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Kathy Welt and Christine Johnston March 7, 1997 (DRAFT) Page 9 4. During November and December 1996, MCC maximized the use of its underground sumps to allow ponds MB-1 and MB-2R to be drawn down in anticipation ofrainfall-runoff and snowmelt events which ofren occur in January through Mazch. Thus, MCC diverted roughly 14 acre-fee[ more than was discharged to the North Fork. However, there was no "call" on the North Fork during that time, so no vested water rights were harmed. Furthermore, because MCC diverted in-priority, and because the Mount Gunnison right has no return flow obligation, MCC had no return flow responsibility. 5. No net depletions to the North Fork aze projected through October 1, 1997, assuming that: (1) the "steady state" for B East Mains fault inflows in 1997 is roughly 85 gpm, (2) the current treatment facilities will accommodate outflows of 200 to 300 gpm and the new mine water treatment facility is operational by October 1, 1997, and (3) MCC releases its Lost Lake Slough water to the North Fork during times of call. 6. After the new treatment facility is operational, MCC will be able to operate the mine so that there will continue to be no net depletions to the North Fork. Ponds MB-1 and MB-2R Ponds MB-1 and MB-2R play a pivotal role in water operations at the West Elk Mine. With the exception of minor F-Seam water and occasional direct dischazges, all outflows from the mine are directed into one of these ponds. The waters are treated and then discharged to dte North Fork in accordance with the NPDES permit that is provided as Exhibit 7 of the permit. The combined volume of these ponds is 27.9 acre-feet. Because the ponds also intercept stormwater and snowmelt nmoff, the optima] mode of pond operations is to keep the pond levels low. MCC is required to maintain the capability to store the full 10-year return frequency, 24-hour duration runoff volume, which totals 6.8 acre-feet for both ponds. One of the major reasons why MCC has utilized the underground sumps is to ensure that there will be sufficient storage capacity in ponds MB-1 and MB- 2R to accommodate large snowmelt and rainfall-runoff volumes. For example, in the spring of 1996, MCC experienced arain-on-snow runoff episode that was more severe than a 1-in-25 year storm event. Discharge volumes from the ponds in 1996 were larger than normal due to the inflows from the B East Mains fault. The combined annual discharge volume from both ponds was 433 acre-feet. Even with the higher dischazges, the volume only represents about O.l percent of the annual flow in the North Fork for 1996. Proposed Loue Pine Gulch Discharge MCC is also planning to add the capability to discharge water from the NW Panel sealed sump into Lone Pine Gulch. MCC has received approval from the CWQCD to add this location as an NPDES discharge point. Discharge to Lone Pine Gulch would begin only when the water level in the sump has reached the seal elevations, and only if MCC determines that it is not feasible to convey the necessary amount of water out of the main mine portals. Long-term obsen~ations of Lone Pine Gulch by MCC staff indicate that it is an ephemeral drainage which flows only in response to snowmelt runoff and intense rainfall runoff events. At this time, it is