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Mountain Coal Comparry 1997 Annual Hydrology Repor[ N'esr Elk A4ine • MCC enlarged the hydraulic capacity of the WWTP to 20,000 gallons per day during WY97. MCC anticipates that the lazger facility will meet future demands. Sedimentation Ponds There are five ponds at the West Elk Mine that are utilized to manage mine outflows, stormwater runoff, and wastewater treatment effluent: MB-1, MB-2R, MB-3, MB-4, and MB-5. 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}. The following discussion focuses on ponds MB-] and MB-2R, because they are the most important ponds at the West Elk Mine and process far more water than ponds MB-3, MB-4, and MB-5. For example, in WY97, ponds MB-1 and MB-2R discharged about 555 acre-feet to the North Fork, while ponds MB-3 and MB-5 together discharged roughly 12 acre-feet. Pond MB-4 did not discharge during WY97. Furthermore, ponds MB-1 and MB-2R played a vital role in managing the lazge volume of mine water associated with the fault inflows. MCC also holds a storage right to retain water in ponds MB-1, MB-2R, and MB-3. 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. With the exception of minor F-Seam discharges, all outflows from the mine are directed into one of these two ponds under normal operating conditions. F-Seam water is • typically directed into one of these two ponds, but on occasion, the water is discharged through the Sylvester Gulch fan outfall (Ol ]A). Pond MB-1 receives mine discharges and stormwater runoff, and can be discharged to either the North Fork or transferred to pond FW-l. Pond MB-2R receives mine discharges, 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 for settling to occur then discharged to the North Fork. The combined capacity of ponds MB-] and MB-2R is 27.9 acre-feet. MCC operates the ponds so that they have the capability to completely store the 10-year return frequency, 24-hour duration rainfall-runoff event for the pond drainage area; this runoff volume is 6.8 acre-feet For both ponds. After subtracting the required stormwater storage volume from the total pond capacity, an "active" combined pond capacity of 21.1 acre-feet is available. Pond dischazges to the North Fork aze made in accordance with the MCC CDPS Dischazge Permit No. CO-0038776, Table 7 summarizes the water chemistry of ponds MB-1 and MB-2R discharges during WY97. Table 8 provides the numeric discharge limits for the sedimentation ponds, as per the CDPS permit. Historically, pond releases have generally complied with the numeric limitations in the permit. As discussed below, due to the unusual circumstances that occurred in WY97, there were three combined discharge permit exceedances from ponds MB-1 and MB-2R. r~ U 831-032.4 age 'rig t oter ngineers