Formatted Title
Prolonged Effects from Short-Term Bio-Trap Deployment in Monitoring Wells at a Chlorinated Solvents Remediation Site
Background/Objectives
Background/Objectives. Historical degreasing operations at an aircraft parts manufacturing facility located near St. Louis, Missouri, resulted in chlorinated solvent groundwater contamination that presented unacceptable risks to occupants of onsite industrial buildings and downgradient, offsite residential dwellings. Elevated levels of tetrachloroethylene (PCE), trichloroethylene (TCE), and their daughter products (i.e., cis-1,2-dichloroethylene and vinyl chloride) are present within the shallow overburden saturated zone. The groundwater plume extends from the area of the former degreaser at the Facility into the adjacent residential and industrial/commercial areas to the south and southeast of the Facility. Treatability studies were completed in 2020 within a low-permeability clay overburden and shallow shale bedrock as part of the response action evaluation for the site. One of the technologies selected for testing during the treatability studies was enhanced in situ bioremediation (EISB). In 2023, 1,500 tons of source material soil was removed from the clay overburden zone.
Approach/Activities
Approach/Activities. Several shallow overburden and shallow bedrock monitoring wells were selected for evaluation during the EISB treatability study. Bio-Trap samplers supplied by Microbial Insights were deployed in both source treatment areas and the overall plume treatment area. Each monitoring well was sampled with an in situ microcosm assembly containing three individual Bio-Trap units containing sampling media to provide microbial, chemical, and geochemical data to assess different biodegradation mechanisms. The three Bio-Trap units used for the treatability study included a monitored natural attenuation (MNA) unit, a bio-stimulation (BioStim) unit, and a bio-augmentation (BioAug) unit. The samplers were deployed for approximately 75 days to allow for an effective and Site-representative microbial community to form on the sample matrix before retrieval. Samples were submitted to a laboratory for analysis of microbial, chemical, and geochemical parameters.
Results/Lessons Learned
Results/Lessons Learned. The results of the treatability study generally indicated that EISB could meet the cleanup goals for the groundwater plume, assuming adequate distribution of nutrients into the low permeability clay material. The MNA and BioStim units showed potential of biodegradation with generally increased biological activity in BioStim units. However, the BioAug units showed levels of biodegradation that were often orders of magnitude higher than those achieved in the MNA unit. Based on this study, the combination of adding an electron donor and culture offered the greatest potential for growth and survival of microbial populations responsible for complete anaerobic reductive dechlorination of Site contaminants. Typically, the effects of Bio-Trap deployment are short lived, and wells are anticipated to return to background conditions in a relatively short time. However, since Bio-Trap deployment and retrieval in 2020, prolonged effects have been detected in the wells where Bio-Trap samplers were deployed. PCE and TCE concentrations have been significantly reduced in these wells. Additional sampling for microbial populations was conducted in 2021, which indicated that the microbial communities introduced in the BioAug units have continued to thrive and proliferate in the wells where the Bio-Trap samplers were deployed. The BioAug culture did also significantly raise the concentration of microbial strains capable of degrading vinyl chloride over the course of the deployment, and more detections of indigenous strains were observed in the August 2021 sampling event. It is anticipated that the effects of Bio-Trap deployment have been continued to be observed due to the initial presence of contaminant-degrading microbes at the site, low groundwater flux throughout the Site, and Site conditions favorable for microbial growth.