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COLORADO DEPARTMENT OF PUBLICHEALTHAND ENVIRONMENT, Water Quality Control Division Rationale-Page S, Permit No. CO-0000132 from a specific event and occurred through automan'c dewatering devices within 48 hours after measurable precipitation has stopped. In addition, to waive settleable solids limitations, it is necessary to prove that an evenrgreater than the magnitude of the 10 year, 14-hour event occurred. For snowmelt to waive TSS and total iron limitations, it is necessary to prove that discharge directly resulted from snowmelt and occurred through automan'c dewatering devices within 48 hours after pond inflow has stopped. In additton, to waive settleable solids limitations, it is necessary to prove that an event greater than the magnitude of the applicable 10 year, 14-hour event occurred. Documentation that the treatment facilities were properly operated and maintained psior to and during the storm event must be submitted with any request for relief. The Division shall determine the adequacy of proof. As part of this determination, the Division shall evaluate whether the permiuee could have controlled the discharge in such a manner thatprimary limitations could have been met, whether proper sediment storage levels were maintained and the ponds had sufficient water and sediment capacity for the storm event plus other relevant factors. Al! manual pond dewatering must meet TSS and total iron limitations unless previous approval has been granted for ponds that have no other method of dewatering. Post-Mining Areas: In conformance with 40 CFR 434.50, commencing at the time active mining has ceased and all surface areas served by a sedimentation pond have been returned to the required contour and revegetation has commenced, applicable discharges maybe eligible for limitations other than those specified in Part I.A.1: In most cases, these post-mining limitations shall remain in effect until bond release, The permittee shall notify the Division at the appropriate time so that consideraa'on ofpermit modifications can be made. Prior to notification and subsequent permit modification, active mining limitations will apply regardless ofactua! mine status. d. Pollutants Limited by Water Quality Standards /Outfa11015 Ong: The water quality assessment in Appendix A contains the evaluation ofpollutants limited by water qualirystnndards applicable to outfal1015. The mass balance equation shown in Section ryofAppendix A was used for metals to calculate the maximum allowable effluent concentration, M„ that could be discharged without causing the water quality standard to be violated. A detailed discussion of the calculations for the maximum allowable effluent pollutant concentraa'ons for metals is provided in Section IV of the water quality assessment contained in Appendix A. The maximum allowable effluent pollutant concentrations determined as pan ofthese calculations represent the calculated effluent limiu that would be protective of water quality. These are also known as the water quality-based effluent limits (WQBEIs). The Permiu Unit evaluated the calculated WQBELs and has made a determination as to whether there rs a reasonable potena'al for the facility discharge to cause or contribute to an exceedance ofa stream standard If there is a reasonable potential for the discharge to contribute to an exceedance, effluent limiu are included in thepermit. For metals, a quantitative approach to reasonable potential evaluations is utilized as discussed in following paragraphs. i. Metals: Un'lizing the assimilative capacities contained in Appendix A, an analysis must be performed to determine whether to include the calculated WQBELs in the permit The guidelines for performing a reasonable potential analysis are outlined in the Division's document, Determination ofthe Renuirement to /nclude Wafer puality Standards-Based Limits in CDPS Permits Based on Reasonable Potential Procedural Guidance dated December 2002. This guidance document utilizes both quantitative and qualitative approaches to establishing reasonable potential depending on the amount of available data. Per the Procedural Guidance, a quantitative determination of reasonablepotential requires a minimum of ten data points. There was not sufficient effluent data to conduct a quantitative analysis for any metal since outfall 015 has not yet discharged. There were no effluent data for total recoverable (TR) arsenic, potentially dissolved (PD) cadmium, TR trivalent chromium, PD copper, dissolved and TR iron, PD manganese, total mercury, PD nickel, PD selenium, PD silver, and PD zinc, and therefore monitoring will be prescribed, as discussed subsequently, prior to the determination of the need for effluent limits. tns(Revised: 4/28/2004