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COLORADO DEPARTMENT OF HEALTH, Water Qunliry Control Division <br />Rationale -Page 2/, Pernut No. CO-0043648 <br />The placement of treated mine tailings and waste rack within the Arequa Gulch drainage upstream of <br />point AG-1 will occur as part ojthe Cresson project. It is possible that such actions may Have a <br />ameliorating effect upon metals concentrations in Arequa Gulch. Therefore, corresponding to the <br />timeframe for the construction of the wasterock area, and allowing for some time for the hydrologic <br />system to stabilize, imposition of effluent limits equal to the stream standards will be temporarily <br />delayed, as noted in the tables of eQluent limitations listed in this permit. Limits for al! metals will be <br />specked, except for lead and nickel acute limits, which are so high that they do not need to be <br />included. <br />5. Antideeradation <br />This permit has been developed based upon the premise that there will be no point source discharge of <br />process water to surface waters. Therefore, the receiving wester is not a "reviewable water", pursuant ro <br />section 3.1.8(3J(aJ ojT3ie Basic Standards and Methodoloeies for Surface Water, based upon a finding of <br />no increased water quality impact. Should a discharge occur in the future, it will need to be reviewed for <br />compliance with the antidegradation requirements. <br />6. Whole Effluent ToziciN (WETI Testine <br />7hir permit hat been developed based upon the premise that there will be no point source discharge of <br />process water to surface waters. The main purpose for the terms and conditions of the permit is to identh <br />any discharges if they occur. And, most of the monitoring that it required by the permit will be for the <br />receiving stream itself. <br />However, discharge point 007A, if it is ever used, will be discrete point source discharge. Therefore, <br />Chronic WET testing requirements will be applied to this point. (See Parts l.A and I.B of the permit.) <br />aJ Puroose of WET Testine: the Water Quality Control Division has established the use of WET testing <br />ar n method jot ident~ing and controlling toxic discharges from wastewater treatment facilities. WET <br />testing is being utilized as a means to ensure that there are no discharges ojpollutants "in amounts, <br />concenrrarions or combinations which are harmful to the beneficial uses or toxic to humans, animals, <br />plants, or aquatic life" as required by Section 3.1.11 (1J of the Basic Standards and Methodoloeies for <br />Surface Waters. <br />bJ lnstrenm Waste Concentration (/WCI: Where monitoring or limitations for WET are deemed <br />appropriate by the Division, chronic instream dilution as represented by the chronic /WC is critical in <br />determining whether acute or chronic conditions shall apply. For those discharges where the chronic <br />IWC is greater [/inn (> J 9. I %, chronic conditions apply, where the /WC is less than or equal to (<_ J <br />9. 1 acute conditions apply. The chronic /WC ic' determined using the following equation: <br />/WC = /FnciliN Flow (FFJ/(Stream Chronic Low Flaw (nmwnlJ + FFJI X 100% <br />77te flows and corresponding IWC for the appropriate discharge point are: <br />DiscJtarge Point Chronic Low Flow, Facility Design Flow, IWC, (%) <br />30E3, (cjs) (cjs) <br />007A 0.0 not known 100% <br />The /WCjor this permit is !00%, which represents a wastewater concentration of 100% effluent and 0% <br />receivin4 ,ctrenm. <br />