Formatted Title
Full-Scale In Situ Bioremediation of TCE-Impacted Groundwater underneath a Building Using Groundwater Recirculation
Background/Objectives
Enhanced reductive dechlorination (ERD) was the chosen remedy for reaching groundwater cleanup goals at an active manufacturing facility in Colorado. Releases of trichloroethene (TCE) have been documented on the upgradient side of the building while suspected releases may have occurred within the building footprint near a parts washer. Passive soil gas sampling was used to identify the source areas in the vadose and groundwater. The total impacted groundwater area measured 15,000 square feet with the highest groundwater concentration of TCE being 54,900 µg/L. Negligible reductive dechlorination was occurring naturally as evidenced by low cis-1,2-DCE and vinyl chloride concentrations. Redox conditions at the site were oxidative (>100 mV) prior to the ERD activities. The soil at the site was gravelly clay to 7 feet bgs, gravelly sand and sandy gravel to 23 feet bgs, and claystone beneath 23 feet bgs. Depth to water at the site varied seasonally with an adjacent downgradient creek between 9 and 12 feet bgs.
Approach/Activities
For treatment of the groundwater plume residing underneath the building footprint, 10,300 lbs of a soluble, nutrient-amended substrate (carbohydrate-based) was distributed using consistent groundwater recirculation. To minimize disturbance to the operations within the facility, only seven vertical injection wells and five vertical extraction wells were used for the groundwater recirculation activity. Three separate groundwater recirculation wells were created for increased control over the distribution of the soluble substrate. Conveyance piping was not winterized, and active remediation was restricted to the summer months at a rate of approximately 5 gpm. The combined active groundwater remediation activity consisted of less than 18 months over three summers.
An SVE system was also operated continuously to remove TCE from the vadose soil.
Results/Lessons Learned
TCE concentrations and total solvent concentrations decreased by 99% in the wells within and outside of the building footprint. After two quarters of operation, reductive conditions less than -50 mV existed in all sampled wells. Reductive conditions persisted during winter shutdowns and following cessation of treatment activities. Complete dechlorination to ethene occurred without bioaugmentation and no cis-1,2-DCE or vinyl chloride accumulation was observed.
Monthly substrate dosing was increased from original calculations early in the treatment due to low organic carbon accumulation. The naturally high sulfate soil supported competition with dehalogenating bacteria for the added substrate. The ERD process was successful even in the presence of sulfate concentrations greater than 100 mg/L.
The presentation will discuss the site conditions, groundwater recirculation setup, operational data, geochemistry, and contaminant reductions.