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Posted 4/10/2012

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By Charles Coyle, Sam Bass, Quang Le, Héctor Santiago and Mark Rothas
Environmental and Munitions Center of Expertise, U.S. Army Engineering and Support Center, Huntsville

A team from the U.S. Army Corps of Engineers’ Omaha District successfully used an innovative method of deep soil mixing to remediate an area contaminated by trichloroethylene at an Atlas missile site on the former Offutt Air Force Base near Arlington, Neb. The steam injection and auger technology used is considered relatively new and can be applied at sites that have difficult soil and groundwater conditions to clean up and where technologies that are typically used do not work well.

Thermal treatment using large-diameter auger soil mixing and placement of zero-valent iron, or ZVI, was effective in treating the source area at the Atlas missile site. The pilot study was primarily intended to address vadose-zone soils impacted with chlorinated volatile organic compounds, trichloroethylene, cis-1,2-Dichloroethylene and  vinyl chloride.

The technology operates one treatment cell at a time by advancing a single 8-foot diameter auger to required depths of up to -60 feet. During active mixing, the soil is homogenized and the permeability increases, allowing for steam and hot air to be injected through ports in the cutting tool of the auger.

Steam heats the contaminated soil, thermally desorbing the VOCs from soil particles and then volatizing the desorbed chemicals. Hot air injected by the process carries some of the volatized contamination to the surface for capture and treatment. Slurried, micro-scale zero-valent iron is also injected as the large diameter auger is withdrawn. The ZVI helps create reducing conditions and facilitates dechlorination of the remaining, dissolved-phase chlorinated solvents.

The treatment zone was about 6,750 square feet, and the depth interval of the treatment zone was from 10 to 40 feet below ground surface. The number of “pushes” by the large diameter auger to achieve coverage of the treatment zone was 163.

Based on data collected after completion of treatment, the combined technology was effective in reducing TCE, cis-1,2-dichloroethylene and vinyl chloride concentrations in source area soil and groundwater. A reduction in TCE and total chlorinated volatile organic compounds concentrations of more than 99 percent was observed in both soil and ground water samples collected within the treatment zone.

Outside of the treatment zone, some locations had increased chlorinated solvent concentrations. These increases indicate that a limited degree of contaminant mobilization occurred during treatment. A five-fold increase in the TCE concentration in soil was observed at one location. Directly below this location, the TCE level in groundwater increased from 92.6 ug/L to 10,900 μg/L, suggesting contaminate mobilization. This sample location was just outside of the treatment area and is one of the two monitoring well locations that are closest to the high strength portion of the source area, suggesting that the contaminant mobilization was probably limited to areas in close proximity to the treatment zone.

At some of the monitoring well locations, substantial reductions in TCE were accompanied by increases in DCE. DCE is an intermediate breakdown product of TCE. Generation of DCE indicates that reductive dechlorination is occurring. Continued monitoring will be needed to confirm that complete dechlorination of DCE is occurring.

It is important to note that the contaminated groundwater at this site will be addressed through a separate remedy. Full-scale groundwater treatment will be accomplished through in situ bioremediation via injection of organic substrate. Also, continued monitoring will be performed to confirm the effectiveness of both remedies.

The soils requiring treatment were mostly silty clay and glacial till. Some modifications to the process were required to facilitate boring through the low-permeability soils. For example, the auger bit was modified to add more aggressive teeth to the top and bottom of the bit to make it more suitable for the clay soils at the site.

Soil treatment operations began in October 2010, but due to winter conditions, the contractor suspended operations in November. They were resumed in March 2011 and completed in May. The contractor was paid $2.033 million to remediate about 7,500 cubic yards, or $271 per cubic yard. The total cost also included project oversight, management and analytical effort. Work was performed under a performance-based contract.