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' COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale - Page 15. Permit No. CO-0027146 VI. TERMS AND CONDITIONS OF PERMIT (continued) A. Effluent Limitations (continued) 5. Whole Effluent Toxicicv ('dET) Testing (continued) Inscream Waste Concentration (IWCI: (continued) The flows and corresponding IWC for the appropriate discharge point are: Discharge Point Stream Low Flow (lE3) Facility Flow IWC 001 1,086 cfs 0.360 MGD (0. 557 cfs) 0.1*X 002 .r 1,086 cfs 0.360 MGD (0 .557 cfs) 0.1*%. .093'-DG"'~ 0 cfs 0.432 MGD (0. 669 cfs) 100X The IWC for oucfalls 001 and 002 is 0.17., which represents a wastewater concentration of 0.1X effluent co 99.9X receiving stream. The IWC for oucfall 004 is 100X, which represer.cs a wastewater concencrac'_or. of 100X effluent co OX receiving stream. Acute Biamonitorine: At this time, the Division believes there is no reasonable potential for the discharges co interfere with attainment of applicable water quality class ificacions or standards. The basis for this belief is the lack of known toxic ~pollucancs in toxic amounts in these discharges. The only known metals in these effluents are iron anz.manganese. Ac this time, there are no data available chat show iron or manganese co be toxic ac the levels authorized in this permit. If, however, an unacceptable level of acute toxicity is identified through 'd ET testing, the permictee is required co conduct a toxicity ~~incidenc response and identify a control program to eliminate that toxicity, as identified in Part I.B. of the permit. Additionally, the permit may be modified co incorporate an acute toxicity limit, as provided in regulation. Chronic Biomonicorine: The determination as to whether or not an individual facility must conduct chronic WET testing is dependent upon two factors: the racio_of the chronic stream low flow_ (30E3) co the design flow of the •.~ascewacer treatment plant and the aquatic life classification of the receiving •aacer. If the receiving water is classified aquatic life class L, chronic WET tasting is always required regardless of che._dilucion ratio. If`~he receiving •aacer is classified aquatic life c'_ass 2 with all appropriate aauacic 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.