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Pittsburg and Midway Coal Company Edna Mine Water Quality Assessment CO-0032638 significantly since reclamation has occurred. Therefore, in accordance with Division procedures, the maximum of the 30-day average flows reported during the past two years for each outfall were used to establish the design flows. This resulted in a design flow of 1.2 MGD (1.9 cfs) at Outfa11004 and 2.6 MGD (4.0 cfs) at Outfall 007. Currently, treatment of waters dischazged via Outfall 004 is accomplished in Moffat Pond and involve physical settling. Although some settling occurs prior to discharges entering Outfa11007, other wastewater is dischazged without treatment. Nearby Sources An assessment of nearby facilities based on EPA's Permit Compliance System (PCS) database found 40 dischargers in the Routt County area. More than one-half of the facilities were discharging to another watershed. Several facilities conducted construction-related operations (such as sand and gravel mining and construction dewatering) and thus had no pollutants of concern in common with the Edna Mine. Other facilities were located more than twenty miles from the Edna Mine and thus were not considered. The nearest dischargers were: • Twentymile Coal Company, CO-0027154, which discharges via multiple outfalls to Fish Creek, Foidel Creek, and Middle Creek--all are tributaries to Trout Creek downstream from the Edna Mine (stream segment COUCYA13b). • Seneca Coal Company, CO-0000221, which discharges via multiple outfalls to tributaries to Grassy Creek, Fish Creek, Dry Creek, and Sage Creek, which are tributaries to Trout Creek downstream from the Edna Mine (stream segments COUCYA13b, COUCYA13d, and COUCYA13e). The ambient water quality background concentrations used in the mass-balance equation account for pollutants of concern contributed by upstream sources, and thus it was not necessary to model upstream dischargers together with the Edna Mine discharges when determining the available assimilative capacities in Trout Creek and its tributaries. Due to the distance traveled, the significant dilution of the receiving streams, and the changes in the characteristics of the receiving streams modeling downstream facilities in conjunction with the Edna Mine discharges was not necessary. Based on available information, there is no indication that non-point sources were a significant source of pollutants of concern. Thus, non-point sources were not considered in this assessment. Metals: Metals may occur naturally in rock. Earth disturbances, such as mining operations, expose rock to air and water creating a potential for these elements to be present in elevated concentrations. Thus, arsenic, cadmium, chromium, copper, iron, lead, manganese, mercury, nickel, selenium, silver and zinc, which have been identified as pollutants potentially present in mine water discharges, are evaluated in this assessment. According to the Agency for Toxic Substances and Disease Registry, chromium in the trivalent form occurs naturally in the environment whereas hexavalent chromium is produced by industrial processes. Industrial processes at this facility would not generate hexavalent chromium and therefore the evaluation for chromium is limited to the trivalent form. For Outfa11004, the full assimilative capacity of the stream for metals was determined to equal the in-stream water quality standazds as set forth in Table A-5. Appendix A ~wQn ur version as rnBL> Page 10 of 19 Last Revised September 2, 2005