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~OLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Qualiry Control Division mendment No. 2 -Rationale -Page 4, Permit No. CO-0000221 For rainfall, to waive TSS and total iron limitations, i1 is necessary fo prove that discharge directly resulted from a specific event and occurred through automatic dewatering devices within 48 hours after measurable precipitation has stopped. !n addition, to waive settleable solids limitations, it is necessary to prove that an event greater than the magnitude of the 10-year, 24-hoar event occurred. For snowmelt, to waive TSS and rota! iron limitations, it is necessary to prove that discharge directly resulted from snowmelt and occurred through automatic dewatering devices within 48 hours after pond inflow has stopped. !n addition, to waive settleable solids limitations, it is necessary to prove that an event greater than the magnitude of the applicable 10-year, 24-hour event occurred. c) Anriderradatian: Since (he receiving water is Use Protected, an antidegradation review is not required pursuant ro section 3.1.8(1)(b) ojThe Basic Standards and Methodoloeies (or Surface Water. d) Whole Effluent Toxirirv (WEI7 Testinx: For outfalls 010 and 011, acute WET resting is required. (See Parts I.A.1. (b) and I.A.2. ojthe permit.) 1) moose of WET Testier: The Water Quality Control Division has established the use of WET testing as a method for identifying and controlling toxic discharges from wastewater treatment facilities. WET resting is being utilized as a means to ensure that there are no discharges of pollutants 'in amounts, concentrations ar combinations which are harmful ro the beneficial uses or toxic to humans, animals, planes, or aquatic life' as required by Section 3.1.11 (!) of the Basic Standards and Melhodoloeies for Surface Waters. 2) /nstream Waste Concentration (IWC): Where monitoring orlimirarions for WET are deemed appropriate by the • Division, chronic instream dilution as represented by the chronic /WC is critical in determining whether acute or chronic conditions shall apply. For chose discharges where the chronic IWC is greater than (>) 9.1 %, chronic conditions apply, where the IWC is less than ar equal to (<_) 9.l acute conditions apply. The chronic IWC is determined using the following equation: IWC = (Faciliry Flow (FF)/(Stream Chronic Low Flow (annual) + FF)/ X 100% The flows and corresponding IWC for the appropriate discharge point follow. Dtscharge.Point:: Chronic Low Flow, Facility. Design Flow,. IWC, (%) . 30E3.. (cfs- (cfsl 010. 011 0 9.2 100 The IWCjor this permit is 100%, which represents a wastewmer concentration of 100% effluent fo O~Y receiving stream. However, (rs addition to the IWC value for determining whether chronic or acute WET testing requirements are applicable, the classification ojthe receiving stream must be considered. According ro the Colorado Water Oualirv Control Division Biomanitorinr Guidance Document. dated July 1, 1993, where the receiving water is classified aquatic life, class 2 without a!! ojthe appropriate aquatic life numeric standards, acute WET testing is required, regardless ofdilution. Since the receiving water is classified as Class 2 Aquatic Lije use, bur does nor bout all of the appropriate aquatic life numeric standards, the Division has determined that acute WET testrng is applicable for this permit (also see following paragraph). 3) Acute WETLimitatians: The Divisionhasjoundthatdischargesofundergroundminedraiwgeandspoilsspring waterjrom coal mines at similar sites have exhibited effiuem toxicity. On this basis, the Division believes there • is reasonable potential for the discharge to interfere with attainmem ojapplicable water quality classifications or standards. Because of this condition, the acute toxicity limit has been incorporated into the permit and becomes efjec(ive immediately. Prior to the acute WET limit becoming effective, the permittee is required to condud routine monitoring, the results of which are to be reported as an LC# If the LCD 5 100` e_Qluenr, the permittee is required to conduct the automatic compliance srhedu/e as ideruified in Part I.B.S. ofthe permit.