2011-10-07_GENERAL DOCUMENTS - M1977300

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Phase 1: Upstream Cutoff Wall and 8 Cubic Feet per Second (cfs) Bypass Pipeline — This phase will involve the construction of an upstream cutoff wall (up to 1.5 -foot wide, 10 to 20 feet deep, 140 feet long) during low flow conditions to create a barrier to alluvial and surface flows in Ralston Creek upstream of areas of potential contamination. This cutoff wall will be constructed at an existing road crossing of Ralston Creek upstream of the southern waste pile. A replacement road crossing is included as part of the cutoff wall design. A short segment of 42- inch diameter steel pipe will be installed in the cutoff wall in preparation for Phase 2. The channel upstream and downstream of this cutoff wall will be armored with boulders and rock to minimize erosion. In conjunction with the upstream cutoff wall, a flexible, 8 cfs bypass pipeline (consisting of an 18 -inch flexible pipeline) will be installed to draw clean water from upstream of the cutoff wall, discharging near the downstream property boundary. The 8 cfs bypass pipeline will run along the north side of the existing unpaved road, with periodic anchoring and /or staking to prevent excessive movement or rolling. The diversion of the creek into the 18 -inch diameter pipeline is listed as "Substantial Completion of the Pipeline" on the attached schedule and will be complete by January 31, 2012, pending agency approvals. The bypass pipeline will isolate the upgradient groundwater and low flow surface water up to 8 cfs from potential areas of contamination on the Site and will improve water quality in the creek downstream of the property. Water that exceeds the 8 cfs bypass pipeline will pass through a 42 -inch diameter steel pipe and flow down the existing channel. Phase 1, listed as "Final Completion of Phase 1" on the attached schedule, will also include the installation of an irrigation system to sustain riparian vegetation surrounding Ralston Creek. According to the "Endangered and Threatened Wildlife and Plants, Revised Critical Habitat for the PMJM (Zapus hudsonius preblei) in Colorado" by the Fish and Wildlife, U.S. Department of the Interior, "The segment of Ralston Creek that passes through the Cotter Corporation's existing Schwartzwalder Mine serves as a connector between areas supporting all PCEs [Primary Constituent Elements] required by the PMJM located upstream and downstream ". Ralston Creek provides linear riparian PMJM habitat through the Site. For more information on PMJM, we can provide the Biological Assessment (BA), if desired. A water supply for irrigation and other Project purposes has been agreed on, in principle, among Cotter, Denver Water, the Colorado State Engineer's Office in Denver, and the Division 1 Engineer's Office in Greeley. Cotter will divert water from Ralston Creek under the approved exchange for beneficial uses, including irrigation of riparian vegetation. Phase 2: Temporary Diversion Pipeline (100 cfs) and Downstream Cutoff Wall — After the upstream cutoff wall and 8 cfs bypass pipeline have been constructed, construction of the 100 cfs Temporary Diversion Pipeline will commence (pending authorization from the applicable regulatory agencies). The pipeline will consist of a 42 -inch diameter steel pipe and will follow a largely aboveground alignment from the upstream cutoff wall through the Site, discharging downstream of the downstream cutoff wall near the property boundary. The downstream cutoff wall will be constructed to key into bedrock to contain residual groundwater (primarily direct precipitation and flows entering the alluvial fill between the two cutoff walls) so that it can be collected via the existing pump- and -treat system. The discharge from the WWTP to Ralston Creek will be below the downstream cutoff wall. Energy dissipation measures will be installed October 7, 2011 Page 2