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
|