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-40- <br />Discharge of Mine Water from Underoround Workings <br />Underground mines in the North Fork area are generally dry. The only <br />operation to discharge mine water from the underground workings on a <br />continuing basis is the Somerset Mine. The Somerset Mine discharges <br />this water into Bear Creek at a site approximately 3/4 mile upstream <br />from its confluence with the North Fork of the Gunnison River. The <br />current permit application anticipates a discharge rate of 115 gpm <br />(0.25 cfs). Review of NPDES monitoring reports on file with the <br />Division indicate that the discharge has averaged 107 gpm (0.24 cfs) <br />over the past 3 years. The data indicate that the discharge rate has <br />increased over the past 2 years. It is unknown whether or not this <br />trend will continue. <br />Since Bear Creek is an ephemeral stream, it is appropriate to consider <br />the effects of this discharge on the North Fork of the Gunnison. The <br />mass balance - mixing model (report by Water, Waste, and Land, Ltd., <br />1980) presented below was used to estimate the downstream <br />concentrations. <br />QuCu + QmCm <br />Cd = Qu + Qm <br />where Cd = downstream concentrations in the river, <br />Cu = upstream concentration, <br />Qu = upstream river discharge, <br />Qm = mine discharge, and <br />Cm = concentration in the mine discharge. <br />U.S. Steel Somerset anticipates amine discharge of 0.25 cfs, and NPDES <br />records indicate that past discharges have a total dissolved solids <br />concentration ranging from 1,890 mg/1 to 4,000 mg/1. Data from the <br />U.S.G.S. gaging station at Somerset shows the lowest monthly mean flow <br />of the North Fork of the Gunnison River to be 45.2 cfs for the water <br />year 1979. Using this low flow value for Qu and a TDS of 4,000, a <br />conservative water quality estimate can be made. Water quality data <br />from the North Fork that same year (1979) show an average TDS <br />concentration of 95 mg/1. <br />Cd = _(45.2 cfs)(95 mg/1) + (0.25 cfs)(4,000 mg/1) <br />45.2 cfs + 0.25 cfs <br />Cd = 116.5 mg/1 <br />This equation predicts a downstream concentration of 211 mg/1 during <br />periods of low flow. This is an increase of 117.3 mg/1 over baseline <br />conditions. This increase in concentration would not limit downstream <br />water use. Under average flow conditions for the past 45 years of 432 <br />cfs, the expected increase would be less than 2.5 mg/1. <br />