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RAG Empire Corporation, Eagle Mine Water Quality Assessment CO-0034142 <br />~tandards for metals are generally shown in the regulations as Table Value Standards (TVS), and <br />these often must be derived from equations that depend on the receiving stream hazdness or species <br />of fish present. The Classification and Numeric Standazds documents for each basin include a <br />specification for appropriate hardness values to be used. Specifically, the regulations state that: <br />The hazdness values used in calculating the appropriate metal standard should be based <br />on the lower 95% confidence limit of the mean hazdness value at the periodic low flow <br />criteria as determined from a regression analysis of site-specific data. Where <br />insufficient site-specific data exists to define the mean hardness value at the periodic <br />]ow flow criteria, representative regional data shall be used io perform the regression <br />analysis. Where a regression analysis is not appropriate, asite-specific method should <br />be used. <br />Hardness data for Williams Fork River near the point ;of dischazge of 'the Eagle Mine were <br />insufficient to conduct a regression analysis based on the ]ow flow. In the absence of a regression <br />analysis, the WQCD's approach to calculating iazdness involves computing a mean hardness. <br />The mean hazdnessin the Williams Fork River was computed to be 224 mg/1 as. CaCO3`based on <br />sampling=conducted from 1981 through 2000 by the Eagle Mine downstream of the outfalls This <br />hardness value and the formulas contained in the TVS were used. to calculate the in-stream water <br />quality standazds for.metals with the results shown in Table A-3. <br />Because the Regulation 37 is expected to incorporate changes to its TVS equations within the next <br />several months, this WQA has also been developed~to reflect the water quality standazds based on <br />the new TVS equations. These are set forth in Table A-4. <br />Ambient Water Quality - . . <br />The WQCD evaluates ambientwater quality based on•a variety of statistical methods as presen'bed in <br />Section 31.8(2)(a)(i) and 31.8(2)(b)(i)(B) of the Colorado Department of Public Health and <br />Environment Water Quality Control Commission Regulation No. 31. Ambient water quality is <br />evaluated in this WQA to determine assimilative capacities for pollutants of concern. <br />To conduct an assessment of the ambient water quality upstream of the Eagle Mine dischazges, data <br />were primarily gathered from the Williams Fork River at sampling location WF-1, which is located <br />just upstream of all the outfalls. Data from this location were available from a period of record from <br />June 1981 through December 2000. Because analytical. results for several metals of concern were <br />not available, the data from sampling location WF-1 were supplemented with ambient water quality. <br />data fur dissolved silver and dissolved zinc from Vv'QCD sampling location 12823_(Morapos Creek <br />Neaz Hamilton). This location was considered to be a comparable watershed for purposes of the <br />Rationale for Classifications, Standards and Designations of Segments of the Lower Colorado River <br />Basin and was likewise considered comparable for this WQA. ~' <br />Appendix A <br />Page 4 of 13 <br />