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Mountain Coa! Compam~ 1999 Annual Hydrology Report West Elk Mine Sedimentation Ponds There are seven ponds, MB-1, MB-2R, MB-3, MB-4, MB-5, RPE pond, and SG-1, at the West Elk Mine that are utilized to manage mine outflows, stormwater runoff, and wastewater treatment effluent. In addition, there is a "freshwater" pond, FW-1, that is used to equalize diversions from the North Fork. Pond FW-1 serves as the raw water supply for the mine for both domestic and in-mine purposes. In WY99, pond MB-2R discharged about 300 acre-feet to the North Fork, whereas ponds MB-3, MB-4 and MB-5 together discharged less than one acre-foot. Pond MB-1 did not discharge during WY99. Pond SG-1, located in Sylvester Gulch, discharged less than one acre-foot during WY99. The pond receives runoff from the ventilation shafts area and water encountered during development of the mine ventilation shafts. The RPE pond did not dischazge during WY99. Ponds MB-1 and MB-2R As depicted in Figure 6, ponds MB-1 and MB-2R play a strategic role in water management at the mine. Pond MB-1 receives mine discharges and stormwater runoff, and can be discharged to either the North Fork or transferred to pond FW-1. During WY99, pond MB-] did not receive mine water discharges and, consequently did not dischazge to the North Fork. Pond MB-2R receives mine dischazges, stormwater runoff and effluent from the wastewater treatment plant (WWTP), and is released to the North Fork or occasionally sent to pond FW-1. Both ponds operate in a "batch" mode; i.e. water is detained while settling occurs then discharged to the North Fork. • Pond MB-1 was re-lined with a HDPE liner in January 1998 to eliminate a suspected source of water to the mine site landslide. It was believed that the previous liner was damaged, thereby allowing water to seep out of the pond and to contribute inFlow to the mine site landslide. The new liner has been extremely effective in eliminating seepage from MB-1, which, in tum, has aided in stabilizing the landslide. Sediment removal was performed on pond MB-2R from October 1, 1998 to December 3, 1998. During this process, flows were diverted to pond MB-3 for treatment and discharge. Pond discharges to the North Fork are made in accordance with the MCC CDPS Discharge Permit No. CO-0038776. Table 8 summarizes the water chemistry of ponds MB-1 and MB-2R discharges during WY99. Table 9 provides the numeric discharge limits for the sedimentation ponds, as per the CDPS permit. Neither pond had a discharge that exceeded numeric standards during WY99. Use of the NW Panels sealed sump and the Lone Pipe pipeline resulted in improved water management in sedimentation ponds MB-1 and MB-2R. This fact is evident by the reduced volume of water treated in the ponds and the lack of dischazge exceedences. The water quality of the North Fork is better protected during upset conditions due to the storage capacity of the NW Panels sealed sump • 831-031.510 Page 17 Wright Water Engineers