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The quality of water in Trout Creek will be measurably affected only by the mining of areas previously undisturbed (primarily in the Moffat Area), and, to a lesser extent, remining of portions of West Ridge. Considering West Ridge first, 68 percent of the West Ridge drainage area had been mined previous to initial permit issuance in 1981. Monitoring of ground water at the toe of the slope and sampling of spoil spring discharges indicates that the concentration of total dissolved solids in the subsurface runoff averages about 3030 mg/l. It has been estimated that the pre-mining concentration of total dissolved solids in drainage from the West Ridge Area would be 2122 mg/l. The TDS concentrations actually monitored at a surface water station on Trout Creek over a three year period averaged 1840 mg/l. The estimation of post-mining water quality of West Ridge drainage uses a similar method, except that in this case nearly the entire West Ridge Area will have been mined. Calculations indicate that mining the remainder of the undisturbed area on West Ridge is expected to increase the mean annual TDS concentration from 2122 mg/1 to 2828 mg/1. Similar computations are made for the Moffat Area. Approximately 800 acres of a total of 1,030 acres in the Moffat Area are expected to be mined. The combined surface and subsurface discharge-weighted mean-annual TDS concentration prior to disturbance is estimated to be 541 mg/1. Ground water quality from the undisturbed Moffat Area is estimated to have a TDS of 570 mg/l. The post-mining spoil water is expected to contain about 3000 mg/1 of total dissolved solids. The post-mining discharge-weighted mean-annual TDS concentration in combined surface and subsurface runoff from the Moffat Area, then, is expected to be 2290 mg/l. The large increase in TDS in the mine site runoff was not expected to create a proportional increase in Trout Creek because of the effect of dilution. The magnitude of the effect can be computed using 1975 data at the downstream stream gaging site. The 1975 discharge of salts was measured as 8,750 tons in a total water volume of 34,900 ac-f t. This converts to a mean concentration of 184 mg/1. It was assumed that an identical flow condition would occur during a given year after mining is completed at the Edna Mine. The stream will receive runoff from the West Ridge and Moffat Areas that contain elevated TDS concentrations as discussed above. The mean annual runoff from West Ridge is estimated to be 2.10 inches over an area of 560 acres or 98 acre-feet. The corresponding volume for the Moffat Area is 180 acre-feet. The post-mining concentration of total dissolved solids from the West Ridge drainage is estimated to be 2928 mg/l. The pre-mining salt _21_