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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division <br />Rationale - Page 2. Permit No. CO-0027154 <br />For this facility, chronic WET testing is required for outfalls 001, 005 and 007. (See <br />Parts I.A and I.B of the permit.) <br />a) Purpose of WET Testing: The Water Quality Control Division has established the <br />use of WET testing as a method for identifying and controlling toxic discharges <br />from wastewater treatment facilities. WET testing is being utilized as a means <br />to ensure that there are no discharges of pollutants "in amounts, concentrations <br />or combinations which are harmful to the beneficial uses or toxic to humans, <br />animals, plants, or aquatic life" as required by Section 3.1.11 (1) of the Basic <br />Standards and MethodoloPies for Surface Waters. <br />b) Instream Waste Concentration (IWC): Where monitoring or limitations for WET are <br />deemed appropriate by the Division, chronic instream dilution as represented by <br />the chronic IWC is critical in determining whether acute or chronic conditions <br />shall apply. For those discharges where the chronic IWC is greater than (>) <br />9.1X, chronic conditions apply, where the IWC is less than or equal to (<) 9.1 <br />acute conditions apply. The chronic IWC is determined using the following <br />equation: <br />IWC - [Facility Flow (FF)/(Stream Chronic Low Flow (annual) + FF)[ X LOOX <br />The flows and corresponding IWC for the appropriate discharge point are: <br />Dischar a Po <br />g.:' rots.:.::.:Chronic.:Lov:.Flov.;.:::.::;:Fa.cility Design.;;.''.<Ii~IC,::(X):.:::::... <br />,,,,.. . <br />,.... <br />' :.::::....:: .::..::::.::::: ':::' :.30E3;:...(cfs).::.:: ^..;.''.!:.'Floe,.:;..;,..:.:::.;:.-.:::.:.:.::::::.:;:..:.;.: <br />,. . <br />001, 005, 007 0 1.5 100 <br />The IWC for outfalls 001, 005 and 007 is 100X, which represents a wastewater <br />concentration of 100X effluent to OX receiving stream. <br />c) Chronic WET Limitations: Because this facility discharges toxic metals in <br />potentially toxic amounts, the Division believes there is reasonable potential <br />for the discharge to interfere with attainment of applicable water quality <br />classifications or standards. On this basis, the chronic limit has been <br />incorporated into the permiC and becomes effective April 1, 1996. Prior to the <br />chronic limit becoming effective, the permittee is required to conduct routine <br />monitoring. The results of the testing are to be reported on Division approved <br />forms. The permittee will be required to conduct two types of statistical <br />derivations on the data, one looking for any statistically significant difference <br />in toxicity between the control and the effluent concentrations and the second <br />identifying the IC25, should one exist. Both sets of calculations will look at <br />the full range of toxicity (lethality, growth and reproduction). If a level of <br />chronic toxicity occurs, such that there is a statistically significant <br />difference in the lethality (at the 95X confidence level) between the control and <br />any effluent concentration less than or equal to the Instream Waste Concentration <br />(IWC) and if the lethality ICZS < the IWC, the permittee will be required to <br />follow the automatic compliance schedule identified in Part I.B of the permit, <br />if the observed toxicity is due to organism lethality. <br />