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-6Z- Cd = QuCU.+ QmCm Qu - Qm where Cm = concentration in the mine discharge, Cd = downstream concentration, Cu = upstream concentration, Q.u = upstream river discharge rate, and Qm = mine discharge rate. was used to estimate the downstream concentrations. Using data from the USGS gaging station at Somerset the lowest monthly mean of 45.2 cfs for the water year 1979, a very conservative estimate can be made. (The average discharge for the past 45 years is 432 cfs.) Waste quality data from that same year show an average TDS of 95 mg/1. Cd = (45.2 cfs) (95 mg/Z) +(0.25 cfs)(3,500 mg/1) (45.2 cfs) + (0.25 cfs) Cd = 113.7 mg/1 This equation predicts a downstream concentration of 113.7 mg/1 during periods of low flow. (This is an increase of 20 mg/1.) This increase in concentration would not limit downstream water use. Under average flow conditions, the expected increase would be less than 2 mg/I. A complete water chemistry of this discharge has not been provided. However, experience at other mining operations would suggest it to be a sodium-bicarbonate type water with relatively low trace metal concentrations. 2. Seepage from Waste Rock/Coal Refuse Piles The North Fork area contains several historic, existing, or proposed waste rock/ coal refuse piles. Mt. Gunnison's currently approved five year permit includes a small waste rock pile ( 1 acre) with a larger pile of approximately 10 acres anticipated in the future. The Orchard Valley mine has an existing waste rock pile (5 acres), and Western Slope Carbon has recently reclaimed a small waste rock pile ( 4 acres). The Somerset mine plan area includes two historic waste rock/ coal waste disposal areas, the Hubbard Creek site was used from 1970 until 1977, and the East Yazd site ( 9 acres) was used as a waste dump prior to 1969 but is currently part of the surface facilities area. All future waste rock at Somerset will be disposed of underground or at a waste disposal site to be established at the area of the C seam mine portals (1 .2 acres) after the final sealing of these portals. Most of these waste rock/coal refuse piles are located in areas away from the North Fork stream - alluvial aquifer system, and seepage could reach the stream - alluvial aquifer system only after considerable dilution. Two sites, however, the Somerset East Yard, and western Slope Carbon's Crystal Meadow site are adjacent to streams, where seepage from the piles might enter the stream - alluvial aquifer system.