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Central Appalachia WWTF Water Quality Assessment CO-003 <br />N. Technical Analysis <br />In-stream background data and low flows evaluated in Sections II and III aze ultimately used to <br />determine the assimilative capacity of the unnamed tributaries to East Salt Creek near the Central <br />Appalachia W WTF for pollutants of concem. 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. For <br />ammonia, it is the standard procedure of the WQCD to determine 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 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 (minhnal <br />dilution), effluent flow and the water quality standazd. The mass-balance equation is expressed as: <br />Mz _ MsQ3 -MiQ~ <br />Qz <br />Where, <br />Q~ =Upstream low flow (lE3 or 30E3) <br />QZ =Average daily effluent flow (design capacity) <br />Qj =Downstream flow (QI + QZ) <br />Ml = 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 standazds) <br />When Qt equals zero, QZ equals Q3, and the following results: <br />Mz = M3 <br />Because the low flow (QI) for the unnamed tributaries to East Salt Creek is zero, the assimilative <br />capacity of the unnamed tributaries for the pollutants of concern is equal to the in-stream water <br />quality standazds. <br />The upstream background pollutant concentrations used in the mass-balance equation will varybased <br />on the regulatory definition of existing ambient water quality. For most pollutants, existing quality is <br />determined to be the 85`" percentile. For metals in the total recoverable form, existing quality is <br />determined to be the SOa` percentile. For pathogens such as fecal coliform, existing quality is <br />determined to be the geometric mean. <br />For non-conservative parameters and ammonia, the mass-balance equation is not as applicable and <br />thus other approaches aze considered where appropriate. Note that conservative pollutants are <br />pollutants that aze modeled as if mass is conserved and there is no degradation, whereas non- <br />Appendix A Page 6 of 9 Draft <br />