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The flow rate and water quality of Trout Creek between Middle Creek and Fish Creek will be affected by the discharge from Site 109. Based upon the TR -32 update to the original modeling, there is a potential for significant impacts to the water quality in this reach. The modeling indicates that during low flow periods, the sulfate level may exceed 250 mg /L when site 109 is discharging. The highest modeled SAR value was 1.6, indicating that there is no significant impact to the SAR level. The highest modeled conductivity value was 970 µmhos /cm, indicating that the changes will not significantly impact irrigation use. Based upon the potenial for elevated sulfate concentrations, TC will commit to maintaining the discharge rate from Site 109 at a level to prevent exceeding applicable stream or effluent standards. In addition to any other required monitoring, TC will monitor the flow rate and conductivity at Site 69 (Trout Creek below Middle Creek) and Site 109 weekly and the conductivity at Site 301 (Trout Creek above Middle Creek) when Site 109 is discharging. The discharge from Sites 109 and 115 will be, adjusted using the Excel based "Mine Discharge Calculator" shown in Exhibit 51, Table E51 -26, or based on compliance considerations under TC's CDPS discharge permits. The flow rate is calculated based upon the mass balance equation shown in Exhibit 51, Table E51 -26. TC will only be required to meet the in- stream sulfate conditions during the period of June through February and consequently the increased monitoring at Site 69 will only be required during the same period. During the winter inonihs, it is often not possible to get a reliable flow rate reading from Sites 69 and 1005. To estimate the flow rate at thesc sites at those times a linear regression has been developed between Sites 69 and 1003 (downstream on Fish Creek) and the Yampa River below Steamboat Springs (09239500). These stations were chosen because a reasonably good correlation was achieved and the flow rate at this station is accessible in real time on the USGS's Internet site. In order to account for estimation errors, a relationship with an intercept two standard errors lower than the calculated one was used. There is insufficient period of record at Site 1005 (Trout Creek below Fish Creek) to perform a regression. Therefore, the flow at Site 1005 was calculated by adding the estimated natural flow from Fish Creek (Site 1003) to the measured discharge from Site 115 and the estimated flow from Trout Creek above Fish Creek (Site 69). The equations are shown on Exhibit 50, Tables E50 -2 and E50 -3 and Exhibit 49, Table E49 -26 and the regression analyses and plots are shown in Exhibit 50 Figures E50 -2 and E50 -3). Since it is difficult to accurately measure sulfate levels in the field, conductivity has been used as the indicator parameter for sulfate. In Exhibit 32, at the Station 69 node, the dry season TDS and sulfate levels were modeled to be 412 and 174 mg/L respectively. Using the conversion factor of 0.75, the TDS is equivalent to a conductivity of 549 µmhos /em. The ratio of the sulfate to conductivity levels is estimated to be 0.32. The stream standard of 250mg /L is therefore equivalent to a conductivity of 780 µ.mhos /cm. Summaries of possible results from the mass balance calculation are shown in Exhibit 51, Tables E451 -23 and E51 -24. Site 109 may be able to discharge at the maximum loading under all cases and at flow rates as low as 50 percent of the mean flow rate. During periods where Site 109 cannot discharge, the flows will be diverted into the sump in the WMD. As discussed above, the sump will be drawn down during periods of high flow to compensate for when Site 115 is pumped at less than 55 -gpm and Site 109 discharge is diverted into the sump_ It should be noted for the period of record before starting the underground mine (October 1980 to December 1984) 21 percent of the sulfate measurements exceeded 250 mg/L. This indicates that there are other sources of sulfate in the system. To account for this, when the conductivity at Site 301 (Trout Creek above Middle Creek) equals or exceeds 740 mhos /cm the discharge from Site 109 will cease. This level is 95 percent of the standard. Lower Trout Creek — Below Fish Creek The flow rate and water quality of Trout Creek below Fish Creek will be affected by the discharges from Site 109 and Site 115. Based upon the TR -32 update to the original modeling, there is a potential for significant impacts to the water quality in this reach. The modeling indicates that during low flow periods the sulfate level may exceed 250 mg/L. The highest modeled SAR value was 2.4, indicating that there is no significant impact to the SAR level. The highest modeled conductivity value was 964 µmhos /cm, indicating that the changes will not significantly impact irrigation use. Based upon the potential for elevated sulfate concentrations, TCC will commit to maintaining the discharge rates from Site 109 and Site 115 at levels to prevent exceeding applicable TRI3 -83 105 -155 11/03/14