2011-08-31_REVISION - M1977300 - Laserfiche WebLink

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IRIS MITIGATION AND DESIGN, INC. SCHWARTZWALDER MINE PHASE 1 - UPSTREAM CUTOFF WALL AND LOW FLOW BYPASS PIPELINE - BIOLOGICAL ASSESSMENT include the continued use of an irrigation system to sustain riparian vegetation surrounding Ralston Creek. Phase 3: Characterization and Long -term Reclamation — Phase 3 of the Project will include further Site characterization, which will allow Cotter to develop and implement a long -term, sustainable reclamation plan for the Site. The long -term plan will include returning flows to Ralston Creek and creation of required mitigation. Since the characterization and reclamation activities would be very difficult to complete until the Site is substantially dewatered, many of the details of Phase 3 have not yet been determined. 1.3 Schwartzwalder Mine History During active mining operations dating back to the 1950's, Ralston Creek was displaced from its historic, meandering alignment to the north side of the valley to accommodate mining activities on the Site. The Site was an active uranium mine between 1953 and 2000. Between 1966 and 2000 the mine was owned and operated by Cotter. After 2000, active mining operations ceased on the Site; however, the Site remains under Cotter's ownership. Based on geologic evaluations described in the EPP, the alluvial material at the Site and adjacent to Ralston Creek varies in depth from 0 to 36 feet. The alluvial material generally is composed of sand to cobble size material. Groundwater flows in the alluvium from northwest to southeast across the Site, generally following the Ralston Creek riparian corridor alignment. A strong hydrologic connection exists between the creek and the alluvium, especially in the vicinity of the Alluvial Fill Area below the WVVfP, creating the potential for up- gradient water in Ralston Creek to come into contact with materials in the Alluvial Fill Area, resulting in mobilization of uranium. The Site has a water treatment system which includes sumps which collect and pump alluvial groundwater from the Site back to a WWTP to remove uranium before it is discharged to Ralston Creek near the downstream property boundary. The cutoff wall and low flow bypass pipeline will prevent up- gradient flows up to 8 cfs from coming into contact with areas of potential contamination on the Site. Based on hydrology data presented in the EPP, the low flow bypass pipeline would be expected to divert all or most of the flow in the creek from July through March. Cotter anticipates using the low flow bypass pipeline as the primary diversion method for 1 to 2 years while the Phase 2 Temporary Diversion Pipeline (100 cfs) is designed, permitted and constructed to allow for clean -up and reclamation of the Site in Phase 3. The low flow Page 3 August 2011