The Johnson Company reviewed approximately six years of existing data from previous investigations to develop a conceptual model for this chlorinated solvent-contaminated site. The conceptual model relied on the following contaminant fate and transport theory:
Dissolved chlorinated solvent contamination in the unconsolidated deposits continues to migrate, and the fractured bedrock contamination has reached a stable steady-state condition downgradient from dispersed DNAPL sources.
The Johnson Company performed site characterization and monitoring to substantiate the contaminant fate and transport theories, and to determine if unacceptable risks to receptors are present. These activities included:
- seasonal sampling of monitoring wells in unconsolidated deposits and bedrock, including analysis of inorganic parameters related to biodegradation potential, and volatile organic parameters to assess plume stability and migration potential
- extensive and continuous water-level monitoring during bedrock aquifer pumping
- semi-annual indoor air monitoring, and
- rapid adaptive drive point groundwater profiling coupled with on-site analysis of samples with gas chromatography to delineate the dissolved chlorinated solvent plume in unconsolidated deposits.
We were able to demonstrate that:
- the dissolved chlorinated solvent plume in the unconsolidated deposits is captured and contained by an easily managed and monitored existing storm drain conduit
- monitored natural attenuation is an appropriate remedy that sufficiently manages risks to building occupants and downgradient receptors
- the dissolved chlorinated solvent plume in bedrock remains at a steady-state condition with no interferences in flow from upgradient bedrock pumping, and
- no unacceptable risks are present as a result of the contamination.
The property owner was able to estimate its future financial liabilities and not embark on a very expensive remedial action.
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