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COLORADO DEPARTMENT OF PURL/C HEALTH AND ENVIRONMENT, Water Quality Control Division Ratiorm[e -Page 7, Permit No. CO-0044776 2) /nstrenm Waste Concentration (IWCI: Where ntonitonng or limitations jot WET are deemed appropriate by rite Division, chronic instreanr dilution as represented by the chronic IWC is critical in determining whe[her acute or chronic conditions shall apply. For those discharges where the chronic IWC is greater than (>) 9.1 %, c)tronic conditions apply, where the IWC is less than or equal to (s) 9.1 acute conditions apply. The chronic IWC is determined using the following equation: IWC =[Facility Flow (FF)/(Stream Chronic Low Flow (annual) + FF)J X 100% The flows and corresponding IYYC jot rite appropriate discharge point are: Discharge Point Chronic Low Flow, Facility Design Flow, JR'C, (%) 30E3, (cfs) (cfs) 002 40 0.22 0.55 The IWC for this permit is 0.55%, which represents a wastewater concentration of 0.55% effluent to 99.45% receiving stream. Thus, acute WET testing is applicable for outfal[ 002 ivl:ert mine drainage is discharged. 3) Acute WET Limitations: Because mine drainage from coal mines at similar facilities ltas shown aquatic toxicity, the Division believes there is reasonable potential for the discharge to interfere with attainment of applicable water quality classifications or standards. Because of This condition, the acute toxicity limit itas been incorporated into the permit and becomes effective immediately. The penniaee is required to conduct routine ntortitoring, the results ajwhich are to be reponed as an LCJa which is defined as the concentration that is lethal to 50% ojthe subject test organisms. If rite LCsv s 100% effluent, the penniaee is required to conduct the automatic compliance schedule as identified in Pan 1.A.4. c. of the perntit. 4) General Information: The permittee should read rite WET testing sections under Part I.A.4. of the permit carefully. The permit outlines the test requirements and the required follow-up actions tfte penniaee must take !o resolve a toxicity incident. The pennitee should read, along with the documents listed in Part /.A.4.a. ojthe permit, the Colorado Water Ounlity Control Division Bionronitoring Guidarsce Document. dared July 1, 1993. This document outlines the criteria used by the Division in such areas as granting relief from WET testing, modifying test met/rods and changing test species. The penniaee should be aware that some of r/te conditions outlined above may be subject to change if the facility experiences a change in discharge, as outlined in Pan /l.A.2. of the permit. Srech changes shall be repaned to the Division immediately. S) Surface Runoff Dsscharges: For discharges of surface runoff alone, WET testing is not a requirement. However, the Division reserves rite right to reopen the permit to include WET testing, should facility conditions change or if new information becomes available. This applies to a[I discharges from outfalls 001 and 003 autltoriZed at this lime, and to outfa[l 002 only when the discharge does not have a mine drainage component. /f at arty time during a mortitorirtg period, the discharge corttalru mine drainage - either alone or commingled with surface runoff; WET testing will be required. The WET testing exemption only applies if mine drainage is not discharged for the entire monitoring period. 3. Stormwater Evalr[ation: Stormwater from active and inactive coal mining sites that has been contaminated by contact with overbrtrden, raw material, intermediate products, byproducts, finished products or waste products located at rite site is required to be covered by a Colorado Discharge Permit System (CDPS) permit in order to be discharged to wa[ers of the State. This coverage nary be obtained under either a CDPS Industrial Wastewater Discharge Pernrit or a CDPS Stormwater Discharge Permit.