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The flow rate and water quality of Trout Creek between Middle Creek and Fish Creek will be affected by the <br /> discharge from Site 109. Based upon the TR -32 update to the original modeling, there is a potential for <br /> significant impacts to the water quality in this reach. The modeling indicates that during low flow periods, the <br /> sulfate level may exceed 250 mg/L when site 109 is discharging. The highest modeled SAR value was 1.6, <br /> indicating that there is no significant impact to the SAR level. The highest modeled conductivity value was 970 <br /> µmhos /cm, indicating that the changes will not significantly impact irrigation use. <br /> Based upon the potential for elevated sulfate concentrations, TCC will commit to maintaining the discharge rate <br /> from Site 109 at a level to prevent exceeding applicable stream or effluent standards. In addition to any other <br /> required monitoring, TCC will monitor the flow rate and conductivity at Site 69 (Trout Creek below Middle <br /> Creek) and Site 109 weekly and the conductivity at Site 301 (Trout Creek above Middle Creek) when Site 109 is <br /> discharging. The discharge from Sites 109 and 115 will be adjusted using the Excel© based "Mine Discharge <br /> Calculator" shown in Exhibit 51, Table E51 -26, or based on compliance considerations under TCC's CDPS <br /> discharge permits. The flow rate is calculated based upon the mass balance equation shown in Exhibit 51, Table <br /> E51 -26. TCC will only be required to meet the in- stream sulfate conditions during the period of June through <br /> February and consequently the increased monitoring at Site 69 will only be required during the same period. <br /> During the winter months, it is often not possible to get a reliable flow rate reading from Sites 69 and 1005. To <br /> estimate the flow rate at these sites at those times a linear regression has been developed between Sites 69 and <br /> 1003 (downstream on Fish Creek) and the Yampa River below Steamboat Springs (09239500). These stations <br /> were chosen because a reasonably good correlation was achieved and the flow rate at this station is accessible in <br /> real time on the USGS's Internet site. In order to account for estimation errors, a relationship with an intercept <br /> two standard errors lower than the calculated one was used. There is insufficient period of record at Site 1005 <br /> (Trout Creek below Fish Creek) to perform a regression. Therefore, the flow at Site 1005 was calculated by <br /> adding the estimated natural flow from Fish Creek (Site 1003) to the measured discharge from Site 115 and the <br /> estimated flow from Trout Creek above Fish Creek (Site 69). The equations are shown on Exhibit 50, Tables <br /> • E50 -2 and E50 -3 and Exhibit 49, Table E49 -26 and the regression analyses and plots are shown in Exhibit 50 <br /> Figures E50 -2 and E50 -3). Since it is difficult to accurately measure sulfate levels in the field, conductivity has <br /> been used as the indicator parameter for sulfate. In Exhibit 32, at the Station 69 node, the dry season TDS and <br /> sulfate levels were modeled to be 412 and 174 mg/L respectively. Using the conversion factor of 0.75, the TDS <br /> is equivalent to a conductivity of 549 µmhos /cm. The ratio of the sulfate to conductivity levels is estimated to <br /> be 0.32. The stream standard of 250mg/L is therefore equivalent to a conductivity of 780 µmhos /cm. <br /> Summaries of possible results from the mass balance calculation are shown in Exhibit 51, Tables E451 -23 and <br /> E51 -24. Site 109 may be able to discharge at the maximum loading under all cases and at flow rates as low as <br /> 50 percent of the mean flow rate. During periods where Site 109 cannot discharge, the flows will be diverted <br /> into the sump in the WMD. As discussed above, the sump will be drawn down during periods of high flow to <br /> compensate for when Site 115 is pumped at less than 55 -gpm and Site 109 discharge is diverted into the sump. <br /> It should be noted for the period of record before starting the underground mine (October 1980 to December <br /> 1984) 21 percent of the sulfate measurements exceeded 250 mg/L. This indicates that there are other sources of <br /> sulfate in the system. To account for this, when the conductivity at Site 301 (Trout Creek above Middle Creek) <br /> equals or exceeds 740 µmhos /cm the discharge from Site 109 will cease. This level is 95 percent of the <br /> standard. <br /> Lower Trout Creek — Below Fish Creek <br /> The flow rate and water quality of Trout Creek below Fish Creek will be affected by the discharges from Site <br /> 109 and Site 115. Based upon the TR -32 update to the original modeling, there is a potential for significant <br /> impacts to the water quality in this reach. The modeling indicates that during low flow periods the sulfate level <br /> . may exceed 250 mg/L. The highest modeled SAR value was 2.4, indicating that there is no significant impact to <br /> the SAR level. The highest modeled conductivity value was 964 µmhos /cm, indicating that the changes will not <br /> significantly impact irrigation use. Based upon the potential for elevated sulfate concentrations, TCC will <br /> commit to maintaining the discharge rates from Site 109 and Site 115 at levels to prevent exceeding applicable <br /> PR09 -08 2.05 -155 04/27/09 <br />