2009-09-03_REVISION - C1980007 (3)

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West Elk Mine 1 Gulch Facilities Area. See Exhibit 46 for the original design specifications and for the current designs for MB-4,. Unit Train Loadout Sedimentation Pond. Pond MB-4 was relocated in 1998 from the east side of the train load-out to an area west of the load-out between two sets of train tracks. The relocated sediment pond is constructed of concrete and is preceded by an oil skimmer and a sediment trap. Design criteria for Ponds MB-5 are contained in Exhibit 47, and an as-built survey is contained in Exhibit 66. Ponds MB-2 and MB-6 no longer exist and were replaced by new pond MB-2R. Design criteria for the Refuse Pile Expansion (RPE) pond, designated the "RPE" pond, are included in Exhibit 70. Hydro-logic Design of Runoff Ponds Seven storm drainage sediment control structures are used as settling ponds for storm water runoff from the mine site. These ponds have been designed according to CDMG reTalations (see Exhibits 43, 47, 66, and 70). These ponds have been constructed to contain an estimated three years of sediment storage volume and a runoff storage volume resulting from the 10-year 24-hour rainfall event. Sediment will be excavated, allowing for additional storage, when sediment has reached the maximum storage capacity. Sediment volumes are estimated by a quarterly visual evaluation. The smaller ponds MB-3, MB-4, and MB-5, usually dry during a portion of the year and sediment levels, relative to the primary discharge structure elevations, can be visually determined. The larger ponds usually contain water and visual evaluations are less accurate. These ponds, MB-1, MB- 2R, and SG-1 will be surveyed at least every three years to determine the actual sediment levels. The CDMG requirements for the emergency spillway sizing are 25-year, 24-hour storm flows, or 100-year, 24-hour storm flows, depending on the size of the structure. The spillways for MB-3, MB-4 and MB-5 were sized based on the 25-year, 24-hour storm, while MB-1, MB-2R, and SG- 1 were sized for the 100-year, 24-hour storm event in accordance with the requirements of the State Engineer. Drainage basins for the spillway sizing are shown on Map 54, Sediment and Drainage Control Facilities. The RPE pond has been designed to completely contain the runoff from a 100-year, 24-hour storm event. Designs are included in Exhibit 70. MB-1 and MB-2R have been classed in the United States Bureau of Reclamation (USBR) Hazard Assumption "B" category. Accordingly, the probable maximum precipitation inflow design flood has been reduced to the "B" assumption. This assumption is justified in that the largest storm drainage/sediment control pond has a storage volume of less than 15 acre-feet. Furthermore, the potential for loss of life is minimal as immediate downstream households are limited in number and could be evacuated quickly. In addition, the impact on the North Fork of the Gunnison River is minimal since the energy from such a small volume of water would be quickly dissipated. The 10-year, 24-hour storm runoff design was adopted in accordance with CDMG regulations. The Revised Universal Soil Loss Equation (RUSLE) was used to predict the sediment yield from three years of runoff. MCC will maintain the ponds in compliance with the designs to effect discharge in compliance with effluent limitations. In some cases, MCC has installed concrete sediment traps above the pond inlets to facilitate sediment removal. These traps typically have 2.05-30 Revised June 2005 PRIO; Rev. January 2006; Rev. March 2006; Rev. April 2006 PRIG, May 2006 PRIG; Rev. January 2009 MR-350