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Mountain Coal and Beaz Coal Water Quality Assessment CO-0038776 and CO-0044377 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 Mountain Coal and Beaz Coal W WTFs when determining the available assimilative capacities in the North Fork of the Gunnison River. Due to the distance traveled, the significant dilution of the North Fork of the Gunnison River, and the changes in the chazacteristics of the receiving stream, modeling downstream facilities in conjunction with Mountain Coal and Bear Coal WWTFs was not necessary. Due to the low flow ofzero, consideration of other sources when allocating assimilative capacities to discharges to Sylvester Gulch, West Sylvester Gulch, the Dry Fork of Minnesota Creek and Lone Pine Gulch was not necessary because the full assimilative capacities of these receiving streams aze equal to the stream standazds. 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. Chlorine: The mass-balance equation was used to determine the assimilative capacity for chlorine for Outfa11004, which receives effluent from the domestic W WTF (Internal Outfa11007). There are no point sources discharging total residual chlorine within one mile ofthe Mountain Coal WWTF Outfa11004. Because chlorine is rapidly oxidized, in-stream levels ofresidua] chlorine are detected only for a short distance below.a source. Ambient chlorine was therefore assumed to be zero. Using the mass-balance equation provided in the beginning of Section IV, the acute and chronic low; .. flows set out in Section III, the chlorine background concentration of zero as discussed above, and the in-stream standards for chlorine shown in Section II, assimilative capacities for chlorine were calculated. The effluent flow, QZ, reflects the design flow from Outfa11004 and Internal Outfa11007 in the respective calculations. The data used and the resulting calculations ofthe allowable dischazge concentration, M2, are set forth below. North Fork o the Gunnison River Parameter QI (cfs) Qz (cfs) Q3 (cfs) Ml (mg/I) M3 (mg/l) Mz (mg/1) Acute Chlorine 32 0.28 32.28 0 0.019 2.2 Chrotic Chlorine 43 0.28 43.28 0 0.011 1.7 Ou a11007 Parameter Q t (cfs) Q Z (cfs) Q q (cfs) Ml (mg/I) M j (mg/1) MZ (mg/1) Acute Chlorine 32 0.016 32.016 0 0.019 38 Chronic Chlorine 43 0.016 43.016 0 0.011 30 Fecal Coliform: There aze no point sources discharging fecal coliform within one mile of the Mountain Coal WWTF Outfall 004. Thus, fecal coliform assimilative capacities for Outfall 004 were evaluated separately. Appendix A Page 14 of 20 Last Revised 3/9/2004