Dry cleaning solvents were found in the soil under a parking lot adjacent to a large dry cleaning plant located in a mixed commercial and residential suburb of Boston, Massachusetts. The contamination was traced back to a storm water catch basin in the parking lot which contained sediment and sludge with high concentrations of the dry cleaning solvent perchloroethene (PCE). The sludge and catch basin were removed, but the contaminated ground water had migrated under the dry cleaning plant and adjacent residential properties. While most of the mass of ground water contamination remained under the parking lot, the low concentrations under residential properties were enough to raise concerns about potential vapor intrusion hazards. CL-Out bioremediation was implemented in the parking lot area to reduce the volume of contamination in the source area and mitigate future potential off-property risks.
The setting of the site is a complex glacial region of interbedded tills and channel deposits. The surface material at the site was mostly interbedded silt and sand lenses that formed a water bearing zone of varying permeability.
The affected aquifer was an unconfined silty sand zone approximately 25 feet thick. Monitoring wells were screened at two levels in the aquifer to assess the potential for DNAPL. Shallow wells were screened at the water table (10 to 20 feet deep). Deep monitoring wells were screened at the bottom of the perched aquifer approximately 25 to 35 feet deep.
The size of ground water plume that resulted from leaching soil contaminants was estimated to be 13,000 square feet. The contamination was mainly PCE, trichloroethene (TCE) and cis 1,2-dichloroethene (DCE).
CL-Out bioremediation was implemented in the ground water in the source area. Four existing monitoring wells were used for the injections. Surrounding monitoring wells were sampled to assess the progress of the remediation. The ground water was periodically inoculated for a period of 18 months. During each inoculation three units (165 gallons) of CL-Out were injected.
After 18 months of treatment, the contaminant concentrations in the source area were reduced by as much as 99%.