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COLORADO DEPARTMENT OF PUBL/CHEALTHAND ENVIRONMENT, Water Quality Control Division Rationale-Page 7, Permit No. CO-0031115 The maximum allowable effluent pollutant concentrations determined as part of these calculations represent the calculated effluent limits that would be protective ofwater quality. These are also known as the water quality-based effluent limiu (WQBELs). The Permits Unit evaluated the calculated WQBELs and has made a determination as to whether there is a reasonable potential for the facility discharge to cause or contribute to an exceedance ofa stream standard. Ifthere is a reasonable potential for the discharge to contribute to an exceedance, effluent limits are inc]uded in the permit. For metals, a quantitative approach to reasonable potential evaluations is utilized as discussed in following paragraphs. Metals: Utilising 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 Reouirement to Include Water Ouality Standards-Based Limits in CDPS Permits Based on Reasonable Potential Procedural Guidance dated December 2002. This guidance document utilizes both quanritative and qualitative approaches to establishing reasonable potential depending on the amount ofavailable data. Per the Procedural Guidance, a quantitative determination of reasonable potential requires a minimum of ten data points. There was not su, jhcient effluent data to conduct a quantitative analysis for any metal at any outfalJ. For this reason, monthly effluent monitoring requiremenu jot 12 months at outfalls Oll, O17, 019, 020, 021, 023 for total recoverable (TR) arsenic, TR cadmium, total trivalent chromium, TR copper, TR lead, TR selenium, and TR zinc are included in the permit. e. Antideeradation: Since the receiving waters are Use Protected, an antidegradation review is not required pursuant 3o Sech'on 31.8(2)(6) of The Basic Standards and Methodologies for Surface Water. f Colorado Mixing Zone Regulations • Pursuant to section 31.10 of The Basic Standards and Methodologies for Surface Water, a mixing zone determination is required for this permitting action. The Colorado Mixing Zone Implementation Guidance, dated April 2002, identifies the process for determining the meaningful limit on the area impacted by a discharge to surface water where standards may be exceeded (i. e., regulatory mixing zone). This guidance document provides for certain exclusions from funkier analysts under the regulation, based on site-specific conditions. The guidance document provides a mandatory, stepwise decision-making process for determining if the permit limits will not be affected by this regulation. Exclusion, based on Extreme Mixing Ratios, may be granted ijthe ratio of the design flow to the chronic low flow (30E3) is greater than 2:1 or if the ratio of the chronic low flow to the design flow is greater than 20:1. Since the ratio of the design flow to the low flow is greater than 2:1, the permtnee is eligible for an exclusion from further analysis under the regulation. g. Salinity Regulations: In compliance with the Colorado River Salinity Standards and the Colorado Dtscharne Permit Svstem Reeulations. the permtttee shall monitor for total dissolved solids on a uarter basis. Samples shall be taken at all authorized effluent discharge points. h. Whole Effluent Toxicity fWETI Testing.• For, 011, 017, 019, 020, 021, and 023, acute WET testing is required. (See Part LA of the permit.) WET Testing is required only when mine water is being discharged. i. Purpose of WET Testing: The Water Quality Control Division has established the use of WET testing as a method for identifying and controlling toxic discharges from wastewater treatment facilities. WET testing is being utilized as a means to ensure that there are no discharges of pollutants "in amounts, concentrations or combinations which are harmful to the beneficia/uses or toxic to humans, animals, plants, or aquatic life" as required by Section 31.1 / (/) of the Basic Standards and Methodologies for Surface Waters. ii. In-Stream Waste Concentration IIWCI: Where monitoring or limitations for WET are deemed appropriate by the Division, chronic in-stream dilution as represented by the chronic /WC is critical in determining whether acute or chronic conditions shall apply. According to the Colorado Water pualiN Control Division Biomonitoring Guidance Document, dated July 1, /993, for those discharges where the chronic IWC is greater than 9.1 % and the receiving stream has a Class I Aquatic Life use or Class 2 Aquatic Life use with all of the last Revisett: 3/21/2004