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Twentymile Coal Company Water Quality Assessment <br />000042161 <br />place on September 30, 2000. Accordingly, the BWQ is calculated based on the following equation: <br />• <br />• <br /> <br />BWQ - MepQep + M~ ~ ,Q~ <br />Qep + Q~ i , <br />where: <br />BWQ = Baseline water quality concentration <br />QW, = Upstream chronic low flow (30E3) <br />Qep = Design flow of 0.78 MGD (1.2 cfs) <br />MW, = Upstream background pollutant concentration at the 85'" percentile <br />Mpg- 30-day average limit or long term average concentration if no limit. <br />Currently, it is the WQCD's approach to evaluate five yeazs of ambient water quality data and <br />effluent data, if available, for the time frame prior to September 30, 2000. A review of five years <br />of ambient water quality data was presented in Table A-4 to establish the MW,. To establish the <br />MAR, several sources were used. An effluent limit was available for total recoverable iron, only. <br />PCS monitoring data front July I, 1998, through June 30, 2000, were available for ammonia and <br />were thus used. There is only one datum on file for manganese and it is from 12/28/93. The value <br />for both total recoverable manganese and dissolved manganese is 80 ug/l. <br />Pursuant to the approach discussed above, the equation for BWQ, and the available data, the <br />BWQs for pollutants of concern aze set forth in Table A-9. <br />Table A-9: Baseline Water Quality Chronic Concentrations <br />Pollutant M,~ (mgA) Q,~(cjs) MW, (85th) QW, (cjs) BWQ <br />(n+S~~ WQS <br />(mBB~ <br />Mn, Dis 0.080 1.2 0.143 1.3 0.11 3.105 <br />Fe, Trec 0.50 1.2 4.6 1.3 2.63 1.000 <br />NH, (Jan) 3.84 1.2 0.070 3.1 1.12 0.7 <br />NH, (Feb) 3.84 1.2 0.070 3.5 1.03 0.6 <br />NH, (Maz) 3.84 1.2 0.070 4.1 0.92 0.4 <br />NH, (Apr) 3.84 1.2 0.070 4.4 0.88 0.4 <br />NH, (May) 3.84 1.2 0.070 2.5 1.29 0.3 <br />NH, (Jun) 3.84 1.2 0.070 1.9 1.53 0.2 <br />NH, (Jul) 3.84 1.2 0.070 1.3 1.88 0.2 <br />NH, (Aug) 3.84 1.2 0.070 1.3 1.88 0.2 <br />NH, (Sep) 3.84 1.2 0.070 1.3 1.88 0.2 <br />NH, (Oct) 3.84 1.2 0.070 1.3 -1.88 0.2 <br />NH, (Nov) 3.84 1.2 0.070 1.6 1.69 0.3 <br />NH, (Dec) 3.84 1.2 0.070 3.1 1.12 0.5 <br />A previous antidegradation review conducted on December 28,1988, established an antidegradation <br />limit of 0.5 mg/1 for chronic total recoverable iron. However, additional antidegradation-based <br />effluent limits have not been established as part of this WQA. This is because Section 31.8(3)(a) of <br />Appendix A <br />Page 11 of 12 <br />