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COLORADO DEPARTMENT OF HEALTH. Water Quality Control Division Rationale -Page 16 Penrrit No. CO-0045675 VI. TERMS AND CONDIT/ONS OF PERMIT B. Determination of Effluent Lirnita[ions 4. Discussiono(ERluentG'mitanons (Cont.) Table V/-3 -- Summary ojMass Balance Calculations - Outfall 0016 (250 gprn discharge flow) (Cont.) Antidegradanon-Based Upstream Segment 28 Antidegradation- &/or WQS-Based # Ambient Mean Water Quality Std Based WQS Effluent Concentraton Parameter Concentration Acute/Chronic Acwe/Chronic Acute/Chronic (mg/l) (MI) c/ (M3) a/ (M3) (M2) Nickel, Potentially Dissolved 0.00050 e/ 0.660/0.068 0.011 2.2# / 0.043 • Radium 226+228, Total, pCi/l 0 d/ 5.0 5.0 g/ 5.0 • g/ Selenium, Total 0.00050 e/ 0.010 (Acute) 0.00193 0.0053 • Silver, Potentially Dissolved 0.000/0 e/ 0.00094/0.000030 0.000030 g/ 0.00029# / 0.000030• g/ Uranium, Total Natural 0 d/ /.5/0.92 0.92 g/ 4.9# / 0.92 • g/ Zinc, Potentially Dissolved 0.0025 e/ 0.080/0.073 0.014 0.26# / 0.051 a/ = The single indicated standards indicated in this column are chronic standards urrless odrendse indicated. b/ _ "0" is assuuted to be ;.ero; the mean was be(nw dre rnininrum detection (ere( for nrese indicated pnrameters. c/ = Far the metals parameters indicated as dissolved or potennal(y dissolved, the nnrbient mean for these pararnerers is for dre dissolved fraction. d/ = Presently, there is either no data or inadequate data collected m evahtate the insveam concentration for these parameters. Wins additimra[ data, these calculations x2(I 6e reevaluated. For the present rune, nre upstream levels are assumed m equal:ero. e/ = Since nrese anrbiem upstream concentrations were below nre minimum levels of detection, Nre MI value represents one-half of the mininwm level ojdetection. j/ = Since the baseline loading at Smtion RS-5 is greater than or equal m dre segment 28 water grutlity standard, the antidegradation-based standard mrd antidegradation-based limitation will revert to the more sMngent value that is equal m dre wmer quality standard. g/ = Since either there is not enough instremn data collected, or the instream data is less dmn the minhnum detection level w/rids is greater thmr the segment 28 water qua(iq• standard, the antidegradation-based standard and anridegradanon-based limitation wilt revert m the more sMngem ratite tlmt is equal m the water qualip• stmrdard. One additional consideration jot antidegradarion reviews is that ija deterndwtion lras been made in accordance with section 31.8(3)(c) that a proposed activity is likely to result in significam degradation of reviewable waters, a determination shall be made pursuant to this section whether the degradation is necessary to accommodate important economic or social development in the area in which the waters are located. The Division's evaluation shows that, based on available data, the resulting antidegradation-based effluent lirnilations are determined to be reasonably related to the economic, environmental, public health, and energy impacts to the public and affected persons. /f the permittee disagrees xdth this finding and/or determines shat the proposed antidegradation-based limits can nor be attained xdth the proposed treatment or other relevant treatment technology, pursuant to section 3l.8(3)(c) ojthe Basic Standards and Methodologies for Surface Water, the pennitree should submit all pertinent information to the Division during the public nonce period. e) Deternination of Parameters Limited for This Permit Metals and /norganics Limitations: For determining which metals and inorganic parameters rill be limited in this permit, the Division evaluated the contributing xastewater sources that are being treated and then will be discharged. A summary of these two contributing sources, which includes the Pit Backfill source and t/re Alluvial Pumping wells source, are included in Appendrz A. The pennittee has estirnared tha! the pit backfill water contributes approximately 75 % ojthe sources to be collected and treated, and the alluvial well water is approximately 25 %. fn addition, data from a previously identified source, the seep/surface water collection system, is included in Appendix A for informational purposes. At Borne point, the downgradient containment wells may also be included in the sources. The two present sources (from the backfill wells and the curtain wells) are summarised in Appendu A and reflect the potential water quality prior to treatment. The Division has compared the quality of these two sources with dre calculated antidegradatian-based allowable permit concentration levels. When the two untreated wastewater source(s) contain concentrations that approach and/or