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21). Based upon the potential to exceed the conductivity limit, TCC will commit to maintaining the discharge rate <br />from Site I IS at a level to prevent material damage. )n addition to any other required monitoring, TCC will <br />• monitor the flow rate and conductivity at Site 16, to be replaced by Site 16A, (Fish Creek upstream of Site 115) <br />and Site I IS weekly when Site 1 IS is discharging. The discharge from Site 1 IS will be adjusted using the Excel© <br />based "Fish Creek Borehole Discharge Calculator" shown in Exhibit 49, Table E49-25. The flow rate is calculated <br />based upon the mass balance equation shown on this table. A summary of possible results from this calculation is <br />shown in Exhibit 49, Table E49-22. Site 115 could potentially be able to be discharged at 55 gpm (the rate <br />required to maintain the pool elevation in the sump) at most instream Flows. Actual discharge rates will be se[ <br />based upon measured in-stream flow and water quality measurements (see Exhibit 14). Site 1 IS could possibly be <br />able to discharge at least 600 gpm during mean Flow periods. This may allow the sump to be drawn down to <br />compensate for periods when Site ] IS is pumped at less than 55 gpm. <br />During the winter months, it is often not possible to get a reliable Flow rate reading from Site 16. To estimate the <br />Flow rate at Site 16 at those times a linear regression has been developed between Site 16 and the Yampa River <br />below Steamboat Springs (09239500). This station was chosen because a reasonably good correlation was <br />achieved and the flow rate at this station is accessible in real time on the USGS's Intemet site. In order to account <br />for estimation errors, a relationship with an intercept two standard errors lower than the calculated one was used. <br />The equation is shown on Exhibit 50, Table E50-] and Exhibit 49, Table E49-25 and the regression analysis and <br />plot are shown in Exhibit 50 (Figure E50-1). <br />The discharge rate from Site 115 will also have to be adjusted based upon potential impacts to Trout Creek. This <br />will be discussed in the Lower Trout Creek sections below. <br />Lower Trout Creek <br />The mine is not expected to impact [he water quality or flow rates in Trout Creek above Middle Creek. There are <br />• no discharges from the Foidel Creek mine to this reach and this reach is not within the angle of draw of the <br />underground workings. <br />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 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 when site 109 is dischazging. The highest modeled SAR value was 1.6, indicating that there <br />is no significant impact to the SAR level. The highest modeled conductivity value was 970 umhos/cm, indicating <br />that the changes will not significantly impact irrigation use. <br />Based upon the potential to exceed the sulfate concentration, TCC will commit to maintaining the discharge rate <br />from Site 109 at a level to prevent exceeding the standard. In addition to any other required monitoring, TCC will <br />monitor the flow rate and conductivity at Site 69 (Trout Creek below Middle Creek) and Site 109 weekly and the <br />conductivity at Site 301 (Trout Creek above Middle Creek) when Site 109 is discharging. The discharge from <br />Sites 109 and 115 will be adjusted using the Excel© based "Mine Discharge Calculator" shown in Exhibit 49, <br />Table E49-26. The flow rate is calculated based upon the mass balance equation shown in Exhibit 49, Table E49- <br />26. TCC will only be required to meet the instream sulfate conditions during the period of June through February <br />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 Intemet site. In order to account for estimation errors, a relationship with an intercept two <br />standard errors lower than the calculated one was used. There is insufficient period of record at Site 1005 <br />AP~~®V~® JUN 2 8 2000 <br />PR 99-OS 2.05-I55 03/28/00 <br />