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<br />COLORADO DEPARTMENT OF HEALTH <br />Water Quality Control Division <br />Rationale - Page 6 <br />Permit No. CO-0027154 <br />Outfall 005 <br />This outfall serves as the discharge point for underground mine drainage, in <br />addition to other continuous sources. Mine drainage flows to a sedimentation <br />pond where commingling occurs with surface runoff from active mining areas. <br />Federal BPT and BAT apply to this discharge. Additionally, SES apply as do <br />WQS for pH and several metals parameters. A comparison is shown in Table 3. <br />Derivation of the WQS based metals limitations is discussed below. <br />TABLE 3 <br />Parameter SES Limitation BPT SAT <br />pH, s.u. 6.0 - 9.0 6.5 - 9.0 6.0-9.0 N/A <br />011 and Grease, mg/1 10 N/A N/A N/A <br />Total Suspended Solids, mg/1 30/45 N/A 35/70 N/A <br />Total Recoverable Iron, mg/1 N/A 1.0 3.5/7.0 a/ 3.5/7.0 a/ <br />Total Recoverable Copper, mg/1 N/A 0.01 N/A N/A <br />Total Recoverable Silver, mg/1 N/A 0.0001 N/A N/A <br />Total Recoverable Zinc, mg/1 N/A 0.05 N/A N/A <br />Settleable Solids, ml/1 N/A N/A N/A 0.5 <br />a/ BPT and BAT limitations are expressed as "total" <br />Water quality standard (WQS) based effluent limitations are calcul ated <br />using the following mass balance formula: <br />EL ' WQS CQ7_10 + QEFF) - (Q7-10 X Cam) <br />QEFF <br />Where: <br />EL. Calculated effluent limitation (mg/1) <br />WQS = Applicable water quality standard (mg/1) <br />Q7-10 = Receiving water 7-day, 10-year low flow (0 cfs) <br />QEFF =Maximum effluent flow (1.5 cfs) <br />CAB 'Ambient instream concentration (mg/1) <br />In calculations where the Q7_10 is zero, limitations are less than or equal <br />to the WQS. Division policy states that limitations never be less than the <br />WQS. Thus, limitations effectively equal WQS for all metals parameters above. <br />