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Pittsburg and Midway Coal Company Edna Mine Water Quality Assessment CO-0032638 <br />BWQ = MQ~Qe~ +MuisQ~ds <br />Qep + Q~s <br />Where, <br />BWQ =Baseline water quality <br />Q,~s =Upstream chronic low flow (30E3) <br />Mss =Upstream background pollutant concentration at the existing quality <br />Qe~- = 2-yeaz average flow <br />Mep = 2-year average effluent pollutant concentration <br />When Q„ is equals zero, the following results: <br />BWQ = Mew <br />Thus, for Outfall 004, the BWQ concentration equals the Mel: A further discussion of the <br />determination ofMepat Outfa11004 is provided in later paragraphs. <br />For Outfall 007, additional data must be compiled to develop the BWQ concentration. The <br />antidegradation requirements outlined in The Basic Standards and Methodologies for Surface Water <br />specify that chronic numeric standards should be used in the antidegradation review; however, where <br />there is only an acute standard, the acute standard should be used. Chronic standards were available <br />for all pollutants except total recoverable trivalent chromium. Thus, the chronic low flows were used <br />for Q„is for all pazameters except total recoverable trivalent chromium when establishing the BWQ <br />concentrations; for total recoverable trivalent chromium, the acute low flow was used. <br />The period of record for the low flows utilized as part of the antidegradation review differ from the <br />low flow data previously set forth in Section III. Low flows, Q,~s, for the antidegradation review <br />were determined based on the same approach and using data from the same location as discussed in <br />Section III, but the period of record was limited to data collected prior to September 30, 2000. The <br />following annual low flows resulted. <br />• Chronic low flow (30E3) = 3.5 cfs <br />• Acute low flow (lE3) = 1.4 cfs. <br />Currently, it is the Division's approach to evaluate five years of ambient water quality data, if <br />available, for the five yeazs prior to September 30, 2000. <br />The period of record for the ambient water quality data evaluated as part of the antidegradation <br />review differ from the ambient water quality data previously set forth in Section II. Ambient water <br />quality data used for the antidegradation review were gathered at the same location as the other data, <br />but were evaluated for a period of record from October 1995 through September 2000. These data <br />are summarized in Table A-8. <br />Appendix A ~wQn Br vers;on a-s Fnsr> Page 14 of 19 Last Revised September 2, 2005 <br />