2009-10-30_PERMIT FILE - C1982056A (5)

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stream or effluent standards. In addition to any other required monitoring, TCC will monitor the flow rate and conductivity at Site 69 (Trout Creek below Middle Creek), Site 115, and Site 109 weekly and the conductivity at Site 301 (Trout Creek above Middle Creek) as described in the Fish Creek and Lower Trout Creek between Middle and Fish Creek discussions above. In addition to any other monitoring requirements, the flow rate and conductivity at Site 1005 will be monitored weekly when either Site 109 or 115 is discharging see Exhibit 14). The discharges from Site 115 and Site 109 will be adjusted using the Excel© based "Mine Discharge Calculator" shown in Exhibit 51, Table E51 -24 or based on compliance considerations under TCC's CDPS discharge permits.. The flow rate is calculated based upon the mass balance equation shown on this table. As indicted in the 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. Since it is difficult to accurately measure sulfate levels in the field, conductivity has been used as the indicator parameter for sulfate. As described above, a conversion factor of 0.32 will be used for converting conductivity to sulfate levels. 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. Site 115 will not be discharged when the calculator or instream measurements indicate that the sulfate standard will be exceeded. As discussed above, the sump will be drawn down during periods of high flow to compensate when Site 115 is pumped at less than 55 gpm and Site 109 is diverted into the sump. Since the discharges from 109 and 115 must be adjusted during periods of low flow, they will not exceed the lowest flow rate determined by the Mine Discharge Calculator and the Fish Creek Borehole Calculator or based on compliance considerations under TCC's CDPS discharge permits. 411 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 given 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. PR09 -08 2.05 -156 04/27/09