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-38- Two other possible surface water effects, increased infiltration in the areas overlying underground workings and increased erosion from disturbed areas, were briefly considered but then deleted from further analysis. Although subsidence might logically increase infiltration rates in an undermined watershed (and therefore decrease runoff), the Division is unaware of any literature documenting that this effect actually occurs. Surface flow from disturbed areas would carry higher sediment concentrations than natural runoff. However, Rule 4.05.2 requires that all disturbed flow pass through a properly designed sediment control pond. Effluent from the pond is required to meet NPDES limitations prior to discharge. During operation, monitoring and inspection by the 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. Discharoe of Mine Water from Underoround 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 96.1 gpm during 1984. The yearly average inflow was approximately 62 gpm. The total discharge for the year 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. Cd = QuCu + QmCm Qu + 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.