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Minrec, Incorporated Water Quality Assessment CO- 0048233 calculate the WQBELs. It is the Division's approach to calculate the WQBELs using the lowest of the monthly low flows (referred to as the annual low flow) as determined in the low flow analysis. The Division's standard analysis consists of steady- state, mass - balance calculations for most pollutants. The mass - balance equation is used by the Division to calculate the WQBELs, 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: Where, M2 - M3Q3 -M1Q1 Q2 Q, = Upstream low flow (IE3 or 30E3) Qz = Average daily effluent flow (design capacity) Q3 = Downstream flow (Q, + Qz) M, = In- stream background pollutant concentrations at the existing quality Mz = Calculated WQBEL M3 = Water Quality Standard, or other maximum allowable pollutant concentration The upstream background pollutant concentrations used in the mass - balance equation will vary based on the regulatory definition of existing ambient water quality. For most pollutants, existing quality is determined to be the 85th percentile. For metals in the total or total recoverable form, existing quality is determined to be the 50th percentile. Calculation of WQBELs Where a WQBEL is calculated to be a negative number and interpreted to be zero or when the ambient water quality exceeds the in- stream standard, the Division standard procedure is to allocate the water quality standard to prevent further degradation of the receiving waters. The Division's Restoration and Protection Unit investigates issues of water quality standard exceedances. This Unit is tasked with determining if the exceedances are valid and placing the receiving stream on the Clean Water Act Section 303(d) list of impaired waters, if appropriate. If the receiving water is placed on the State's 303(d) list, the Assessment Unit is tasked with developing the Total Maximum Daily Loads (TMDLs) and the Waste Load Allocations (WLAs) to be distributed to the affected facilities. Metals, Boron, and Sulfide: Using the mass - balance equation provided in the beginning of Section VI, the low flows and background concentrations contained in Section IV, and the in- stream standards for metals and sulfide as shown in the previous section, the WQBELs were calculated. The data used and the resulting WQBELs, Mz, are set forth in Table A -7a for chronic standards and in Table A -7b for acute standards. Appendix A (WQa v 7.0) Page 13 of 25 Last Revised Sept 14, 2009 /ES