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0 <br />COLORADO DEPAR77l1ENT OF HEALTH, <br />Rationale -Page 2, Permit No. CO-0032751 <br />Water Quality Control Division <br /> <br />For this facility, chronic WET testing is required. (See Para I.A and I.B of the permit.) <br />aJ Puroose of WET Testine: The Water Quality Coturol .Division has established the use of WET testing as a <br />nuthod for identifying and controlling toxic discharges from wastewater treatment facilities. WET testing <br />is being utilized as a means to ensure that there are rw discharges of pollutants "in amounts, <br />concentrations or combinations which are hatmfid to the beneficial uses or toxic to humans, animals, <br />plants, or aquatic life' os required by Section 3.1.11 (1J of the Basic Standards acrd Methodoloeies for <br />Surface Waters. <br />bJ Instream Waste Concentration (lA'C): Where monitoring or limitations for WET are deemed appropriate <br />by the Division, chronic instream dilution as represented by the chronic TWC it critical in determining <br />whether acute or chronic conditions shall apply. For those discharges where the chronic IWC is greater <br />than (>J 9.1 %, chronic conditions apply, where the IWC is less than or equal to (SJ 9.1 acute conditions <br />apply. The chronic IWC is determined using the following equation: <br />IWC =(Facility Flow (FFJ/(Stream Chronic Lox Flow (annual) + FFJJ X 100%a <br />The seasonal low-flows and corresponding INrC's far discharge point 001 are: <br />Seaton ..:. Chfanic Law Flow, Facil0y Design Flow, IR'C, (90) <br /> 30E3, (MGI)) (MGD) <br />August-March 0.040 0.036 47.4 <br />April-June 0.054 0.122 69.3 <br />July 0.30 0.093 23.7 <br />The most critical seasonal IWCjor this permtt is 23.7%, which represents a wastewater concentration of <br />23.7% effluent to 76.3% receiving stream <br />However, in addition ro the IWC value for determining whether chronic or acute WET testing requirements <br />are applicable, the classifuation of the receiving stream must be considered. According to the Colorado <br />_Water Oualiry Control Division BiomonitorinJ> Guidance Document. dated July 1, 1993, chronic WET <br />resting is required for facilities where the receiving stream has a Cars I Aquatic Life use or Class 2 <br />Aquatic Life use with al[ of the appropriate aquatic life numeric standards; otherwise, acute WET testing is <br />required. Since this receiving water is clarsjried as Gass I Aquatic Lfe use, site Division has determined <br />that chronic WET testing is applicable for this permit (see following paragraph!. <br />cJ Chronic WET Limitations: Based upon previ.~us WET testing and chemical analysis of the efJlrtem, the <br />Division believes there is reasonable potential for the discharge to interfere with attainment of applicable <br />water quality class fu;ations or standards. 7herefare, pursuam ro section 6.9. Z. (SJ of the Reeulations (or <br />State Diseharee Permit Svstem (5 CCR 1002-~, the chronic limit has been incorporated imo the permit <br />and becomes effective itnmetfiately. The perrnittee is required to conduct routine monitoring. The results <br />of the testing are to be reported on Division itpproved forms. The permittee will be required to conduct a <br />statistical derivation on the data, Looking for any statistically sign fuant difference in toxicity between the <br />control and the effluent concentrations. This set of calculations will look at the full range of toxicity <br />(lethality, growth and reproduction!. Ifa level of chronic toxicity occurs, such that there it a statistically <br />sign~am difference in the lethality (at she 9.5% confidence level) benveen the control and any effluent <br />concentration less than or equal to the Insrre~vn Waste Concentration (IWC), the permiaee will be required <br />to follow the automatic canpliance schedule ~dent~e.I in Pan LB. ojthe permit, if the observed toxicin~ is <br />due to organism lethality. <br />