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Mountain Coal and Beaz Coal Water Quality Assessment CO-0038776 and CO-0044377 <br />Table A-8 <br />Low Flows for Sylvester Gulch, West Sylvester Gulch, the Dry Fork of Minnesota Creek, <br />and Lone Pine Gulch at the Mountain Coal WWTFs <br />IE3 <br />30E3 <br />Chronic <br />IV. Technical Analysis <br />In-stream background data and low flows evaluated in Sections II and III are ultimately used to <br />determine the assimilative capacity of the North Fork of the Gunnison River, Sylvester Gulch, West <br />Sylvester Gulch, the Dry Fork of Minnesota Creek and Lone Pine Gulch near the Mountain Coal and <br />Bear Coal W WTFs for pollutants of concern. For all pazameters except ammonia, it is. the WQCD's <br />approach to conduct a technical analysis of stream assimilation capacity using the lowest of the <br />monthly low flows (referred to as the annual low flow) as calculated in the low flow analysis. <br />For ammonia, it is the standazd procedure ofthe WQCD to detemune assimilative capacities for each, .' <br />month using the monthly low flows calculated in the low flow analysis, as the regulations allow the:. <br />use of seasonal flows when establishing assimilative capacities. <br />The WQCD's standazd 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 />WQCD to calculate the maximum allowable concentration ofpollutants iri the effluent, and accounts <br />for the upstream concentration of a pollutant at the existing quality, critical low flow (minimal <br />dilution), effluent flow and the water quality standazd. The mass-balance equation is expressed as: <br />M x = MsQa - M iQ~ <br />Qz <br />Where, <br />Q1=Upstream low flow (lE3 or 30E3) <br />Qz =Average daily effluent flow (design capacity) <br />Q3 =Downstream flow (Q, + Qz) <br />Ml = In-stream background pollutant concentrations at the existing quality <br />Mz =Calculated maximum allowable effluent pollutant concentration <br />M3 =Maximum allowable ia-stream pollutant concentration (water quality standards) <br />When QI equals zero, QZ equals Q3, and the following results: <br />Mz = Ms <br />Appendix A Page 11 of 20 Last Revised 3/9/2004 <br />