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

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Based on the potential to have a high conductivity, TCC will commit to maintaining the discharge rate from Site 115 at a level to prevent material damage. In addition to any other required monitoring, TCC will monitor the flow • rate and conductivity at Site 16. to be replaced by Site 16A, (Fish Creek upstream of Site 115) and Site 115 weekly when Site 115 is discharging. The discharge from Site 115 will be adjusted using the Excel© based "Fish Creels Borehole Discharge Calculator" shown in Exhibit 49, Table E49-25, 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. A summary of possible results from this calculation is shown in Exhibit 49, Table E49-22. Site 115 could potentially be able to be discharged at 55 gpm (the rate required to maintain the pool elevation in the sump) at most instream flows. Actual discharge rates will be set based upon measured in-stream flow and water quality measurements (see Exhibit 14). Site 115 could possibly be able to discharge at least 600 gpm during mean flow periods. This may allow the sump to be drawn down to compensate for periods when Site 115 is pumped at less than 55 gpm. During the winter months, it is often not possible to get a reliable flow rate reading from Site 16. To estimate the flow rate at Site 16 at those times a linear regression has been developed between Site 16 and the Yampa River below Steamboat Springs (09239500). This station was 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.. The equation is shown on Exhibit 50, Table E50-1 and Exhibit 49, Table E49-25 and the regression analysis and'. plot are shown in Exhibit 50 (Figure E50-1). The discharge rate from Site 115 will also have to be adjusted based upon potential impacts to Trout Creek. This will be discussed in the Lower Trout Creek sections below. 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 of the underground workings. 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 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 rate from Site 109 at a level 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) 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 49, Table E49-26, or based on compliance considerations under TCC's CDPS discharge permits. The flow rate is calculated based upon the mass balance equation shown in Exhibit 49, Table E4926. TCC 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 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 MR08-230 2.05-155 09/05/08