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Envirocon completed six phases of work at this refinery site closure and redevelopment project in Casper, Wyoming. One of the phases of work was the Soda Lake Inlet Basin Remediation which included excavation, stabilization, transportation, and disposal of 250,000 cubic yards of refinery wastewater impacted sediments deposited in the inlet basin at the upper end of Soda Lake. The Inlet Basin used to function as a sediment trap for solids entrained in the refinery's wastewater discharge stream. It is physically separated from the main lake by rock weir, though it remains hydraulically connected. Since wastewater discharge operations began in 1958, an estimated 250,000 cubic yards of impacted sediments were discharged to the basin which has developed into an environment similar to manmade wetlands. The sediment resided under a blanket of brackish water and wetland plant species. Removal of the impacted sediments was a component of a State and EPA approved remedy for the Casper Refinery site, however extreme care was taken not release impacted sediments into the adjacent main lake which had previously received a No Further Action (NFA) memorandum. The impacted sediments were readily distinguishable from native soils by distinct differences in color, texture, and physical properties.
Envirocon, having successfully implemented a variety of cost savings ideas on other projects at the Casper site, was asked to submit a proposed methodology and budgetary estimate for the remediation of the Soda Lake Inlet Basin. The primary remediation methods under consideration to that point had been capping or removal options requiring hydraulic dredging. Both the hydraulic dredging and capping options had total estimated costs in excess of $20M. Envirocon proposed passive dewatering and drying of the Inlet Basin followed by excavation and placement of the impacted sediments in a CAMU cell at a savings of well over 50%. Independent treatability studies conducted by RETEC, the project engineer, further validated the methodology. Envirocon's approach was subsequently incorporated into the approved Remedy Decision for the Inlet Basin signed in January 2002.
Once approval was obtained for the approach, Envirocon collaborated with RETEC and the client to develop an approved work plan for remediation of the Inlet Basin. The plan developed by RETEC and Envirocon began with the removal of surface water from the Inlet Basin. This was achieved by pumping water directly from the Inlet Basin to the main lake, installing cut-off trenches up gradient from the Inlet Basin to cut ground and surface water flow, and cutting "dendritic" trenches into the sediments once uncovered to release entrained "pore" water. Once sufficiently dry to support low ground pressure equipment, Envirocon selectively excavated sludge from the basin to the discernable sludge/subgrade interface. Removed sediments were transported to the nearby on-site CAMU for treatment (if necessary) and disposal. The removal procedures and confirmation of completion was based upon achieving line and grade, visual observation, and limited confirmation sampling. Passive and active dewatering activities began during the summer of 2002. Removal, transportation, and CAMU cell placement of Inlet Basin sediments commenced in March of 2003. This project was an example of the Envirocon/RETEC Team's ability to collaborate with their client's best interests in mind to render an approach that was not only practical and compliant, but that also saved money.
As part of the Casper redevelopment Envirocon constructed a kayak course in the North Platte River. The kayak course required strategic placement of large imported boulders in conjunction with the construction of four river-wide low-head structures designed to channel flow into recreational whitewater features suitable for slalom racing or play boating. To protect the fishery and to take advantage of the lowest river flows, the course was constructed during the winter months. Flows averaged between 400 to 450 cfs for the duration of the project with most of the work taking place directly in the river. Inflatable water bladders were used as temporary diversion dams to direct flow around areas where the structures were being built, typically blocking half the river at any given time. Construction began with the uppermost feature and proceeded downstream. Clean, recycled concrete from the refinery foundation removal project was used as base material for each of the structures. Once established, each feature was completed with imported river rock and grouted in place.
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