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COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale -Page 8, Permit No. CO-0042161 • 3) Chronic WET Limitations: This outfall may discharge toxic metals in potentially toxic amounts. On this basis, the Division believes there is reasonable potential for the discharge to interfere with anainment of applicable water quality classifications or standards. Because of this condition, the chronic limit has been incorporated into the permit and becomes effective immediately. The permittee is required ro conduct routine monitoring. The results of the resting are ro be reported on Division approved forms. The permirree will be required to conduct a statistical derivation ors the data, looking for any statistically signifcam difference in toxicity between the control and the effluent concentrations. This set of calculations will look at the full range of toxicity (lethality, growth and reproduction). /f a level of chronic toxicity occurs, such that there is a statistically significant difference in [he lethality (at the 95'X, confidence level) between the control and any e.Qluent concentration less Than or equal to the Insrream Waste Concentration (/WC), the permirree will be required ra follow the automatic compliance schedule identified in Part /.B of the permit, if the observed toziciry is due to organism lethality. Only exceedance of the limitation specified in Part /.A.2. will trigger the requirement for conducting the automatic compliance schedule idemified in Part I.B. of the permit. If the toziciry 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 permirree will be required. However, this incidem, along with other WET data, will be evaluated by the Division and may form the basis for reopening the permit and including additional WET limits or other requirements. 4) General Information.• The permirree should read the WET testing sections under Part I.A.??1 of the permit carefully. The permit outlines the test requirements and the required follow-up actions the permittee must take to resolve a toziciry incident. The permirree should read, along with the documents listed in Part /.A.4.a. of the permit, the Colorado Water Qua[iry Controf 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 resting, modifying test methods and changing test species. The permirree 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 II.A.2. of the permit. Such changes shnll be reported to the Division immediately. 5. Whole E1lluenr Tozicirv (WET) Testing: Because ourfa[[ 002 does nor directly discharge to surface waters, WET testing is not a requirement for this outfall. However, the Division reserves the right to reopen the permit ro include WET testing for this ourfalf, should facility conditions change or if new information becomes availabfe that indicates the presence or potential for e,Qluem toziciry. 4. Stormwater Evaluation: Stormwater discharge permits are required for active and inactive coal miningjacilities as covered under Standard Industrial Classification (SIC) Code /1. Division records indicate that the Twentymile Coal Company has coverage for stormwater discharges for the Foidel Creek mine under a General S~ormwater Discharge Permit for stormwater discharges associated with Light /ndusrrial activities, number COR-040116. Stormwater permitting issues for these mines will 6e 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 Quality Comrol Act required the Division to "determine whether or not any or all of the water quality standard based eJJluent limitations are reasonably related to the economic, environmental, public health and energy impacts to the public and affec[ed persons, and are in furtherance of the policies set forth in sections 25-8-I92 and 25-8- 104. " • The Colorado Discharge Permit System Regulations, Regulation 61.0, funkier define this requirement under section 6/. /1 and stare: "Where economic, environrnemal, public heahh and energy impacts to the public and affected persons have been considered in the classifications and standards setting process, permits wriaert to meet the standards may be presumed to have taken into consideration economic factors unless: