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_,~,- The rather large increase in TL1S in the runoff will net create a proportional increase in Trout Creek, of course, because of dilution. The rough magnitude of the effect can be computed using the 1975 data at Station C-6. The 1975 discharge of salts at Station C-6 was measured to be 8,750 tons in a total volume of water equal to 34,900 ac-ft. This converts to a mean concentration of 184 mg/1. Sf an identical condition were to prevail during some year after mining was complete on the Edna mine, the stream would receive runoff from the [Jest Ridge and Pfoffat areas that contain increased TDS concentrations. The Sarre area would have contributed the same volume of water prior to mining, but with a smaller TDS concentration. 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 F?offat area is 180 acre- feet. The pre-mining concentration of dissolved solids in c:rainage from !Jest Ridge is 2122 mg/2 and the post-mining concentration is es timatecl to be 282amg/1. Thus, the pre-mining salt load contributed to Trout Creek from tfre [Jest Ridye is 282 tons, and the post-mining salt load is 387 tons, for an increase of 94 tons of dissolved solids. The corresponding computations for. the P?offat area show the pre-mining salt loading to be 132 tons, and a post-mining salt loading of 559 tons, giving an increase of 427 tons. The post-mininy man annual concentration of 7'DS in Trout Creek is I96 mg/1, indicating an increase due to miring of 12 mg/1. The model upon which the above computaticns are based was derived for long-term average conditions. Strictly speaF:ing, it does not apply to the analysis of wet or dry years that deviate significantly from the average because the assumption of zero net change in watershed storaye may be violated. lior/ever, one can estimate the effect on Trout Creek at lower floi,~s by assuming that average conditions prevail on the mined lands, but less than average runoff occur.; on the upper portion of the watershed. This unlikely occurrence provides an overestimate of the salt load con- tribution to the Trout Creek and a corresponding overestimate of the mean TDS con- centration, estimates an increase of 20 mg/1 above the conditions with no mining in a dry year. In summary, this calculation shows that one-half the average runoff from the upper portion of the watershed i:•ith normal runoff from the mined lands will cause the mean TDS in Trout Creek to increase from 164 to ?30 mg/l. iio:/ever, on Ll 20 mg/1 of this increase is attributable to mininrr the west Ridye an~i F?offac areas. The remainder is from previously mined lands. The time distribution of dissolved solids concentration i.n Trout Creek as indicated by EC is shown in Exhibit 2.5-d of the permit application. It i.s noted that the peak TDS con ccntra ti on occurs in the ca r.Ly spring. This is duo to the inflow of salts from the mined area carried by the snowmelt runoff on the mined area and the fact that snowmelt on the upper portions of the Trout Crecy. drainayc has not yet begun in earnest. These data show that the large fraction of tiro increased salt load will be contributed in April and May, when increases to stream flow due to mining are expected to be 31 mg/1 and 39 mg/1 respectively. The earliest diversions from Trout Creek for irrigation occur about June 1. By this date, flow in Trout Creek is sufficient to dilute the effects of the salt load from the mine. The increase in salt contribution to Trout Creek resulting from the proposed mining would have no adverse effects upon irrigation.