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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale - Page 18. Permit No. CO-0027154 VI. TERMS AND CONDITIONS OF PERMIT A. Effluent Limitations (continued) Instream Waste Concentration (IWC): As a condition of the permit, the permittee will be required to conduct routine monitoring for acute toxicity. Acute toxicity occurs when there is a statistically significant difference in the mortality observed, for Ceriodanhnia 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) oz should a species mortality in any dilution of effluent (including 100X effluent) exceed 50X, the permittee is required to conduct a series of timely tests to identify and ultimately eliminate or treat the toxicant. The IWC is determined using the following equation. IWC - Facility Flow (FF) X 100 Stream Low Flow (annual) + FF The flows and corresponding IWC for the appropriate discharge point are: Discharge Point Stream Low Flow (lE3) Facility Flow (dry weather) IWC 001 0 cfs 005 0 cfs 007 0 cfs 0.6 MGD 100X 0.2 MGD 100X 0.46 HCD 100X The IWC for outfalls 001, 005 and 007 is 100X, which represents a wastewater concentration of 100X effluent to OX receiving stream. Acute Biomonitorine: At this time, the Division does not find a reasonable potential for the discharges to interfere with attainment of applicable water quality classifications or standards. The basis for this belief is the lack of known toxic pollutants in toxic amounts in these discharges. The only known metals in these effluents are Sron and manganese. At this time, there are no data available that show iron or manganese to be toxic at the levels authorized in this permit. If, however, an unacceptable level of acute toxicity is identified through WET testing, the permittee is required to conduct a toxicity incident response and identify a control program to eliminate that toxicity, ae identified in Part I.B. of the permit. Additionally, the permit may be modified to incorporate an acute toxicity limit, as provided in regulation. Chronic Biomonitoring: The determination as to whether or not an individual facility must conduct chronic WET testing is dependent upon two factors: the ratio of the chronic stream low flow (30E3) to the design flow of the wastewater treatment plant and the aquatic life classification of the receiving water. If the receiving water is classified aquatic life c ass 1, chronic WET testing is always required regardless of the dilution ratio. If the receiving water is classified aquatic life c ass with all appropriate aquatic life-based numeric standards and if the chronic stream flow-to-design flow ratio is less than 19:1, chronic WET testing is required. If the receiving water has no aquatic life classification, chronic monitoring is not required.