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-40- (45.2 cfs)(95 mg/1) + (0.25 cfs)(3,500 my/1) Cd = (45.2 cfs + 0.25 cfs %d = 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 2U mg/1 over baseline conditions. This increase in concentration would not limit downstream water use. Under average flow conditions for the past 45 years of 432 cfs, the expected increase would be less than 2 mg/l. 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, sulfate type water with relatively low trace metal concentrations and neutral pH. The North Fork region contains several historic, existing, or proposed waste rock/coal refuse piles. The Mt. Gunnison Mine facilities 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 life-of-mine area includes two historic waste rock/coal waste disposal areas. The Hubbard Creek site was used from 1970 until 1977. The East Yard 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 the Somerset Mine 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. Three sites, however, the Somerset East Yard, and Western Slope Carbon's Crystal Meadow and East Mine Bench sites are adjacent to streams, where seepage from the piles will eventually enter the stream - alluvial aquifer system. The effects of the waste piles on dissolved solids concentrations in the North Fork of the Gunnison can be evaluated using the mass balance - mixing model shown above. This requires estimating the total dissolved solids concentration of the leachate from the pile and estimating the rate of discharge to stream - alluvial aquifer systems. The total dissolved solids concentration can be estimated by using a conservatively high concentration of 3,000 mg/l. The chemistry of the leachate would probaoly be a sodium-bicarbonate, sulfate type water with relatively low total metal concentrations and neutral pH. The rate of discharge can be estimated from the water balance provided in 'the Orchard Valley permit application. This water balance estimated subsurface flow to be 2.8 irr/yr. Assuming that most of the water for subsurface flow and deep seepage is the result of snowmelt, it may be that this discharge is limiteu to a 60-day period. The rate of discharge can be calculated.