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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division <br />Rationale - Page 6. Permit No. CO-0042161 <br />DISCUSSION: (Cont.) <br />H. Whole Effluent Toxicity (WET) Testing <br />For this facility, acute and chronic WET testing are required. (See Parts <br />I.A and I.B of the permit.)' Monitoring shall be performed commencing with <br />the first full calendar quarter following the permit effective date. <br />Section 6.9.7 of the permit regulation, passed by the Water Quality <br />Control Commission (WQCC), has established the use of WET testing as a <br />method for identifying and controlling toxic discharges from wastewater <br />treatment facilities. WET testing is being utilized as a means to ensure <br />that there are no discharges "in amounts, concentrations or combinations <br />which are harmful to the beneficial uses or toxic to humans, animals, <br />plants, or aquatic life" as required by Section 3.1.11 (1)(d) of the Basic <br />Standards and Methodologies. <br />Chemical analysis of effluent has provided only a partial evaluation of <br />the potential impact a discharge could have on the receiving stream. <br />Also, chemical analysis cannot evaluate the synergistic or antagonistic <br />effect of compounds. There are also compounds for which an accurate or <br />reproducable method of chemical analysis has not yet been developed, as <br />well as compounds which are just beginning to be evaluated for toxic <br />effects. WET testing will provide a more comprehensive means of <br />evaluating the toxicity of a discharge than could othercrise currently be <br />accomplished. <br />As a condition of the permit, the permittee will be required to conduct <br />routine monitoring for acute toxicity. Acute toxicity occurs when there <br />is a statistically significant difference in the mortality observed, for <br />Ceriodaphnia sp. (water flea) and fathead minnows, between the control and <br />any effluent concentration. Should acute toxicity be detected in any <br />effluent concentration less than or equal to the Instream Waste <br />Concentration (IWC) or should a species mortality in any dilution of <br />effluent (including 100% effluent) exceed 50X, the permittee is required <br />to conduct a series of timely tests to identify and ultimately eliminate <br />or treat the toxicant. The IWC is determined using the following equation: <br />IWC = (Facility Flow (FF)/(Stream Low Flow (annual) + FF)] X 100% <br />Using the facility design flow of 1.44 MGD and the acute stream low flow <br />(1E3) of 0.9 MGD, the IWC for the permittee is therefore 62%, which <br />represents a wastewater concentration of 62% effluent to 38% dilution <br />water. <br />The Colorado Permit Regulations on biomonitoring call for several <br />approaches on limits which are objectionable to EPA. In an effort to <br />. maintain continuity of the permit issuance process, EPA and Colorado have <br />agreed to a temporary accommodation which calls for the imposition of <br />monitoring and a series of triggered responses for control of WET if <br />detected rather than immediate WET limits for selected Colorado permits. <br />