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-45- Division ensures that the sediment control system is maintained to function as designed. At the close of operations, surface disturbances would be reclaimed and the sediment control system would be removed following the ensuing liability period. As a result, the increased sediment load carried by disturbed flows would not be expected to reach receiving streams. Discharge of Mine Water from Underground Workings Underground mines in the North Fork area are generally dry. The only operation to discharge mine water from the underground workings on a continuing basis is the Somerset Mine and Blue Ribbon Mine. The Somerset Mine discharges this water into Bear Creek at a site approximately 3/4 mile upstream from the confluence of Bear Creek with the North Fork of the Gunnison River. The current permit application anticipates a discharge rate of 115 gpm (0.25 cfs). Review of NPDES monitoring reports on file with the Division indicate that the discharge has averaged 107 gpm (0.24 cfs) over the past 3 years. The data indicate that the discharge rate has increased over the past 2 years. It is unknown whether or not this trend will continue. The Blue Ribbon Mine experienced inflows ranging from 37.9 gpm to 96.7 gpm during 1984. The yearly average inflow was approximately 62 gpm. The total discharge for the yeas is approximately 95.5 acre-feet. Water is pumped to a sediment pond before being discharged to Hubbard Creek, a tributary to the North Fork of the Gunnison. Since Bear Creek is an ephemeral stream, it is appropriate to consider the effects of the Somerset discharge on the North Fork of the Gunnison. The mass balance - mixing model (report by Water, Waste, and Land, Ltd., 1980) presented below was used to estimate the downstream concentrations. QuCu + QmCm ~- 4u+Qm where Cd = downstream concentrations in the river, Cu = upstream concentration, Qu = upstream river discharge, Qm = mine discharge, and Cm = concentration in the mine discharge. U.S. Steel Corporation anticipates a mine discharge of 0.25 cfs NPDES records indicate that past discharges have a total dissolved solids concentration ranging from 1,890 mg/1 to 4,000 m9/1. Data from the U.S.G.S. gaging station at Somerset shows the lowest monthly mean flow of the North Fork of the Gunnison River to be 45.2 cfs for the water year 1979. Using this low flow value for Qu and a TDS of 4,000 mg/1, a conservative water quality estimate can be made. Water quality data from the North Fork that same year (1979) show an average TDS concentration of 95 mg/1.