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Zero-Valent Iron


Camp Lejeune, NC: In-Situ Treatment And Stabilization, Marine Corps Base Camp Lejeune

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Geo-Con was contracted to perform in-situ soil remediation at the Marine Corps Base Camp Lejeune in North Carolina. Soil remediation was undertaken utilizing Geo-Con's Shallow Soil Mixing (SSM) technique at Site 89 on the Marine Corps Base. SSM is an effective soil improvement technique used to treat soils in-situ without excavation or removal. Treatment of the contaminated soils involved implementation of a proprietary method to deliver Zero Valent Iron (ZVI) suspended in a slurry of water and Texas Sodium Bentonite clay to the TCE/PCE contaminated soils at the site. ZVI has only recently been deployed via soil mixing methods, having historically been utilized as a reagent in the application of permeable reactive barriers. The SSM technique first utilizes Bentonite clay slurry alone as drilling fluid applied through Geo-Con's hollow-stem augers. ZVI is added to the slurry once a column has been drilled to depth. This new mixture is applied in a similar fashion and the column stroked to achieve a homogenous mix. Bentonite clay slurry acts not only as a drilling fluid and reagent transport media but also minimizes groundwater flow through the treatment zone following the treatment process. Geo-Con's scope consisted of in-situ treatment of 30,000 cubic yards of soil with ZVI.

In this application, Geo-Con used a Calweld crane-mounted drill rig to implement the soil mixing operation. A wet or hollow Kelly bar connects the mix auger with the drill turntable and is connected to a hose (via swivel) that carries slurry from the plant to the auger. A ten-foot diameter mixing-auger was used to drill, inject the media at the mixing head and mix to produce a homogeneous soil mix column. ZVI and Bentonite clay were applied at specific ratios in three designated areas to a depth of 25 feet. The ten-foot diameter columns were spaced to provide complete coverage in those treatment areas.

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Bentonite clay slurry was prepared in Geo-Con's on-site mixing plant to be injected into the soil during the drilling operation. The Bentonite slurry was injected at the mixing head as the mixing auger was advanced downward to the maximum treatment depth of 25 feet to create the appropriate soil mix proportions within the column. SSM columns were laid out to provide coverage of the specific ZVI application rate. Once the prescribed amount of Bentonite clay was injected and the column thoroughly mixed, iron was introduced from a second batching system set up specifically for that purpose. The column was then remixed from top to bottom while ZVI was added.

A total of 515 columns were successfully installed to treat the designated areas. A sampling program was implemented to verify the amount of iron installed. Selected columns were sampled a 10-foot intervals with Geo-Con's SSM sampling tool. Soil mix samples were tested for iron content using a wet wash and iron separation test. Results of sampling indicated the average iron content of each column was within specifications and no individual samples were more than +10% from each other.

Control of emissions resulting from the reaction between the ZVI and TCE/PCE was achieved utilizing Geo-Con's emission extraction hood system. The system hood remains in constant contact with the ground surface being treated and extracts reaction emissions as well as nuisance odors. Vapors collected through the system were routed through activated carbon canisters which were properly disposed of at the completion of this successful project.



 

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