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COLORADO DEPAR77NEM OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Comro! Division Rationale -Page 7, Permit No. CO-O~P44776 O 2) Lnstream Waste Concentration OWCI: Where monitoring or limitations for WET' are deemed appropriate by the Division, chronic insrream dilution as represented by the chronic IWC is critical in determining whether acute or chronic conditions shall apply. For those discharges where the chronic IA'C is greater than (>) 9.13'0, chronic conditions apply, where the IWC is less than or equal to (s) 9.1 acute conditions apply. The chronic LWC is determined using the following equation: IWC =[Facility Flaw (FF)/(Stream Chronic Low Flow (annual) + FF)] X 10090 the flows and corresponding IWC for the appropriate discharge point are: Duchage Point Chronic Low Flaw, Facility Design Flow, IfVC, (36) 30Is'3, (cfs) '. (cfs)..:- ...: 002 40 0.22 0.55 The IWC for this permit is 0.553io, which represents a wastewater concentration of 0.55`% e,(Jluent to 99.45'90 receiving stream. Thus, acute WET testing is applicable for outfall 002 when mine drainage is discharged. 3) Acute WET Limitations: Because mine drainage from coal mines at similar facilities has shown aquaric toxicity, the Division believes there is reasonable potential for the discharge to interfere with attainment of applicable water quality classiftcarions or standards. Because of this condition, the acute tonicity limit has been incorporated into the permit and becomes effective immediately. The permittee is required to conduct routine monitoring, the results of which are to be reported as an LCD which is defined as the concentration that is lethal to 50% of the subject test organisms. If the LCS0 s 100`% effluent, the permittee is required to conduct the automatic compliance schedule as identifted in Pan LA.4.c. of the permit. 4) General Information: The permittee should read the WET testing sections under Part LA.4. of the permit carefully. The permit outlines the test requirements and the required follow-up actions the permittee must take to resolve a toxicity inridem. The permittee should read, along with the documents listed in Part I.A.4.a. of the permit, the Colorado Water Quality Control Division Biomonitorinr guidance Document. dated July 1, 1993. This document outlines the criteria used by the Division in such areas as granting relief from WET testing, modifying test methods and changing test species. The permittee should be aware that some of the conditions outlined above may be subject to change if the facility experiences a change in discharge, as outlined in Part lI.A.2. of the permit. Such changes shall be reported ro the Division immediately. S) Surface Runo Discharq_e~: For discharges of surface runoff alone, WET testing is not a requirement. However, the Division reserves the right to reopen the permit to include WET testing, should facility conditions change or if new information becomes available. This applies to all discharges from outfalls 001 and 003 authorized ar this time, and to outjall 002 only when the discharge does not have a mine drainage component. If at any time during a monitoring period, the discharge contains mine drainage - either alone or commingled with surface runoff, WET testing will be required. The WET testing exemption only applies ijmine drainage is not discharged jot the entire monitoring period. 3. Stormwater Evaluation: Stormtivater from active and inactive coal mining sites that has been contaminated by • contact with overburden, ratio material, intermediate products, byproducts, ftnished products or waste products located at the site is required to be covered by a Colorado Discharge Permit System (CDPS) permit in order to be discharged to waters ojthe State. This coverage may be obtained under either a CDPS Industrial Wastewater Discharge Permit or a CDPS Stormwater Discharge Permit,