EOS Permeable reactive barriers successfully treat a large Trichloroethene plume - Case Study

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Courtesy of EOS Remediation, LLC

Historical use of trichloroethene (TCE) and other chlorinated solvents at a former au-tomotive parts manufacturing plant in Indiana resulted in a large groundwater plume, stretching approximately 2 miles from the site. TetraTech GEO evaluated sev-eral remedial alternatives and selected enhanced in situ bioremediation to reduce contaminant concentrations and eliminate continued plume migration at the site.

The Challenge
Historical releases of TCE from multiple sources within the 36-acre manufacturing plant has re-sulted in a plume roughly 9,000 ft long and 1,100 ft across, traversing an unconfined aquifer to a depth of 50 to 75 ft below ground surface (bgs) (Figure 1). Over time, the west-to-east trending plume has migrated to a river downgradient of the site. Groundwater velocity ranges from 1.4 to 2 ft/day. A large area of the plume contained TCE concentrations greater than 1,000 ìg/L.

The Strategy
Initial bench-scale testing by TetraTech GEO determined that enhanced in situ bioremediation would be an effective and economical technology to treat chlorinated solvents in site groundwater. As part of follow-up field testing, TetraTech GEO injected EOS 598B42 emulsified oil substrate, along with a commercially-available dechlorinating bacterial culture, to validate the technology at the site. A groundwater circulation system was operated for 5 months to enhance distribution of substrate and bacteria within the treatment zone. After 1 year, complete dechlorination of TCE was observed, leading to regulatory acceptance of the full-scale remedial design.

Full-Scale Design
Six 1,000-ft long permeable reactive barriers (PRBs) were constructed to intercept contaminant migration at various points along the plume transect (Figure 2). Various EOS products, including EOS 598B42, EOS 450 and EOS LS, were injected through a combination of permanent, temporary and direct push injection wells (typically spaced 15-25 ft on-center) to build the PRBs. A Dosatron system provided in-line dilution of EOS substrate prior to injection (Figure 3). Maintenance injections were conducted approximately 1 to 2 years after the initial injections to replenish substrate both consumed by enhanced bioactivity and diluted by rapid groundwater flow. Approximately 824,000 lbs of EOS substrates were injected to build and maintain the six PRBs.

Results
The geochemistry of the aquifer downgradient of each PRB was transformed to conditions that support reductive dechlorination within 2 months of initial EOS substrate injection. Oxidationreduction potential (ORP) measurements were mostly below -100 mV and there were significant reductions in both nitrate and sulfate concentrations. Figure 4 shows the rapid response to treatment measured in performance monitoring well MW-21S, located 50 ft downgradient of the Airport PRB. Daughter products including vinyl chloride (VC) and ethene formed soon after injection.

Regulatory Approval
The installation of six PRBs along the plume effectively cut off the downgradient migration of TCE from each PBR (e.g., Airport PRB; Figure 5) and several portions of the plume are compliant with site remediation goals. The documented improvement and continued success has allowed the Indiana regulators to endorse the cleanup technology and environmental goals for the site.

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