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COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division <br />Rationale - Page 15, Permit No. CO-0000906 <br />For standards based upon the total and total recoverable methods of analysis, the limitations are based upon <br />• the same method as the standard, except for total recoverable arsenic and total recoverable trivalent chromium. <br />For arsenic, the total recoverable analyses may be performed using a graphite furnace. This method may <br />produce erroneous results and may not be available to the permittee. Therefore, the total method of analysis <br />will be specified at Outfalls MONI and MON9 instead of the total recoverable method. For total recoverable <br />trivalent chromium at Outfall MONI, the regulations indicate that the standard applies to the total of both the <br />trivalent and hexavalent forms. Therefore, monitoring for total chromium, which is the equivalent to total <br />recoverable chromium (representing both valences), will be required. Similarly, at Outfall MON9, monitoring <br />for total chromium is specified for consistency and for the permittee's ease and because hexavalent chromium <br />concentrations are not expected to be present and therefore the total chromium concentrations will be <br />equivalent to the total trivalent chromium concentrations. Finally, to ensure that adequate data are gathered to <br />determine reasonable potential, effluent monitoring at Outfall MONI for total mercury at low-level detection <br />methods will be required by the permit. <br />For metals with aquatic life-based dissolved standards, specifically those listed under MONI, effluent limits <br />and monitoring requirements are typically based upon the potentially dissolved method of analysis, as required <br />under Regulation 31, Basic Standards and Methodologies for Surface Water. Thus, effluent limits and/or <br />monitoring requirements for dissolved cadmium, dissolved copper, dissolved lead, dissolved manganese, <br />dissolved nickel, dissolved selenium, dissolved silver and dissolved zinc, will be prescribed as the `potentially <br />dissolved "form. Note that because total arsenic will be monitored, the analytical results will be used for <br />dissolved arsenic as well in lieu of separate testing for dissolved arsenic. <br />e. Antidegradation - For discharges via Outfall 009, since the receiving water is Use Protected an antidegradation <br />review is not required pursuant to Section 31.8(2)(b) of The Basic Standards and MethodologiesLor Surface Water. <br />For discharges via Outfall 001, as set forth in Section V1 of the water quality assessment contained in Appendix A, <br />an antidegradation review was conducted for pollutants when water quality impacts occurred and when the impacts <br />• were significant. Although the antidegradation review requirements set forth in the water quality assessment <br />contained in Appendix A indicate that antidegradation-based average concentrations (ADBACs) werepotentially <br />applicable for all pollutants, the reasonable potential analysis discussed above indicates that ADBACs do not apply. <br />Thus, no permit limitations or conditions for antidegradation apply to discharges from Outfal1001A at this time. <br />f. Colorado Mixing Zone Regulations - Pursuant to section 31.10 of The Basic Standards and Methodologies for <br />Surface Water, a mixing zone determination is required for this permitting action. The Colorado Mixing Zone <br />implementation Guidance, dated April 2002, identifies the process for determining the meaningful limit on the area <br />impacted by a discharge to surface water where standards may be exceeded (i.e., regulatory mixing zone). This <br />guidance document provides for certain exclusions from further analysis under the regulation, based on site-specific <br />conditions. <br />The guidance document provides a mandatory, stepwise decision-making process for determining if the permit limits <br />will not be of%cted by this regulation. Exclusion, based on Extreme Mixing Ratios, may be granted if the ratio of <br />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 <br />flow is greater than 20:1. Since the ratio of the design flow to the chronic low flow is 3:1 for discharges from <br />Outfall 001 to the Middle Fork of the Purgatoire River and the ratio of the design flow to the chronic low flow is <br />greater than 2: I for discharges from Outfall 009 to the Left Fork of Apache Canyon Creek, the permittee is eligible <br />for an exclusion from further analysis under the regulation. <br />g. Whole Effluent Toxicity (WET) Testing <br />For this facility, both acute and chronic WET testing is required. (See Part I.A.4, and Part I.A.5. of the permit.) <br />i. Purpose of WET Testing -The Water Quality Control Division has established the use of WET testing as a <br />method for identifying and controlling toxic discharges from wastewater treatment facilities. WET testing is <br />being utilized as a means to ensure that there are no discharges of pollutants "in amounts, concentrations or <br />combinations which are harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life" as <br />required by Section 31.11 (1) of the Basic Standards and Methodologies for Surface Waters. <br />ii. In Stream Waste Concentration (IWC) - Where monitoring or limitations for WET are deemed appropriate by <br />the Division, chronic in-stream dilution as represented by the chronic IWC is critical in determining whether <br />acute or chronic conditions shall apply. According to the Colorado Water Ouality Control Division