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-43- Cumulative Hydrologic Impact Assessment - Surface Fla ter Discharge of Afine Water fz~om Underground Workings Underground mines in the North Fork area are generally dry. The only operation to discharge mine water from the underground workings on a continuing basis is the Somerset Mine. The Somerset Mine discharges this water to Bear Creek at a site approximately 3/4 mile upstream from its confluence with the North Fork of the Gunnison River. The current permit application anticipates a discharge rate of I15 gpm (0.25 cfs). Review of NPDES monitoring reports on file with the C^fLRD indicate that the discharge has averaged 107 gpm (0.24 cfs) over the past 3 years. The data indicate that the discharge rate has increased over the past 2 years. It is unknown whether or not this trend will continue. Since Bear Creek is an ephemeral stream, it is appropriate to consider the effects of this discharge on the North Fork of the Gunnison. The mass balance - mixing model (report by [dater, [caste, and Land, Ltd., 1980) presented below was used to estimate the downstream concentrations. Cd = QuCu + QmCm Qu + Qm where Cd = downstream concentration in the river, Cu = upstream concentration, Qu = upstream river discharge, Qm = mine discharge, and Cm = concentration in the mine discharge. U.S. Steel Somerset anticipates a mine discharge of 0.25 cfs, and NPDES records indicate that past discharges have averaged a total dissolved solids concentration of approximately 3,500 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 value for Qu, a very conservative estimate can be made (the average discharge for the past 45 years is 432 cfs). Water quality data from the North Fork that same year show an average TDS concen- tration of 95 mg/1. Cd = (45.2 cfs)(95 mg/I) + (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/1. 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.