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COLORADO DEPARTMENT OFPUBLIC HEALTHAND ENVIRONMENT, Water Quality Control Division Rationale -Page 7, Permit No. CO-0031115 calculated effluent limiu that would be protective ojwater quality. These are also known as the water quality-based effluent limits (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 steam standard. If there is a reasonable potential for the discharge to contribute to an exceedance, effluent limits are included in the permit. For metals, a quantitative approach to reasonable potential evaluations is utilized as discussed in following paragraphs. Metals: Utilizing 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 oftee Reouirement to Include Water OualiN Standards-Based Limits in CDPS Permits Based on Reasonable Potential Procedural Guidance dated December 1001. This guidance document utilizes both quantitative and qualitative approaches ro establishing reasonable potential depending on the amount of available data. Per the Procedura/ Guidance, a quantitative determination of reasonable potential requires a minimum often data points. There was not sufficient effluent data to conduct a quantitative analysis for any metal at any outfall. For this reason, monthly effluent monitoring regutremenu for 11 months at outfalls OII, 017, 019, 010, 011, 013 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. Antidegradah'on: Since the receiving waters are Use Protected, an antidegradation review is not required pursuant ro Secdon 31.8(1)(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 Apri11002, identifies theprocess 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 documeniprovides for certain exclusions from further analysis under She regulation, based on site-specific condifions. The guidance document provides a mandatory, stepwise decision-mala~ng process for determining if the permit limits will not be affected by this regulation. Exclusion, based on Extreme Mixing Ratios, may be granted if the rato 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 10:1. Since the ratio of the design flow to the low flow is greater than 2:1, the permtttee 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 Dsscharge Permit System Regulations. the perminee shall monitor for total dissolved solids on a quarterly basis. Samples shall be taken at all authorized effluent discharge points. h. Whole Effluent Toxicity /WET) Testing.• For, 011, 017, 019, 020, 011, and 013, acute WET testing is required. (See Part I.A of the permit.) WET Testing is required only when mine water rs being dischar 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 ofpollutants "in amounts, concentrations or combinations which are harmful to the benefcial uses or toxic to humans, animals, plane, or aquatic life" as required by Section 31.11 (1) of the Basic Standards and Methodologies for Surface Waters. ii. In-Stream Waste Concentration /IWC): Where monitoring or limitations for WET are deemed appropriate by the Division, chronic in-stream dilution as represented by the chronic IWC is critical in determining whether acute or chronic conditions shall apply. According to the Colorado Water OualiN Control Division Biomonitoring Guidance Document. dated July 1, 1993, for those discharges where the chronic IWC is greater than 9.1 % and the receiving stream has a Class 1 Aquatic Lije use or Class 2 Aquatic Life use with all of the appropriate aquatic life numeric standards, chronic conditions apply. Where the chronic IWC is less than or Last Revised: 4/292004