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Pittsburg and Midway Coal Company Edna Mine Water Quality Assessment CO-0032638 <br />Standards for metals are generally shown in the regulations as TV S, and these often must be derived <br />from equations that depend on the receiving stream hazdness or species of fish present. The <br />Classification and Numeric Standards documents for each basin include a specification for <br />appropriate hardness values to be used. Specifically, the regulations state that: <br />The hardness values used in calculating the appropriate metal standard should be based <br />on the lower 95% confidence limit of the mean hardness 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 hazdness value at the periodic <br />low flow criteria, representative regional data shall be used to perform the regression <br />analysis. Where a regression analysis is not appropriate, asite-specific method should <br />be used. <br />There are no ambient water quality data available for the unnamed tributary to Trout Creek. <br />Hardness data for Trout Creek near the points of discharge of the Edna Mine Outfali 007 were <br />insufficient to conduct a regression analysis based on the low flow. Therefore, the Division's <br />alternative approach to calculating hardness was used, which involves computing a mean hardness. <br />In the absence of direct hardness data, hardness values were calculated from calcium and magnesium <br />data using the following formula recognized by the Division: <br />Hardness (mg/l) = 2.497 x Ca (in mg/1) + 4.116 X Mg (in mg/1) <br />The mean hardness of Trout Creek was computed to be 407 mg/1 based on sampling data from the <br />Edna Mine station TR-D, located on Trout Creek approximately four miles downstream from Outfall <br />007, and approximately one mile downstream from the confluence of the unnamed tributary <br />containing the discharge from Outfall 004. The Basic Standards and Methodologies for Surface <br />Water indicates that hardness must be capped at 400 mg/1 when determining in-stream metal water <br />quality standards using the equations in the TVS. This maximum hardness value and the formulas <br />contained in the TVS were used to calculate the in-stream water quality standards for metals, with <br />the results shown in Table A-3. Note that this hardness value was also assumed to be representative <br />of the unnamed tributary to Trout Creek and therefore Table A-3 also reflects the standards <br />applicable to the unnamed tributary to Trout Creek. <br />Ambient Water Quality <br />The Division evaluates ambient water quality based on a variety of statistical methods as prescribed <br />in 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 for use in determining assimilative capacities and in completing <br />antidegradation reviews for pollutants of concern, where applicable. <br />The ambient water quality was not assessed for the unnamed tributary to Trout Creek because the <br />background in-stream low flow condition is zero upstream of the Edna Mine Outfa11004 discharge. <br />Appendix A ~wQn Br vas;°„ a-a rner> Page 5 of 19 Last Revised September 2, 2005 <br />