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As part of the closure of the former Vint Hill Farms Station Army Intelligence Base, volatile organic compound (VOC) contamination was identified in the saturated overburden soils at a location designated as Area Requiring Environmental Evaluation (AREE) 34.
Based on this condition, AREE 34 was remediated using an innovative ZVI-Clay technology which is an in-situ source remediation technology that involves admixing reactive media (zero valent iron), stabilizing agents (clay), and soil containing chlorinated compounds using specialized soil mixing equipment. Through this process, chlorinated compounds react with ZVI and are depleted and the hydraulic conductivity of the targeted zone is reduced, resulting in a reduction in future releases of contaminants from the source area. This patented technology was developed by Dupont and is currently being further developed by Colorado State University (CSU). Prior to the field work, CSU performed a treatability study to confirm proper application rates.
To supplement the AVI-Clay process, it was also determined that biostimulation, a treatment technology that enhances the natural biodegradation of contaminants by the addition of emulsified vegetable oil, would further optimize treatment performance.
Working with the prime contractor, Envirocon developed a proper delivery system for the reagents using the soil mixing technology and assisted in the development of the necessary work plans prior to the start of field work.
The source area identified at AREE 34 consisted of a 50-foot diameter area partially truncated due to the presence of an active electrical conduit. Treatment depth was limited by bedrock depth which was 16 to 18 feet below the ground surface. The soil profile consisted primarily of a reddish-brown clay and silt overlying a brown, saprolitic, silt and fine sand (weathered bedrock).
Soil mixing was accomplished using a Delmag RH-18 rotary drill rig and a 8-1/2 foot diameter mixing auger. A total of 35 overlapping columns were installed to the top of bedrock to treat the required source area. Initial drilling and mixing was accomplished using a bentonite clay slurry as a drill fluid and mixing media. The clay slurry was blended and injected through the hollow drill kelly bar using an on-site slurry mix plant and progressive cavity pump. Following initial drilling, a minus 50 mesh ZVI Cast Iron aggregate and emulsified vegetable oil was blended into each column with a minimum of two complete mixing strokes.
Challenges included working within an active manufacturing and office complex, directly adjacent to a new residential housing plan, and within a confined work area. Daily management of spoils (excess swell material generated from the soil mixing process) was accomplished using a track excavator. This material was initially stabilized with the addition of 5% Portland Cement and temporarily stockpiled adjacent to the treatment area. After treatment operations were complete, the stockpiled material was placed on top of the treatment area to assist in the consolidation process of the underlying treated soils. Upon verification testing, this material was regraded to match the natural topography and final restoration was completed by the prime contractor.
The relationship that Envirocon and the prime contractor developed over the projects duration contributed to its success. The prime contractor’s Project Manager remarked “Envirocon performed at an exceptional level to meet project requirements and completed the field work ahead of schedule. Envirocon’s Project Manager, provided a high level of technical expertise and was extremely diligent in accelerating the project schedule as dictated by the U.S. Army Corp of Engineers.”
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