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COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Comrol Division Rationale -Page 10, Permit No. CO-0027146 fl Whole Effluent Toxicity tWEI'1 Tesrin¢.• Far this facility, acute WET testing is required jot outfalls 001, 002, 004 and 015. (See Part /.A.4. of the permit.) !) Purpose ojWET Testin¢: 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 testing is being utilized as a means to ensure that there are na discharges of pollutants "in amounts, concentrations or combinations which are harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life" as required by Section 31.11 (1) of the Basic Standards and Methodologies for Surface Waters. 2) Instream Waste Concentration tIWCI: Where monitoring or limitations for WET are deemed appropriate by the Division, chronic insrream dilution as represemed by the chronic IWC is critical in determining whether acute or chronic conditions shall apply. For those discharges where the chronic IWC is greater than (> ) 9.1 %, chronic conditions apply; where the /WC is less than or equal to (s) 9.1 acute conditions apply. T&e chronic IWC is determined using the following equation: IWC =[Facility Flaw (FF)/(Stream Chronic Low Flow (annual) + FF)j X l00% The flows and corresponding /WC for the appropriate discharge point are: Discharge Point :.: .Ghronic.Gow Flow;' Faeility.Design Flow; . :.:.IWC, (%) 30E3, (cjs) (c fs) , . 001 and 002 412 0.46 0.11 004 0 0.25 100 015 0 0.6 100 The combined IWCfar outfalls 001 and 002 is 0.I1 %„ which represems a wastewater concentration of 0.11 % effluent to 99.89% receiving stream. thus, acute WET testing is applicable to these discharges. However, in addition to the IWC value for determining whether chronic or acute WET testing requiremems are applicable, the classification of the receiving stream must be considered. According to the Colorado Water Oualirv Control Division Biomonitorin¢ Guidance Document. dated July 1, 1993, where the receiving water is classified aquatic life, class 2 without all of the appropriate aquatc life numeric standards, acute WET testing is required, regardless of dilution. Since the receiving waters for outfalls 004 and OIS are classified as Class 2 Aquatic Life use, but without all of the appropriate aquatic life numeric standards (segment 4), the Division has determined that acute WET testing is applicable for these outfalls (also see following paragraph). 3) Acute WET Limitations: The Division hat found that discharges of mine drainage, pit pumpage, spoils spring water and similar sources from coal mines at similar sites have exhibited effluem toxicity. On this basis, the Division believes there is reasonable potential for the discharges from outfa!!s 001. 002, 004 and 015 to interfere with attainrrtem of applicable water quality classifications or standards. Because of this, the acute toxicity limit has been incorporated into the permit and becomes effective immediately. The permittee is required to conduct routine monitoring, the results of which are to be reported as an LCto- If the LCs¢ s 100% e~'luem, the permittee is required to conduct the automatic compliance schedule as identified in Parr I.A.4.c. of the permit. 4) General Information: The permittee should read the WET testing sections under Parts I.A.4. of the permit carefully. The permit outlines the test requiremems and the required follow-up actions the permittee must take to resolve a toxicity incidem. The permittee should read, along with the documems listed in Pans I.A.4.a. of the permit, the Colorado Water OualiN Control Division Biomonitorirlg Guidance Documem, dated July 1, 1993. This document outlines the criteria used by the Division in such areas ar granting relief from WET testing, modifying test methods and changing test species. The perminee should be aware that some of the conditions outlined above may be subject to change if the facility experiences a change in discharge, at outlined in Part /I.A.2. of the permit. Such changes shall be reported to the Division immediately.