Case Study: Aerobic Cometabolism of Vinyl Chloride in Central Ohio
Site Summary: Chlorinated solvents were found in ground water at a former automobile repair shop. The solvents were thought to have been released from an underground storage tank (UST). After the UST and associated contaminated soil were removed, CL-Out® bioremediation was implemented to reduce the levels of contamination to site-specific cleanup goals. Geology and Hydrogeology The site is set in the area of central Ohio where the ground moraine is relatively thin. The underlying bedrock is limestone with a very porous and heavily fractures surface. While the till of the ground moraine is relatively impermeable the bottom of the UST cavity was very close to the bedrock surface. The affected ground water was in the weathered surface of the bedrock, which is a relatively prolific formation. The depth to the top of water was approximately 9 feet and the monitoring wells were screened to a total depth of 15 feet. Contamination The size of ground water plume that resulted from leaching soil contaminants was estimated to be 4,000 square feet. The original contaminant was suspected to be PCE, but the main contaminants at the time of the remediation were cis 1,2 DCE and vinyl chloride. There may have been naturally occurring organisms that degraded the PCE to DCE and vinyl chloride. CL-Out bioaugmentation was implemented to accelerate the natural process to achieve timely site closure. Remediation Design CL-Out® bioremediation was implemented through temporary injection wells in the source area. Two applications of CL-Out were made between February and August 2008. Monitoring wells were sampled in the treatment area to assess the progress of the remediation. The following table shows the contaminant concentration treads in the monitoring well with the highest pre-treatment contaminant concentrations. Maintaining Aerobic Conditions CL-Out bioremediation is aerobic cometabolism that uses dextrose as the growth substrate. Oxygen is required for the metabolism of the dextrose and to supply oxygen for the oxygenase enzyme that is responsible for the destruction of the DCE and vinyl chloride molecules. During the project dissolved oxygen levels (DO) and redox conditions (ORP) were monitored. Prior to CL-Out remediation the DO average was 1.4 mg/L and the ORP was -62. With clearly anoxic site conditions, CL-Out bioremediation was selected, but the redox conditions were monitored to be sure the conditions did not fall out of the effective range for CL-Out bioremediation. As the remediation progressed, the DO and ORP decreased to 0.42 and -225 respectively. During the August 12, 2008 CL-Out application, ORC® socks were installed in two monitoring wells to maintain aerobic conditions for the remainder of the project. Microbial Population CL-Out consists of naturally-occurring Pseudomonas sp. organisms. The background population of Pseudomonas sp. was tested prior to CL-Out bioaugmentation. The background population was 19,000 cfu/ml. The target population for effective remediation is 1,000,000 cfu/ml. Thirty days after CL-Out bioaugmentation, the population of Pseudomonas sp. was 840,000 cfu/ml in a down gradient monitoring well. Results After two small treatments, the contaminant concentrations in the source area were reduced to acceptable levels. The cis 1,2-DCE concentration decreased from 10.0 ug/L to 1.3 ug/L and the vinyl chloride concentration decreased form 80.0 ug/L to less than 1.0 ug/L. There was no rebound in the four months of post treatment monitoring. Remediation and post treatment monitoring took less than 10 months. The product cost for CL-Out treatment was less than $5,000.