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Twentymile Coal Company Water Quality Assessment 000042161 An analysis was conducted to determine the assimilative capacity of Fish Creek near Twentymile • Coal Company. The WQCD's standard analysis consists ofmass-balance calculations for most pollutants and modeling for pollutants such as ammonia. The mass-balance equation used by the WQCD accounts Cor the upstream concentration of a pollutant, critical low flow (minimal dilution), effluent flow, effluent concentration and the water quality standard. The mass-balance equation is expressed as: MaQ~ - M~Q~ Ma= Q~ Q, =Upstream low flow (IE3 or 30E3) from Table A-5 Q1=Average daily effluent flow (design capacity) Q, =Combined downstream flow (Q, + QZ) M, = In-stream background pollutant concentrations from Table A-4 M, =Maximum allowable effluent pollutant concentration M, =Maximum allowable in-stream pollutant concentration (water quality standards from Tables A- 2and A-3) For non-conservative parameters and ammonia, the mass-balance equation is not as applicable and thus other approaches are considered where appropriate. A more detailed discussion ofthe technical analysis for these parameters is provided in the pages that follow. • Pollutants of Concern During assessment of the Twentymile Coal Company, nearby facilities and receiving stream water quality in the watershed were also evaluated when establishing pollutants ofconcem. The following pollutants of concern were identified by the permit writer and were provided as part of the WQA request form: • Iron • Manganese • Ammonia. No additional pollutants of concern were identified during the data identification and evaluation phase of this assessment. Nearby Sources Based on a review of data available in EPA's Permit Compliance System (PCS) database, 46 dischargers were found in the Routt County area and seven were coal mining dperations. These dischargers were assessed to determine their impacts on the assimilative capacities available to Twentymile Coal Company in Fish Creek. Ultimately, nearby dischargers did not require consideration in this WQA for the reasons summarized below and thus Twentymile Coal Company Outfa11001 was assessed separately in this WQA. Appendix A Page 7 of 12