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Pittsburg and Midway Coal Company Edna Mine Water Quality Assessment CO-0032638 <br />Table A-10 <br />BWQ Concentrations at Outfa11007 for Potential Pollutants of Concern <br />Pollutant Me~~." 'Qep~(cfs} M;~S- Q'urs.(rfs}' BK'Q H'QS ' <br />Fe, TR (ug/I) 115 1.0 100 3.5 103 1000 <br />Mn, Dis (ug/1) 417 1.0 12 3.5 99 2618 <br />Hg, Tot (ug/1) 0 1.0 0 3.5 0 0.010 <br />New or increased impacts on the receiving stream are expected as a result of this permit issuance <br />because the proposed water quality-based effluent limits (WQBELs), either loading or concentration, <br />are greater than the current permitted load or concentration for the following pollutants: <br />• Fe, TR (Outfalls 004 and 007) <br />• Mn, Dis (Outfalls 004 and 007). <br />Note that the evaluation of the current permitted load and the WQBEL load is further discussed at <br />the end of this assessment. <br />For the above listed pollutant, the antidegradationreuiew procedure must continue to determine if <br />impacts are significant. Impacts are deemed to be significant if the calculated assimilative capacity <br />exceeds the calculated antidegradation-based average concentration (ADBAC). ADBACs aze <br />calculated using the significant concentration threshold (SCT), which is the additional amount of <br />pollutant above BWQ concentration that would not cause significant degradation. Section 31.8 <br />(3)(c) specifies that the discharge of pollutants should not be considered to result in significant <br />degradation of the reviewable waters if one of the following summarized conditions is met: <br />• For bioaccumulative toxic pollutants such as mercury, the new or increased loading from <br />the source under review is less than 10 percent of the existing total load to that portion of <br />the segment impacted <br />For all other pollutants <br />o the flow rate is greater than 100:1 dilution at low flow; or <br />o the new effluent load is less than 15 percent of the remaining assimilative capacity; or <br />0 only a temporary change in water quality will result. <br />The SCT for most pollutants equals the BWQ concentration plus 15 percent of the remaining <br />assimilative capacity, and is calculated by the following equation: <br />Appendix A ~wQn sr version a-s rna~> Page 16 of 19 Last Revised September 2, 2005 <br />