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<br />load contributed to Trout Creek <br />and the post-mining salt load is <br />of total dissolved solids. <br /> <br />from the West Ridge is 282 tons, <br />387 tons, an increase of 94 tons <br />Corresponding computations for the Moffat Area show the pre-mining <br />salt load to be 132 tons, and a post-mining salt load of 559 tons, <br />an increase of 427 tons. The post-mining mean-annual concentration <br />of TDS in Trout Creek is 196 mg/l, an increase of 12 mg/1 from <br />baseline conditions. <br />The model upon which the above computations were based was derived <br />from long-term average conditions. Strictly speaking, it does not <br />apply to the analysis of wet or dry years that deviate <br />significantly from the average because the assumption of zero net <br />change in watershed storage may be violated. However, the effect <br />on Trout Creek at lower flows was estimated by assuming that <br />average conditions prevail on the mined lands but less than average <br />runoff occurs on the upper portion of the watershed. This <br />calculation showed that one-half the average runoff from the upper <br />portion of the watershed with normal runoff from the mined lands <br />would cause the mean TDS in Trout Creek to increase from 184 to 330 <br />mg/l. However, only 20 mg/1 of this increase is attributable to <br />the proposed extent of mining of the West Ridge and Moffat Areas. <br />The remainder is from previously mined lands. <br />The time distribution of dissolved solids concentration in Trout <br />Creek shows that the peak TDS concentration occurs in the early <br />spring. This is due to the inflow of salts from the mined area <br />carried by the snowmelt runoff on the mined area and the fact that <br />snowmelt on the upper portions of the Trout Creek drainage has not <br />yet begun. These data show that the large fraction of the <br />increased salt load will be contributed in April and May, when <br />increases to stream flow due to mining are expected to be 31 mg/1 <br />and 39 mg/1 respectively. The earliest diversions from Trout Creek <br />for irrigation occur about June 1. By this date, flow in Trout <br />Creek should be sufficient to dilute the effects of the salt load <br />from the mine. <br />Section 2.5.3.5 of the Edna permit (Hydrologic 8alance> and the <br />Division's January 14, 1987 Findings of Compliance for the Edna <br />Mine contain several predictions of hydrologic impacts from mining <br />operations based upon baseline data, projections and water quality <br />standards. These include: <br />a> a prediction that maximum total dissolved solids (TDS> <br />concentrations in Trout Creek surface water in the <br />permit area will not exceed 607 mg/1 (page 21 of <br />Findings), <br />b> a prediction that the mean annual TDS concentration of <br />Trout Creek surface water in the permit area during dry <br />years will equal 330 mg/1 (page 2.5-104) and will equal <br />196 mg/1 during "normal" years (page 2.5-103), and <br />_2L_ <br />