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-37- <br />cfs). Review of NPDES monitoring reports on file with the Division indicate <br />that the discharge has averaged 107 gpm (0.24 cfs). The data indicate that <br />the discharge rate has increased over the past few years. It is unknown <br />whether or not this trend will continue. <br />Blue Ribbon has encountered inflows at the working face in their mine. <br />Previously a dry mine, Blue Ribbon encountered inflows in a sudden two day <br />period in spring 1983. Initial inflows were 150 gpm but has reduced to around <br />64 gpm. <br />Since Bear Creek is an ephemeral stream, it is appropriate to consider the <br />effects of Somerset Mine discharge on the North Fork of the Gunnison. The <br />mass balance - mixing model (report by Water, Waste, and Land, Ltd., 1980) <br />presented below was used to estimate the downstream concentrations. <br />Cd = QUCo + pnCm <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 a mine 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 U. S.G. S. <br />gaging station at Somerset shows the lowest monthly mean flow of the North <br />Fork of the Gunnison River to be 45.2 cfs for the water year 1979. Using this <br />low flow value for Qu and a TDS of 4,000, a conservative water quality <br />estimate can be made. Water quality data from the North Fork that same year <br />(1979) show an average TDS concentration of 95 mg/1. <br />(45.2 cfs)(95 mg/1) + (0.25 cfs)(4,000 m /1) <br />Cd= cs + cs <br />Cd = 211 mg/1 <br />This equation predicts a downstream concentration of 211 mg/1 during periods <br />of low flow. This is an increase of 117.3 mg/1 over baseline conditions. <br />This increase in concentration would not limit downstream water use. Under <br />average flow conditions for the past 45 years of 432 cfs, the expected <br />increase would be less than 2.5 mg/1. <br />This discharge is likely a sodium-bicarbonate, sulfate type water with <br />relatively low trace metal concentrations and neutral pH. <br />As part of their Annual hydrologic Report, Blue Ribbon calculated that the TDS <br />content of Hubbard Creek had increased an average of 2% as the result of mine <br />discharge. The mine discharge showed an approximate TDS level of 1000 mg/1. <br />This increase in TDS poses no burden on existing Hubbard Creek wate~ uses. <br />Arty impact to the North Fork River would be even less due to dilution. <br />