Building a Robust Conceptual Site Model for Remedy Optimization of a Former Dry Cleaner Contaminant Plume

Groundwater beneath and downgradient from a former dry cleaner contains elevated concentrations of tetrachloroethene (PCE), including dense non-aqueous phase liquid (DNAPL). In 2008, a remedial action was implemented in the source area that included in situ bioremediation using a groundwater recirculation strategy to distribute amendment. The remedy resulted in groundwater concentration reductions greater than 95% on average; however, PCE concentrations remained elevated in discrete areas in situ. The conceptual site model was thus considered incomplete and additional information including the vertical and lateral extent of residual DNAPL and impacts of matrix diffusion on plume contaminant loading and longevity was necessary to inform decisions about technology implementation and optimization.

Historical investigation efforts indicated that the subsurface geology was highly heterogeneous with a stratified, high permeability sand and gravel upper aquifer underlain by a lower permeability stratified sand, silt, and clay in the lower portion of the shallow aquifer. Observations from existing monitoring wells suggested that the stratified geology and low permeability layers likely resulted in both heterogeneous vertical distribution of DNAPL in the source area and sorbed mass is diffusing into the dissolved phase plume, acting as secondary sources of contamination. Therefore, the source area recirculation treatment system was primarily delivering amendment to the higher permeability groundwater, resulting in effective treatment in the transmissive zones, but contaminant mass in the low permeability layers was not as effectively treated.

A post-remediation site investigation was performed to evaluate PCE mass distribution vertically and assess the magnitude of residual, sorbed, and diffused mass in the stratified soils. A TRIAD approach was applied using a mobile laboratory for fast-turnaround PCE data in both groundwater and soil. The results from this investigation profoundly changed the conceptual site model and will be used to determine the most appropriate next steps for remedy optimization of this PCE plume.