2009-08-10_PERMIT FILE - C1982056A (5)

Home Browse Search

(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 umhos/cm. The ratio of the sulfate to conductivity levels is estimated to be 0.32. The strearri standard of 250mg/L is therefore equivalent to a conductivity of 780 umhos/cm. Summaries of possible results from the mass balance calculation are shown in Exhibit 49, Tables E49-23 and E49- 24. Site 109 may be able to discharge at the maximum loading under all cases and at flow rates as low as 50% ofd 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 % 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 umhos/cm the discharge from Site 109 will cease. This level is 95% 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 umhos/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 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 49, Table E49-24 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 49, Table E49-23 and E49- 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. MR08-230 2.05-156 09/05/08