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uORP.DO DEPARTMENT OF MEAL, Water Quality Control Divisio• tionale - Page 10 Permit No. CO-0043168 Instream Waste Concentration (IWC); As a condition of the permit, the permittee will be required to conduct routine monitoring for acute toxicity. An unacceptable level of acute toxicity occurs when 1) there is a statistically significant difference in the mortality (at the 95X confidence level) observed, fot Ceriodaohnia sp. (water flea) and fathead minnows, between the control and any effluent concentration. Should acute toxicity be detected in any effluent concentration less than or equal to the Instream Waste Concentration (IWC) or 2) a species mortality in any dilution of effluent (including 100X effluent) exceed 50X. The IWC is determined using the following equation: IWG - [Facility Flow (FF)/(Stream Low Flow (annual) + FF)~ X 100X The flow and corresponding IWC for discharge point 001 is; Discharge Point Stream Low Flow (lE3) Facility Flow IWC 001 0.013 MGD 0.216 MGD 94,3 X The IWC for this permit is 94.3 X, which represents a wastewater concentration of 94.3X effluent to 5.7X dilutior. eater or receiving stream. P.cute Biomonitorine: .Section 6.9.7.(4)(a)(i) of the "Regulations for the State Discharge Permit System", specifies that an acute toxicity limit and requirements for testing effluents for WET testing shall be incorporated into all new industrial discharge permits. Thus, because of these requirements, the acute toxicity limit has been incorporated into the permit and becomes effective immediately. If an unacceptable level of acute toxicity is identified, the permittee is required to conduct a toxicity incident response and identify a control program to eliminate that toxicity, as identified in Part I.B, of the permit. Once the acute toxicity limitation is effective, the permittee is required to proceed with the automatic enforcement compliance schedule, as identified in Part I.B. of the permit, should a violation occur. Chronic Biomonitorine: The determination as to whether or not an individual facility must conduct chronic WET testing is in part dependent upon the type of discharge and the receiving water classifications. In accordance with Section:6.9.7(5) of the "Regulations for the State Discharge Permit System", chronic WET testing is required for discharges to receiving waters that are classified for Class 2 Aquatic Life with numeric standards where the ratio of the chronic stream low flow (30E3} to the maximum flow from the facility is less than 19:1. In this case, since the ratio (0.013:0.216 - 0.06:1) is less than 19;1, chronic WET testing is required for this facility.