2008-04-15_REVISION - M1980244 (274)

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STORM WATER HYDROLOGY Back round and Prior Draina a Desi ns N The drainage plan for the mine has previously been designed by others and approved by the Colorado Division of Reclamation, Mining and Safety. The drainage plan for much of the mined area was approved in Amendment No. 8 to Mined Land Reclamation Permit M-1980-244 in 2000. That drainage plan addressed the storm water management of SGOSA and the Valley Leach Facility (VLF) Phase IV. Golder Associates, Inc. subsequently prepared revisions to that drainage plan as were required by extension of the East Cresson Mine in May 2004. These revisions focused on the storm water diversions and ponds in Vindicator Valley drainage basin, EMP011 and EMP012. Methodology and Basin Descriptions The SEDCAD4 computer program was used to model the hydrology because of its ease of use in modeling the system and summarizing results. Attachment B includes summaries of the SEDCAD4 results, the network arrangements and sub-watershed results and additional reports that describe the geometry of the sediment ponds, also known as the Enhanced Management Practice (EMP) structures. Three storm water basins associated with the MLE activities are reconsidered. The watershed storm water basins and the management rationale are described below. The East Cresson Mine and ECOSA will collect runoff from the most northerly limit of the mine to the east border above Grassy Creek. That area was treated as a basin because the interim EMPs for operations can also be used after closure. • The North Cresson Mine area (Poverty Gulch) storm water basin is the west flank of the mine north of Squaw Gulch. Most of that area, up to the affected land boundary, will be mined as part of the MLEI. Storm water management is necessary during the growth medium salvage and initial development for mining, but the basins will be removed during mining and the drainage will flow back toward the backfilled mine area after reclamation. Therefore, the EMP008b and EMP009 are designed for operation and will not.be needed after closure. • Some revisions to the SGOSA will be necessary for the proposed MLE. The northwest area of the SGOSA will be extended further west than in previous plans to provide additional storage of overburden materials (the SGOSA Extension). The north and east areas of SGOSA include backfilled mine areas (depressions) rather than overburden impoundments as previously approved. The drainage ditches that previously flowed north to Grassy Creek (per Amendment No. 8) will no longer be Client:CC&V Gold Mining Co. 4/11/08 Title: Storm Water Management, MLE Page 4