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N. Technical Analysis <br />In-stream background data and low flows evaluated in Sections II and III are ultimately used to <br />determine the assimilative capacities of Sanborn Creek and Elk Creek neaz the Oxbow Mining, LLC <br />WWTF for pollutants of concern. For all parameters except ammonia, it is the WQCD's approach to <br />conduct a technical analysis of stream assimilation capacity using the lowest of the monthly low <br />flows (referred to as the annual low flow) as calculated in the low flow analysis. For ammonia, it is <br />the standard procedure of the WQCD to determine assimilative capacities for each month using the <br />monthly low flows calculated in the low flow analysis, as the regulations allow the use of seasonal <br />flows when establishing assimilative capacities. <br />The WQCD'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 />WQCD to calculate the maximum allowable concentration ofpollutants in 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 standard. The mass-balance equation is expressed as: <br />M3Q3 - M~Q~ <br />Mz= Q <br />z <br />Where, <br />Q1=Upstream low flow (lE3 or 30E3) <br />Qz =Average daily effluent flow (design capacity) <br />Qj =Downstream flow (Q~ + 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-stream 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 (QI) for Sanborn Creek and Elk Creek is zero, the assimilative capacity of <br />Sanborn Creek and Ells Creek for the pollutants of concern is equal to the in-stream water quality <br />standazds. <br />Appendix A Page 6 of 10 Draft <br />Oxbow Mining WWTF Water Quality Assessment CO-0000132 <br />