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West E!k Mine Mine Water System `• The mine water management system (as of December 1998) is shown schematically in Figure 22. The discussion of the mine water system is divided into surface water management and underground water management. Surface Water Manaeement In general, surface water management consists of raw water diversions from the North Fork, management of the sedimentation ponds, and operation of the water and wastewater treatment plants. All of these activities aze conducted in the context of rules and regulations from (primarily) the Colorado Water Quality Control Division (CWQCD) for NPDES permits and the SEO for water rights. North Fork Water - In 1976, MCC conditionally appropriated 15 cfs for the Mount Gunnison Pipeline for industrial, domestic, mining, and reclamation uses. In 1986, 0.88 cfs of this right was made absolute, and in 1993, another 0.12 cfs was made absolute for a total of 1.0 cfs absolute. When this water right is in priority, water can be pumped from the intake point to the freshwater storage pond(s) within the surface facilities area. In average years, this water right is typically out of priority from July to September. The 10-acre-foot capacity fresh water pond (FW-1) provides for water storage during the current permit term. One more pond is anticipated for subsequent permit terms. Average withdrawals are not expected to exceed 400,000 gpd during the term of permit. All water supplies are obtained from MCC's water rights in accordance with Colorado Water Law. The freshwater pond is filled during spring runoff and kept full until water rights are out of priority, then the leased water from the Lost Lake Slough (a.k.a. the East Beckwith Reservoir operated by the Fire Mountain Canal and Reservoir Company) is used. The current maximum withdrawal at the intake is 450 gpm. The large fault inflows experienced by MCC in 1996 and early 1997 have not significantly changed the amounts or schedules of water diversions from the North Fork. The potential effect of diversions on North Fork flows have been quantified for both flow rates and flow volumes. Percent changes in flow rates in the North Fork for combinations of streatnflow and diversion conditions are provided in Table 47 and Table 48. In most cases, the percent change is within the estimated 5 percent error of streacnflow measurement at the USGS gaging station on the North Fork. With the existing pump capacity, the diversion would result in measurable change in flow only for low flows with return periods of about 200 years or greater. During these extreme low flow periods, MCC's water rights would be out of priority and diversions would not be made. Normally MCC's water rights would be out of priority when river flows drop below approximately 300 cfs during the growing season. In this case, percent changes in river flows would be less than the 0.4 percent as shown in Table 48. 2.05-180 RevisedJ,m. 1995 PR06; Revised Nov. J998 TR80; 1/98PR08