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

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Similar calculations were performed to predict the sodium adsorption ratio conditions that might occur in Fish Creek and Trout Creek because of discharge of mine inflows to Fish and Foidel Creeks. This was performed primarily to evaluate possible effects on the use of Fish and Trout Creek waters for irrigation purposes. Average • values for the respective components were used. Calculations were performed for Fish Creek below the proposed Fish Creek dewatering discharge point and for Trout Creek at two points (equivalents of Sites 69 and 1005) below the confluences with Middle Creek and Fish Creeks. The results of these analyses are shown in Exhibit 51, E51 -19 to E51 -21, Table 63. The results indicate that SAR conditions may reach up to 6.2 in Fish Creek and up to 3.0 in Trout Creek. Since water with a SAR of below 10 is considered low hazard, the discharges should have no significant impact on agricultural use of the water. Lower Middle Creek By virtue of being downstream of one of the mine discharge points Site 109, and because of the relatively limited flows in Foidel and Middle Creeks, impacts of mine flows could be more significant in this reach. In the original modeling, the increase in specific conductance and dissolved solids at Middle Creek increased by approximately 20 percent, and sulfate concentration increased by over 50 percent. This was based on a flow of 1 cfs and a conductivity of 3,250 umhos/cm (yielding a conductivity load (flow times conductivity) of 3,250 cfs - umhos/cm). The maximum load from Case 1 is 4,228 cfs - umhos /cm, while the maximum loads for Cases 2 and 3 are less than 3,250 cfs- umhos /cm. While the potential for exceedance of the sulfate standard for Trout Creek, Site 109 discharges was previously a concern (flows were maintained below Case 1 flow rates when the "Mine Discharge Calculator" indicated that the sulfate standard will be exceeded), the sulfate standard is no longer applicable, so discharge rates are no longer limited except as necessary to meet other applicable stream standards. Since this reach is not classified as drinking water, and there is no flood irrigation of AVF's for this reach, the respective sulfate standard and conductivity limit do not apply. . Lower Fish Creek Based on the TR -32 update to the original modeling, there is a potential for significant impacts to the water quality in Fish Creek downstream of Site 115. The modeling indicates that during low -flow periods, a conductivity of 1,500 µmhos /cm could be exceeded (Exhibit 51, Table E51 -14). The highest modeled SAR value was 6.2, indicating that it is highly unlikely that a SAR of 10 will be exceeded (see Exhibit 51, Tables 19 to 21). 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 Creek Borehole Discharge Calculator" shown in Exhibit 51, Table E51 -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 51, Table E51 -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 in Exhibit 50, Table E50 -1 and Exhibit 51, Table E51 -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. RN08 -05 2.05 -154 03/12/10