2010-06-04_PERMIT FILE - C1982056A (10)

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previous section, flow rates in Trout Creek will be estimated based upon a regression relationship with the stream gaging station on the Yampa River below Steamboat Springs. • Summaries of possible results from the mass balance calculation are shown in Exhibit 51, Table E51 -23 and E51 -24. Modeling indicates that Site 115 should be able to discharge at the maximum loading under all modeled cases and at the modeled mean flow rate. Discharges from Sites 109, above 900, and 115 may be adjusted, as needed, during periods of low flow to meet applicable stream standards. Yampa River (Below Diversion near Hayden) As would be expected at this downstream location on the Yampa River, streamflow increases only nominally (less than 0.1 percent), and concentrations for all mine periods were not affecting Yampa River constituent levels, except for specific conductance and sulfate concentration during the initial mine period (that is, 1984 through 1991 water years). These increases were approximately 1 µmhos /cm for specific conductance and 1 mg/L for sulfate concentration. Model prediction results for streamflow, dissolved solids concentration and sulfate concentration for each of the four mining periods are summarized in Exhibit 32, Quantity and Quality Modeling Analyses of Surface -Water Resources of the Trout Creek Basin. A comparison of predicted increases in concentrations due to the 32 -year time span of mine- related impacts downstream on the Yampa River (USGS Station 09244410) was provided for dissolved solids and for sulfate (Exhibit 32, Quantity and Quality Modeling Analyses of Surface -Water Resources of the Trout Creek Basin). The original modeling indicated that the differences would be barely discernible. The year -by -year, as well as the time trend of decreasing incremental differences in average baseline versus mine impacted concentrations, is apparent from these tabular comparisons. New modeling indicates that with the increased salt loading of the discharges some impacts can be observed in the modeling. In the modeling developed for TR -32 (June, 1999) the maximum load to the Yampa River from the underground mine discharges is 22 tons per day (8,300 tons /year). The anticipated dissolved solids loads contributed to the • stream system from mine flows will impact average ambient conditions in lower Trout Creek (about 70 tons /day) and to a lesser degree, the Yampa River (approximately 250 tons /day). On an annual basis, these averaged ambient dissolved solids loads would reflect over 25,400 tons /year for lower Trout Creek and about 89,600 tons /year for the Yampa River near Hayden. The impact will be to increase the dissolved solids loading in lower Trout Creek 31 percent and in the Yampa River by 9 percent. Similar increases in the sulfate levels will also occur. Stream Standards Comparisons The State of Colorado stream standards applicable for the study area consist of: • Segment 2c: Yampa River, mainstem from confluence with Oak Creek to confluence with Elkhead Creek • Segment 13a: Trout Creek and tributaries not within a National Forest, excluding Segments 13b, 13c, and 13f, from its source to confluence with Fish Creek • Segment 13b: Foidel Creek and all tributaries and Fish Creek and Middle Creek and all tributaries, from County Road 27 to confluence with Trout Creek • Segment 13c: Trout Creek from headgate of Spruce Hill Ditch to confluence with Fish Creek and tributaries to Trout Creek from County Road 27 to County Road 179 • Segment 13f: Trout Creek and tributaries from confluence with Fish Creek to confluence with Yampa River • The segments have similar water quality standards, except that Segments 2c, 13a, and 13f include drinking water standards if they are being utilized for active water supply, Segment 13b does not include drinking water standards, and Segment 13c includes drinking water standards from June through February. The effect of these segment - specific standards is that the sulfate, chromium I11, and iron drinking water standards (active drinking RN08 -05 2.05 -156 03/12/10