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i• _ Blue Mountain Energy, Inc. W WTP Water Quality Assessment CO-0038024 Agency (EPA) DFLOW softwaze. The output from DFLOW provides calculated acute and chronic low flows for each month. Flow data from January 1, 1989 through September 30, 1999 were available from the gage station. The gage station and time frames were deemed the most accurate and representative ofcurrent flows and were therefore used in this analysis. • During the months of March and October, the acute low flow calculated by DFLOW exceeded the chronic low flow. Tn accordance with WQCD standard procedures, the acute low flow was thus se[ equal to the chronic low flow for these months. IV. Technical Analysis In-stream backgound data and low flows evaluated in sections II and III aze ultimately used to determine the assimilative capacity of the unnamed tributaries to Red Wash and Scullion Gulch, and the White River neaz the Blue Mountain Energy, Inc. WWTP for pollutants of concern. For all parameters except ammonia, it is the WQCD's approach to conduct a technical analysis of stream assimilation capacity using the lowest of the monthly low flows (referred to as the annual low flow) as calculated in the tow flow analysis. For ammonia, it is the standazd procedure of the WQCD to detemrine assimilative capacities for each month using the monthly low flows calculated in the low flow analysis, as the regulations allow the use of seasonal flows when establishing assimilative capacities. The WQCD's standard analysis consists of steady-state, mass-balance calculations for most pollutants and modeling for pollutants such as ammonia. The mass-balance equation is used by the WQCD to calculate the maximum allowable concentration ofpollu[ants in the effluent, and accounts for the upstream concentration of a pollutant at the existing quality, critical low flow (minimal dilution), effluent flow and the water quality standard. The mass-balance equation is expressed as: M~Q~-M~Q~ Mz= Qz . Appendix A Page 9 of 15 Based on the low flow analysis described previously, the upstream low flows available [o the Blue Mountain Energy, Inc. W WTP in the White River were calculated and are presented in Table A-7.