2010-04-20_REVISION - M1977300 (2)

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Whetstone Associates Technical Memorandum Water in the mine pool may be seeping into the Ralston Creek Alluvium at very low flow rates through near-surface fractures and collapsed historical exploration boreholes that were drilled into the deposit from the valley floor. A slight increase in permeability between the mine and the alluvium would dominate the flow regime in a bedrock system having a background hydraulic conductivity of 2.8x10-7 cm/sec. As described in the revised EPP, the mine pool does not appear to impact Ralston Creek via the Schwartz Trend. This conceptual model will be verified with the drilling of one or two additional bedrock monitoring wells, as described in the revised EPP. Although the flooded mine workings are not the primary source of chemical loading to the alluvium adjacent to Ralston Creek', the alluvium and fill are the most practical and feasible regime environment location to mitigate potential releases from the mine pool. The alluvium and fill are both primary and secondary sources of molybdenum and uranium loading to shallow groundwater. The fill is composed of waste rock from the mine was that was placed over approximately seven acres in the valley and leveled for the construction of mine facilities. Although those facilities have been removed, the fill remains in place in the valley floor and has been reclaimed. Project activities over several decades released uranium which traveled in shallow groundwater and was sorbed onto fine grained particles in the alluvium and fill, creating a secondary source for chemical loading. Cotter agrees that the EPP should focus on protection of Ralston Creek and mitigation of groundwater in the alluvium. Potential mitigation options for groundwater in the alluvium and fill are included in the revised EPP, along with a proposed schedule for implementation. 2) The Operator must comply with Rule 6. 4.10(6) pertaining to designated chemicals and materials handling. The Operator must describe the procedures for the disposal, decommissioning, detoxification, or stabilization for the toxic forming materials in the underground. Specifically describe measures to be taken to prevent any unauthorized release of pollutants to the environment. Options for management of the mine pool might include (a) in-situ treatment to immobilize or perhaps recover the uranium, or (b) pumping and treatment of discharge. Cotter disagrees that in-situ treatment of the mine pool water is feasible or necessary. Humans and wildlife are not exposed to water in the mine pool. The main adits are bulkheaded in accordance with the Closure Plan, and access to the mine is limited to crawling down into the workings through the Sunshine Adit. The hydrogeologic conceptual model shows that water in the mine pool has limited potential connection to Ralston Creek and alluvium through fractures and possibly unplugged boreholes. Mixing calculations presented to DRMS in the March 19 meeting indicate that the mine pool could be contributing no more than 0.8% - 8% of the chemical load in the alluvial groundwater. Water quality monitoring and gamma surveys conducted in and along Ralston Creek near the Schwartz Trend crossing (1.5 miles downstream from the mine site) indicated no evidence of chemical loading from the mine pool via the Schwartz Trend. The results of these studies are included in the revised EPP. Water level elevations from new monitoring wells proposed for installation in 2010 are expected to confirm the hydrogeologic conceptual model. Increases in sulfate and uranium concentrations in alluvial groundwater that were observed in Fall 2008 and Spring 2009, suggest that the flooded mine could contribute 0.8 to 8% of the alluvial flow. 4109C.100416 2