The U. S. Army Corps of Engineers Louisville District recently used a bio-soil mixing technology to treat groundwater at the former Nike C-32 Missile Site in Porter County, Indiana.
The Department of Defense acquired the site in 1956 and used it as an Ajax missile battery through 1959, and as a Hercules missile battery from 1959 until the site was deactivated in 1974. The Nike site operated to protect the city of Chicago from any possible invading attacks. Although the Nike C-32 site never launched a missile, it employed nearly 30 personnel who worked on-site and were prepared in case of an attack. Unfortunately, many types of degreasers, such as trichloroethylene, also referred to as TCE, used for cleaning the parts for the missile silos infiltrated into the ground and contaminated the groundwater.
The U.S. Army is responsible for environmental restoration of all properties that were formerly owned by, leased to, or otherwise possessed by the United States and under the jurisdiction of the Secretary of Defense. The Corps has the responsibility to investigate and clean up these sites nationwide.
“The Corps has been working on the Nike C-32 FUDS site for several years conducting the Remedial Investigation/Feasibility Study,” said Clayton Hayes, Louisville District project manager. “Which led to the recently completed remedial action that effectively reduced groundwater contaminate levels below the maximum allowable concentration limit.”
The innovative technology involved mixing soil with zero valent iron, also referred to as ZVI.
“A pilot test was performed during the feasibility study phase in September 2014 to evaluate the effectiveness of soil mixing with ZVI and demonstrated effectiveness in reducing of groundwater contaminate levels,” said Brooks Evens, Louisville District technical manager for the project. “A rotary blender mounted on a hydraulic excavator was used for the mixing operation.”
It was determined during this phase that removal of source contamination (soil removal) and treatment of the groundwater would be the most effective remedial action to close the site quickly.
“The Army could have chosen to just monitor the site but reducing the TCE to below Maximum Contaminate Levels, or MCL (EPA clean-up goals), would have taken over 500 years and cost more than being proactive in remedial treatment,” Evens said. “So the Army determined this to be the best alternative to treat the contamination.”
The remedial action for the project removed the upper 8-feet of soil which was determined to be clean and then it was stockpiled. Soil from 8 to 24-feet was to be removed as contaminated soil.
“After the excavation, the contractor conducted bio-soil mixing by removing the upper 8-feet and then used an auger to mix the soil like a blender,” Evens said. “Once all data came back that the ZVI had been mixed to a 2-3 percent ratio by weight, the backfilling began.”
This effort took five weeks to complete, from site mobilization to preliminary site restoration.
The final restoration will occur in May 2020. The Army will begin long term monitoring in November of 2020 to ensure that the remedial action has been effective. After eight consecutive quarters of performance monitoring showing that cleanup goals have been met, the Army will close out the project at Nike C-32.