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Minrec, Incorporated Water Quality Assessment <br />CO- 0048233 <br />For mercury, the WQBEL load is greater than the TL, and the WQBEL Conc is greater than the TL Conc, <br />and therefore additional consideration of the TL must occur. If the permittee accepts the TL, the AD <br />evaluation continues with the calculation of the SCT and ADBACs in the same manner as the other non - <br />bioaccumulative parameters, using the TL Conc in place of the WQBEL. This would result in an ADBAC <br />of 0. <br />If the TL is not acceptable, an AD Alternatives Analysis must be completed. For more information <br />regarding an Alternatives Analysis, refer to the Antidegradation Guidance and to Regulation 31.8. <br />Significant Concentration Threshold <br />The SCT is defined as the BWQ plus 15% of the baseline available increment (BAI), and is calculated by <br />the following equation: <br />SCT (0.15 X BAI) + BWQ <br />The BAI is the concentration increment between the baseline water quality and the water quality standard, <br />expressed by the term (WQS — BWQ). Substituting this into the SCT equation results in: <br />Where, <br />SCT 0.15 X (WQS -BWQ) + BWQ <br />WQS Chronic standard or, in the absence of a chronic standard, the acute standard <br />BWQ = Values from Table A -10 or Table A -6 (total recoverable and dissolved arsenic, and <br />dissolved zinc) <br />When the BWQ concentration is equal to zero, the following equation results: SCT 0.15 X WQS <br />The SCTs are shown together with the calculations of the ADBACs, in Table A -12. <br />Determination of the Antidegradation Based Average Concentrations <br />The antidegradation based average concentrations (ADBACs) are determined for all parameters except <br />ammonia, by using the mass - balance equation, and substituting the SCT in place of the water quality <br />standard, as shown in the following equation: <br />Where, <br />ADBAC = <br />SCT X Q3 - M, X Qi <br />Q2 <br />QI = Upstream low flow (lE3 or 30E3 based on either the chronic or acute standard) <br />Qz = Current design capacity of the facility <br />Q3 = Downstream flow (QI + Qz) <br />MI = Current ambient water quality concentration (From Section III) <br />SCT Significant concentration threshold <br />The ADBACs were calculated using the SCTs, and are set forth in Table A -12. <br />Appendix A (WQa v 7.0) Page 21 of 25 Last Revised Sept 14, 2009 /ES <br />