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-39- 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 mg/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. Cd = (45.2 cfs)(95 mg/1) + (0.25 cfs)(4,000 mg/1) -45.2 cfs + 0.25 cfs Cd = 116.5 mg/1 This equation predicts a downstream concentration of 211 mg/1 during periods of low flow. This is an increase of 117.3 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 2.5 mg/l. A complete water chemistry of this discharge has been provided. The water is 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 has a temporary storage pile with a new permanent pile approve but yet to be built. The Orchard Valley Mine has an existing waste rock pile (1 acre), 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 established at the area of the "C" seam mine portals (1.2 acres). 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, namely 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 5,000 mg/1. (This is the highest spoil water value seen in Colorado.) The chemistry of the leachate would probably be a sodium-bicarbonate, sulfate type