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COLORADO DEPARTMENT OF PURL/C HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale-Page 8, Permit No. CO-0032638 recently there has not been mt effective methodfar nronitotinglow-level total tnercury concentrations in either the receiving stream or the facility effluent. To ensure thad adequate data are gathered to determine reasonable potential, semi-annual monitoring a[ the MON outfalls using the low-level detection method is required. There were no data for [he remaining water quality standard-based metals (TR arsenic, D cadmium, D trivalent, D copper, D lead D nickel, D selenium, D silver, and D zinc); thus, a quantitative analysis based on the Division's procedures could not be performed. Because effluent data are not available to conduct a quantitative analysis of reasonable potential in accordance with Division procedures, monitoring will be prescribed, at Outfall MON/ and MON2, to determination of the need for effluent limits. For standards based upon the total and total recoverable methods afanalysis, the limitations are based upon the same method as the standard, except for arsenic. For this metal, the total recoverable analyses must be performed using n graphite furnace. This method may produce erroneous results and may not be available to the permtttee. Therefore, the total method of analysis will be specified instead of the total recoverable method. For metals with aquatic life-based dissolved standards, effluent limits and monitoring requirements are typically based upon the potentially dissolved method of analysis, as required under Regulation 3I, Basic Standards and Methodoloeies for Sur ace Water. Thus, effluent limits and/ar monitoring requirernenls for dissolved cadmium, dissolved copper, dissolved trivalent chromium, dissolved lead, dissolved manganese, dissolved selenium, dissolved silver and dissolved zinc, will be prescribed as the 'potentially dissolved"form. e. Antideg_radatton: Since the receiving water is Undesignatetl, an antidegradation review is required pursuant to Section 31.8 of The Basic Standards and Methodoloeies for Surface Water. As set forth in Section Y of the water quality assessment contained in Appendix A, an antidegradation review was conducted for pollutants when water quality impacts occurred and when the impacts were stgnfficara. Based on the antidegradation review requirements, antidegradation-based average concentrations (ADBACs) were potentially applicable as limits for dissolved manganese, total recoverable iron, and total mercury at Outfnlls 004A and 007A.. According to WQCD procedures, the facility has three options related to antidegradation-based effluent limits: (1) the facility may accept ADBACs as permit limits (see Section V of Appendix A); (2) the facility may select permit limits based on their non-impact limit (NIL), which would result in thefacility not being subject to an antidegradation review and thus the antidegradation-based average concentrations would not apply (the NILs are also contained in Section Y ofAppendix A); or (3) the facility may complete an alternatives analysis as set forth in Section 31.8(3)(d) of the regulations which would result in alternative antidegradation-based effluent limitations. The effluent must not cause or contribute to an exceedance of a water quality standard and therefore [he WQBEL must be selected if it is lower than the NIL. Where the WQBEL is not the most restrictive, the discharger may choose between the NIL or the ADBAC: the NIL results in no increased water quality impact,' the ADBAC results in an `insignificant" increase in water quality impact. For total recoverable iron at Outfall 004A, the WQBEL was the most stringent [imitation and was therefore applied. For dissolved manganese, the non-impact limit was the likely choice and was applied. For both total recoverable iron and dissolved manganese at Outfa11007A, the NTLs would be the most likely selection and was therefore applied. f. Colorado Mixing Zone Regulations: Pursuant to section 31.10 of The Basic Standards and Methodoloeies for Surface Water, a mixing zone determination is required for this permitting action. The Colorado Mixinv Zone Implementation Guidance dated Apri12002, ident~es the process for determining the meaningful limit on the area impacted by a discharge [o surface water where standards may be exceeded (i. e., regulatory mixing zone). This guidance document provides for certain exclusions from further analysis under the regulation, based on sire-specifc 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 if the ratio of the design flow to the chronic low flow (30E3) is greater than 2:1 or if the ratio of the chronic lowflow to the design flow is greater than 20:1.