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

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Lower Trout Creek The mine is not expected to impact the water quality or flow rates in Trout Creek above Middle Creek. There • are no discharges from the Foidel Creek mine to this reach, and this reach is not within the angle -of -draw for the underground workings. The flow rate and water quality of Trout Creek between Middle Creek and Fish Creek may be affected by the discharge from Site 109 and above Site 900. Based on the TR -32 update to the original modeling, there is a potential for limited 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periods, the sulfate level may exceed 250 mg/L when Site 109 is discharging, however, since the sulfate standard is no longer applicable, this is not significant. 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 [,mhos/cm, indicating that the changes will not significantly impact irrigation use. During the winter months, it is often not possible to get a reliable flow rate reading from Sites 69 and 1005. To estimate the flow rate at these 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 /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 any periods where Site 109 cannot discharge and still meet applicable stream standards, 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. Lower Trout Creek — Below Fish Creek The flow rate and water quality of Trout Creek below Fish Creek may be affected by the discharges from Site 109, above Site 900, and Site 115. Based upon the TR -32 update to the original modeling, there is a potential for limited impacts to the water quality in this reach. The modeling indicates that during low flow periods the sulfate level may exceed 250 mg/L however, since the sulfate standard is no longer applicable, this is not significant. The highest modeled SAR value was 2.4, indicating that there is no significant impact relative to SAR levels. The highest modeled conductivity value was 964 µmhos /cm, indicating that the changes will not significantly impact irrigation use. 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). As indicated in the RN08 -05 2.05 -155 03/12/10