Formatted Title
Bioremediation of Industrial Chemical Plant Using Multiple Injectates: Enhanced Reductive Dechlorination, Chemical Reduction, and Bioaugmentation
Background/Objectives
The Site is located in Seattle, Washington, and was a chemical manufacturing facility from 1920 to 1990. Solvents included mineral spirits imported via rail and chlorinated solvents imported via drums or by tanker, which were transferred into bulk storage tanks and mixing vessels. By the 1980s, the facility was using over 600 chemicals to manufacture over 300 products. Chemicals manufactured included dry detergents, liquid detergents, organic solvents, cleaners, bleach, and petroleum-based solvents. Historical land use resulted in a release of hazardous substances to soil and groundwater at the Site including halogenated volatile organic compounds (HVOCs), primarily cis-1,2-dichloroethene and vinyl chloride.
The Site is located proximate to a river and has shallow groundwater that varies between 1 to 7 feet below ground surface. Based on previous slug testing, hydraulic conductivity ranges from 10-5 to 10-7 m/s, corresponding to values for soil types ranging from low permeability sand to silt, which are consistent with soil types observed.
Pilot studies for reductive dechlorination and interim action injections using various substrates and bioaugmentation have been conducted at this Site since 2005. The last injection event was conducted in 2021 and involved the injection of an edible oil substrate and bioaugmentation with Dehalococcoides (DHCs) bacteria.
The objective of the cleanup action at the Site is to achieve regulatory closure under Washington State Department of Ecology (Ecology) Voluntary Cleanup Program in accordance with the provisions of the Washington State Model Toxics Control Act Cleanup Regulation (MTCA).
Approach/Activities
Interim action work conducted at the Site in 2005 and 2010 included injection of an electron donor in four targeted areas of the Site. The interim action work resulted in accelerated biodegradation of the HVOCs present in groundwater at the Site. However, the rate of biodegradation of the HVOCs slowed due to electron donor depletion and/or insufficient population of DHCs bacteria.
In May 2015, a bioaugmentation pilot test was conducted to evaluate whether injection of DHC bacteria cultures could achieve an accelerated rate of reductive dechlorination without the injection of an electron donor. The results of the bioaugmentation pilot test indicated that injecting only the DHC bacterial culture did not sufficiently increase the microbial populations capable of biodegrading HVOCs in the subsurface. Addition of an electron donor would be necessary to increase DHC populations and sustain growth to effectively accelerate the HVOC biodegradation processes.
In September 2021, a full-scale injection event was conducted that included in situ enhanced bioremediation, bioaugmentation, and in situ chemical reduction (ISCR). An emulsified vegetable oil (EVO), EOS Pro, manufactured and distributed by EOS Remediation, LLC, was selected as the electron donor. EOS Pro includes a combination of fast- and slow-release electron donors (i.e., fermentable carbon) as well as a blend of micronutrients and vitamin B-12 designed to promote rapid and sustained growth of DHCs. Bioaugmentation using KB-1®, a microbial culture that includes DHC, provided by SiREM, a division of Geosyntec Consultants, Inc., was used to accelerate dechlorination rates. Application of ISCR in the form of zero valent iron (ZVI) was used in select areas with higher residual HVOC concentrations in groundwater to add an abiotic component to the cleanup action intended to further accelerate the cleanup time frame.
The amendments and DHC culture were injected via direct-push drilling throughout each treatment area. Targeted injection depths ranged from approximately 3 to 30 feet below ground surface, depending upon the area of the Site and historical groundwater data.
Performance groundwater and geochemical analyses in 2022 confirmed successful application of the substrate, increase in microbial populations, and reduction of HVOCs in treatment zones.
Results/Lessons Learned
Performance groundwater monitoring was conducted at 6 months, 1 year, and 2 years post injection event. The latest round of groundwater monitoring was conducted in October 2023 and laboratory analytical results are still pending. Concentrations of cis-1,2-dichloroethene and vinyl chloride detected in groundwater decreased by one to two orders of magnitude 6 to 12 months following injections. The groundwater throughout the treatment areas is also monitored for geochemical parameters, metabolic acids, and DHC microbial populations.
Based on the results of the October 2023 groundwater monitoring event, Site conditions will be assessed to determine if additional targeted injections are necessary to meet the cleanup standards at the point of compliance for the Site.