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COLORADO DEPARTMENT OF PUBc..,. HEALTH AND ENVIRONMENT, Water Qu4..q~ Control Divisiai Rationale -Page 7, Permit No. CO-0036684 . Discharge Point Chroriic Law Flow, Facility Design Flow, IWC, (%) 30E3, (cfs) (cfs) 001 0.25 0.7 74% The /WC jot this permit is 74%, which represents a wastewater concentration of 74% effluent to 16% receiving stream. 3) Chronic WET Limitations: The facility has had several WET failures -indications of toxicity in the discharge, at least on an intermittent basis. Thus, the Division finds a reasonable potential jot the discharge to interfere with attainment of applicable water quality classifications or standards. On this basis, the chronic limit has been incorporated into the permit. Because at least three years of manitoritig have occurred ro date, the [imitation becomes effective immediately. The permittee is required to conduct routine monitoring. The results of the testing are to be reported on Division approved forms. The permittee will 6e required to conduct a statistical derivation on the data, looking for any sratisrica[!v significant difference in toxiciry between the control and the effluent concentrations. This set of calculations will look at the full range of toxicity (lethality, growth and reproductionJ. IJ a level of chronic toxiciry occurs, such that there is a statistically significant difference in the lethality (at the 95% confidence leti~e!) benveen the control and any effluent concentration less than or equal to the /nsrreani Waste Concentration (IWC), the permittee will be required ro follow the automatic compliance schedule identified in Pan I.A.2.c. of the permit, if the observed toxicity is due to organism lethality. Only exceedance of the limitation specified in Part /.A.l. will trigger the requirement for conducting the automatic compliance schedule identified in Part I.A.S.c. of the permit. If the toxiciry is due ro differences in the growth of the fathead minnows or the reproduction of the Ceriodaphnia, no immediate action on the part of the pennirree will be required. However, this incident, along with other WET data, will 6e evaluated 6y the Division and niav • jorrri the basis for reopening the permit and including additional WET limits or other requirements. 4) General /n(ormarion: The permittee should read the WET testing sections under Part /.A. of the permit carefully. The permit outlines the rest requirements and the required follow-up actions the permittee must rake to resolve a toxiciry incident. The permittee should read, along with the documents listed in Parr /.A.2. ojthe permit, the Colorado Water Oualirv Control Division Bionionitorine Guidance Document. dared July !, 1993. This document outlines the criteria used by the Division in such areas as granting relieJJrom WET resting, modifying test methods and changing test species. The permittee should be ax~are Char some of rile conditions outlined above may 6e subject to change ijrhe facility experiences a change in discharge, as outlined in Part II.A.2. of the permit. Such changes shall be reported to the Division immediately. 4. Stormwater Evaluation: Stormwater from active and inactive coal mining sites that has been conrmninated by contact with overburden, raw material, intermediate products, byproducts, finished products or waste products located at the site is required to 6e covered by a Colorado Discharge Permit System (CDPS)..permit in order ro 6e discharged to Waters of the State. This coverage may be obtained under either a CDPS Industrial Wastewater Discharge Permit or a CDPS Stormwater Discharge Permit. Division records indicate that Twenrymile Caa! has coverage for srormwarer discharges from the Fish Creek Tipple under a General Stormwater Discharge Permit for Stormwater discharges associated with meta! mining activities, number COR-040126. The permittee has requested that the general permit coverage for Stormwater discharges continue. Stormwater permitting issues for this Jadliry will be handled separately by the Division's Stormwater Unit, although this permit may be reopened at a later date to incorporate Stormwater provisions, iJdeemed appropriate. • 5. Economic Reasonableness Evaluation: Section 25-8-503(8) of the revised (June 1985) Colorado Water Orraliry Control Ac! required the Division to "determine whether or not any or all of the water quality standard based effluent limitations are reasonably related to the economic, environmental, public health and energy impacts to the public and affected persons, and are in furtherance of the policies set forth in sections 25-8-192 and 25-8-l04. "