Site Summary

Historical dry cleaning solvent spills from an active dry cleaners contaminated soil and ground water under a shopping center. The migration of contaminants in shallow ground water spread contamination from under the building into a residential subdivision. The resulting plume of contaminated ground water was more than 1,000 feet long.

Geology and Hydrogeology

The geology of the site was a complex of interbedded marine sediments. The surface material was interbedded silt and sand lenses. The water table was approximately 20 feet deep. The affected aquifer was an unconfined silty sand zone approximately 16 feet thick. The underlying formation was a dense clay aquitard.

Ground Water Contamination

The size of ground water plume that resulted from leaching soil contaminants was estimated to be 150,000 square feet. The contamination was tetrachloroethene (PCE) and its breakdown products trichloroethene (TCE), cis 1,2-dichloroethene (cis 1,2-DCE), and vinyl chloride. The following table shows the pre-treatment concentrations at the source area, mid-plume and at the front of the ground water plume.

Remediation Design

With the high concentrations in the source area, a chemical oxidation treatment was implemented. For the downgradient plume, CL-Out bioremediation was selected and injected at key locations to effectively treat the plume. Ten units of CL-Out (5,500 gallons) were injected into the contaminated ground water through vertical injection wells scattered throughout the centerline of the ground water plume.

Results

Within one year of the application of CL-Out, the contaminant concentrations were significantly reduced throughout the plume. The following table shows the contaminant reductions in the source area, mid-plume and down gradient plume front.

Please refer to the able below for the pre- and post-treatment sampling results. The contaminant concentrations were reduced by 97% in the source area, 86% at the leading edge of the plume, and 65% mid-plume. This effectiveness was the result of the concentration of effort in the source area and the front of the plume, where most of the injection took place and the greatest benefits were to be gained.

Bioremediation Results

Background

The chlorinated solvent trichloroethylene (TCE) was discovered in ground water at a manufacturing facility in eastern Massachusetts. ISCO was selected for remediation of the ground water. Although ISCO reduced TCE concentrations, the residual post-ISCO TCE concentrations were above the applicable remediation standards. The environmental consultant chose CL-Out® bioaugmentation to treat the residual ground water contamination. The CL-Out® treatment successfully achieved the remediation goal quickly and at a low cost.

Geology and Hydrogeology

The contaminated ground water was in a shallow, sandy, water-table aquifer. The residual contamination was located under a manufacturing building and adjacent to a river. The main receptor concerns were vapor migration into the building and contaminated ground water migration into the river.

TCE was the only contaminant in ground water. Post-ISCO TCE concentrations were as high as 54 µg/L. Daughter products were not detected. CL-Out® was selected for the treatment because it is capable of cometabolizing the TCE without producing daughter products such as vinyl chloride. Furthermore, the CL-Out® organisms are aerobic, so they were compatible with the natural oxygenated conditions of the aquifer.

Remediation Design

CL-Out® was introduced into the contaminated ground water by injection through temporary well points. Three drums of the CL-Out® were injected into the ground water through 14 injection points over an area of 1,200 square feet.

The ground water treatment results were monitored by analysis of ground water samples from five locations. The 30-day monitoring results show an immediate decrease in the TCE concentrations without the production of DCE or vinyl chloride. After 30 days as much as 97% of the TCE was destroyed and the ground water met applicable standards.

Bioremediation Results

Dissolved oxygen (DO) concentrations and oxidation-reduction potentials (ORP) were also monitored to assess the growth environment for CL-Out® organisms. Before the bioaugmentation the DO concentration was 0.47 mg/L. After the bioaugmentation, the DO concentration dropped to 0.04 mg/L, which confirms active aerobic microbial metabolism. The ORP was 208 before and 272 after bioaugmentation, which shows positive redox potential was maintained even though oxygen was being utilized.

Ground water samples were tested for CL-Out® organisms by plate count analysis 30 days after the injection. The CL-Out® population was greater than 300,000 cfu/ml in 4 of 5 monitoring points 30 days after the inoculation, confirming a good distribution of the organisms and that the population was sustained at the planned level.

Conclusions

This project demonstrates several of the benefits of aerobic bioaugmentation. Bioaugmentation provides active control of the site as effective organisms compatible with the site conditions were injected where they were needed. Furthermore, the CL-Out® aerobic cometabolism was compatible with the residual conditions following ISCO treatment. The TCE was destroyed without production of daughter products, which accelerated site closure at minimal cost.