modeling. This project will also produce a set of stochastic reaction models that will demonstrate how uncertainty in rate constants leads to the uncertainty in contaminant flux exiting an LPZ-HPZ system, and how this uncertainty can be used to make management decisions regarding cleanup at Pease AFB and other relevant DoD sites, including when to transition from more active to more passive remedial measures.