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Envirocon is presently implementing a major design-construct project that provides for all facets of project management, design, and construction at the Milltown Reservoir Sediments Operable Unit near Missoula, Montana. This project was acquired through a competitive bid process that required Envirocon, on a fast-track basis, to develop a comprehensive design-construct process for remediation of a highly complex Superfund site subject to significant regulatory, political, and community scrutiny.
The Milltown Dam was originally constructed to provide hydropower electricity for a large regional sawmill in 1907. In 1908, a 100-year flood resulted in an overbank condition that washed tailings from a major copper mining operation in the Butte and Anaconda area approximately 100-miles upstream of Milltown. The reservoir backwater condition created by the dam resulted in the aggradation of an estimated 6 million cubic yards of sediment. A portion of this sediment, estimated to equal 2.6 million cubic yards, has resulted in the degradation of groundwater quality within the adjacent area, principally due to the leaching of arsenic at concentrations greater than the maximum contaminant level allowed.
Envirocon's contracted scope includes: the lead role for technical regulatory negotiations including negotiation of the Statement of Work; technical and legal support to the Client on the Consent Decree negotiations; field investigations and data collection; design; work plan generation; construction; and monitoring. Envirocon's approach to the project execution includes:
- Negotiating a Consent Decree that encompasses this Scope of the Work and meets the goals of the client.
- Constructing a 3,700 foot long bypass channel and diversion to prevent scour of contaminated sediments in the Clark Fork River channel;
- Lowering the reservoir level and demolishing the dam;
- Constructing and operating railcar loading and unloading facilities;
- Dry excavation, rail transport, and disposal of 2,600,000 cubic yards of sediments from behind Milltown Dam; and
- Constructing the final Clark Fork River channel alignment and associated floodplain and re-routing of the river back into its permanent location.
Under the Superfund process, the EPA developed a plan to manage the impacted sediments which included hydraulic dredging to recover the sediments and convey them to a nearby repository. Upon evaluation of the plan, Envirocon developed an alternate that provides for early dam removal so that sediments can be "dry excavated" thus providing for higher production. Due to traffic considerations locally, and the potential for beneficial use of the sediments as capping material once excavated and removed from the existing submerged condition, a rail haul alternative was developed.
Since being selected for the project, Envirocon has worked closely with a number of agencies by providing the technical evaluations necessary to support issuance of a Record of Decision that conforms to Envirocon's "dry excavation" approach. These efforts have included the following:
Evaluation of the environmental effects of the project related to scour volumes, rates of release, toxicity, and the continuum of sediment migration;
The development of a by-pass channel concept to limit sediment scour of contaminated sediment;
- Preparing various excavation sequences and approaches to accommodate sediment recovery and channel regrading;
- Preparing several Sampling and Analysis Plans (SAP) to support development of design data;
- Negotiating and writing a Statement of Work for the project; and
- Completing Consent Decree negotiations.
Challenges associated with this project include:
- Reaching consensus among the NRD trustees, EPA, and PRPs on novel plans to integrate restoration with remediation;
- Completion of the final Statement of Work and Consent Decree;
- Obtaining FERC dam license surrender approval;
- Maintaining current scheduled milestones for design and start-up of field work; and
- Researching and refining innovative approaches to the work to add value to the final construction approach.
Preparation of the draft Statement of Work began in earnest in the fall of 2003, including a comprehensive field sampling effort which started in December, 2003. Design activities are scheduled to continue through most of 2007 with drawdown of the reservoir scheduled to begin in early 2006 and construction to begin in late 2006. The schedule is ultimately determined by the regulatory processes, including Superfund, and Natural Resources Damage programs. Currently, all site work is scheduled to be completed by the end of 2010. The project will proceed with a phased design approach allowing construction of certain portions or the work while other portions are being designed.
Construction Summary
The first step in construction will be an initial lowering of the reservoir level approximately 10 feet. Dewatering wells will be installed along the footprint of the bypass channel. Coupled with the initial drawdown the dewatering wells will sufficiently drain the sediments, improving their shear strength, and allowing the bypass channel to be excavated in relatively dry conditions.
Once the bypass channel is completed, a diversion structure will be built across the Clark Fork River forcing its flow into the bypass channel. This event signals the start of the next phase of the reservoir drawdown. The turbines in the powerhouse will be removed and the reservoir will be lowered another 5 feet. Because the Clark Fork River is in a bypass channel, scour of the contaminated sediments in the old channel due to the reservoir drawdown is prevented.
Upon removal of the turbines a cofferdam will be constructed upstream of the dam isolating the spillway section. The spillway section will be removed in dry conditions and a new channel constructed in its footprint. Once the new channel is constructed, the cofferdam will be removed allowing the river to flow freely through the dam. This will be the final stage of reservoir drawdown.
The final stage of reservoir drawdown begins the process of dewatering of the sediments. Without the reservoir's artificial head keeping groundwater levels high in the sediments, they will drain, allowing excavation in relatively dry conditions. Over a two-year period the sediments will be excavated, loaded onto rail cars and transported some 100 miles upstream where the sediments will be spread as capping material on an existing tailings facility. Because of the high organic content of the sediments and relatively low metals levels, the sediments make excellent capping material that will sustain vegetation with little maintenance.
During the two-year sediment excavation period, the remaining portions of the dam will be removed. Because the river is freely flowing through the dam, the remaining portions of the dam can be excavated in dry conditions. Most the demolition debris will be buried on site.
Upon completion of the removal of the sediments and dam, a new river channel and floodplain will be constructed in the footprint of the sediments. After a period of time has passed to allow the new floodplain vegetation to establish, the river diversion will be removed and the river placed into its new channel. The bypass channel will then be regraded marking the conclusion of the project.
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