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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division <br />Rationale - Page 10. Permit No. CO-0000132. <br />VI. TERMS AND CONDITIONS OF PERMIT <br />b) Instream Waste Concentration (IWC): As a condition of the <br />permit, the permittee will be required to conduct routine <br />monitoring for acute toxicity. Acute toxicity occurs when there <br />is a statistically significant difference in the mortality <br />observed, for Ceriodaphnia sp. (water Elea) and fathead minnows, <br />between the control and any effluent concentration. Should <br />acute tonicity be detected in any effluent concentration less <br />than or equal to the Instream Waste Concentration (IWC) or <br />should a species mortality in any dilution of effluent <br />(including 100X effluent) exceed 50X, the permittee is required <br />to conduct a series of timely tests to identify and ultimately <br />eliminate or treat the toxicant. The IWC is determined using <br />the folloxing equation: <br />IWC = [Facility Flow (FF)/CStream Low Flow (annual) + FF)] X 100X <br />Since the acute stream low flow (lE3) for Sanborn Creek is 0 <br />MGD, the low flow of the first downstream segment classified for <br />as aquatic life (the North Fork of the Gunnison) shall be used <br />for the upstream low flow. Using the facility design flow of <br />0.216 MGD for 011 and the acute stream low flow for the North <br />Fork of 26.5 HGD,the IWC for the permittee is therefore 1X, <br />which represents a wastewater concentration of 1X effluent to <br />99X dilution water. <br />c) Chronic WET Testing: The determination as to whether or not an <br />individual facility must conduct chronic WET testing is <br />dependent upon the ratio of the chronic stream low flow (30E3) <br />to the design flow of the wastewater treatment plant, as well as <br />the receiving waters classifications. The immediate receiving <br />water, Sanborn Creek, is not classified as Aquatic Life Class 1, <br />or Class 2 with inorganic and metal numeric standards. However, <br />since the creek discharges almost immediately into the North <br />Fork of the Gunnison River, which is classified as Aquatic Life <br />Class 1, the impacts on the North Fork must also be examined. <br />In calculating the ratio of the chronic stream low flow (30E3) <br />to the design flow of the effluent,if the ratio is less than <br />19:1, chronic WET testing is included as a permit requirement. <br />In this case the ratio was determined to be 84:1. Therefore, <br />chronic WET testing is not required at this time. <br />d) Waiver of Requirements: Industrial facilities having simple <br />effluents of conaietent quality, which can be totally <br />characterized chemically, may request a full or partial waiver <br />of WET testing for the remainder of the life of the permit, <br />subject to Division approval, if the first four quarters of <br />testing show no pattern of toxicity. <br />