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Pittsburg and Midway Coal Company Edna Mine Water Quality Assessment CO-0032638 <br />The Division's standard analysis consists of steady-state, mass-balance calculations for most <br />pollutants and modeling for pollutants such as ammonia. The mass-balance equation is used by the <br />Division to calculate the maximum allowable concentration of pollutants in the effluent, and <br />accounts for the upstream concentration of a pollutant at the existing quality, critical low flow <br />(minimal dilution), effluent flow and the water quality standard. The mass-balance equation is <br />expressed as: <br />Mz = MaQ3-M~Qi <br />Qz <br />Where, <br />Q~ =Upstream low flow (lE3 or 30E3) <br />Qz =Average daily effluent flow (design capacity) <br />Q3 =Downstream flow (Ql + QZ) <br />M, = In-stream background pollutant concentrations at the existing quality <br />MZ =Calculated maximum allowable effluent pollutant concentration <br />M3 =Maximum allowable in-streanr pollutant concentration (water quality standards) <br />When Q, equals zero, QZ equals Q3, and the following results: <br />Mz = M3 <br />Because the low flow (Q,) for the unnamed tributary to Trout Creek is zero, the assimilative capacity <br />of the unnamed tributary to Trout Creek for the pollutants of concern is equal to the in-stream water <br />quality standards. Note that this affects Outfall 004 only. <br />For Outfall 007, the upstream background pollutant concentrations used in the mass-balance <br />equation will vary based on the regulatory definition of existing ambient water quality. For most <br />pollutants, existing quality is determined to be the 85`"percentile. For metals in the total recoverable <br />form, existing quality is determined to be the 50`h percentile. <br />Amore detailed discussion ofthe technical analysis for these parameters is provided in the pages that <br />follow. <br />Pollutants Evaluated <br />The following parameters were identified by the Division as pollutants to be evaluated for this <br />facility: <br />• Metals. <br />During assessment of the facility, nearby facilities, and receiving stream water quality, no additional <br />parameters were identified as pollutants of concern. <br />Edna Mine: The Edna Mine is located in Sections 7, 18, 19; T4N, R85W and Sections 12, 13, 14, <br />23; T4N, R86W; Latitude 40° 19' ] 9"North, longitude 106° 58' 35" West; at 26900 CO Hwy 131, in <br />Routt County. There is no stated design capacity for either outfall and design flows have changed <br />Appendix A twQn sv vas;°„ as ras[ y Page 9 of 19 Last Revised September 2, 2005 <br />