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COLORADO DEPAR77V1EM OF HEALTH, Water Quality Comrol Division
<br />Rationale -Page 14, Permit ,No. CO.0034142
<br />The settleable sodds and pH limitations apply to discharges from rulamation areas (40 CFR Part
<br />434.52(a)). Outfalis 004, 007, 009, 011, 012, 013, 014, 0I5, 016, 017, 018, 019, 020, and 021 are
<br />subject to these limitations. Pursuant to 40 CFR Past 434.63(d)(1J, any discharge or increase in the
<br />volume oja ,discharge from a rulamation area, caused by pruiprtarion within any 24-hour period greater
<br />than the 10-pear, 24-hour pruipitation event (or snowmelt of equivalau xolume) may comply with the pH
<br />limitation only. _
<br />The meal iron, pH, and 7SS limitations apply to discharges of alkaline mine drainage (40 CFR Pan
<br />434.42). Outfalis 003, 022, and 024 are subject to these limitations. Since these discharges emanate
<br />from underground mines that are not rnmrningled with other discharges eligible for alternate limitations
<br />for precipitaton events, pursuant to 40 CFR Part 434.63(0)(1 J(iJ, akernate ej/Itteu limitations for
<br />precipitation events are not applicable to OutfalLs 003, 022, and 024. -
<br />Since the water quality limitations jot pH, as shown in section IILA.3 are more stringent than the federal
<br />standard pH limitations for discharges from reclamation areas, water quality-based /imitations for pH, as
<br />done in the previous permit, will be retained in the proposed permit jot Outjalls 004, 007, 009, and 011 -
<br />021. Similarly, the water quality limitations for pH are more strngem than the federal standard pH
<br />limitations for discharges of alkaline mine drainage. Therefore, inter quality-based limitations jar pH, as
<br />done in the previous permit, will be retained in the proposed permit for Otrtfalis 003 and 022.
<br />For Outfalls 023 and 024, the previous permit had a pH limit of between 6.5 s. u. and 9.7 s. u. Akhough a
<br />pH of 9.7 in the discharge does not impact the instream pH of the Williams Fork River, as indicated in the
<br />previous permit rationale, the permittee must comply with the technology-based standards. Therefore, a
<br />pH limitation of between 6. S s. u. and 9.0 s. u. , (6. S s. u. based on water quality and 9.0 s. u. based on
<br />federal stamardsJ, will be included in the proposed permit. Additionally, based upon a review of the
<br />DMR data submitted for Outfall 023 and 024, no compliance problems are expected since the permittee
<br />did not exceed a pH of 9.0 in either outfall.
<br />Since the water quality limitations for total iron, as discussed in Section V1.A.3.c are more stringent than
<br />the federal guidelines for discharges from reclanation areas and alkaline mine drainage, water qualiry-
<br />based limits, as done in the previous permit, will be incorporated into the proposed permit.
<br />c. Pollutants Limited by Water OualiN Standards - No pollutants were limited by water quality standards,
<br />with the exception of pH, for the discharges entering the Yampa River (OutfaILs 013, 014, 016, 018, 019,
<br />020, and OZ.!J.
<br />Additionally, no pollutants were limited !ry water quality standards, with the aception of pH, for the
<br />discharges from post mining areas to the Williams Fork River (Outfalis 004, 007, 009, 011, 012, 015, and
<br />017).
<br />For the parameters shown in Table V/-2, the mass balance equation shown in VI-A.2 was used to calculate
<br />the allowable e,(Jluent limitations that xnuld not cause the water quality standards to be violated for the
<br />discharges that enter the Williams Fork River (Outfa!!s 003, 022, 023, and 024). These limitations are
<br />shown as the values for Ml in Table V/-6. The values for Qt, Q= and Qr for chronic and acute limitations,
<br />taken from section I//.B.2. ojthir rationale, and used in the calculations for water quality limited
<br />parameters are shown in Table VI-5.
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