2018-04-23_REVISION - M2007031 (4)

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t COLORADO Department of Public Health b Environment Water Quality Control Division Fact Sheet,Permit No. C00048939 Stormwater discharges were not contemplated during development of the Protection of Irrigated Crops policy; therefore, EC and SAR monitoring is not applicable to stormwater-only discharges. b. Whole Effluent Toxicity- 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 beneficial uses or toxic to humans, animals, plants, or aquatic life" as required by Section 31.11 (1)of the Basic Standards and Methodologies for Surface Waters. The requirements for WET testing are being implemented in accordance with Division policy, implementation of the Narrative Standard for Toxicity in Discharge Permits Using Whole Effluent Toxicity (Sept 30, 2010). Note that this policy has recently been updated and the permittee should refer to this document for additional information regarding WET. 4. Water Quality Regulations, Policies, and Guidance Documents a. Antidegradation - Since the receiving water is reviewable, an antidegradation review is required pursuant to Section 31.8 of The Basic Standards and Methodologies for Surface Water. As set forth in Section VII of the WQA, an antidegradation evaluation was conducted for pollutants when water quality impacts occurred and when the impacts were significant. Based on the antidegradation requirements and the reasonable potential analysis discussed below, antidegradation-based average concentrations (ADBACs) may be applied. According to Division procedures, the facility has three options related to antidegradation-based effluent limits: (1)the facility may accept ADBACs as permit limits (see Section VII of the WQA); (2) the facility may select permit limits based on their non-impact limit (NIL), which would result in the facility not being subject to an antidegradation review and thus the antidegradation-based average concentrations would not apply (the NILs are also contained in Section VII of the WQA); 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 the 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. The ADBAC limits are imposed as two-year average limits. b. Antibackslidinq -As the receiving water is designated Reviewable, and the Division has performed an antidegradation evaluation, in accordance with the Antidegradation Guidance, the antibacksliding requirements in Regulation 61.10 have been met. c. Determination of Total Maximum Daily Loads (TMDLs)- The receiving stream to which the Scott Blue II mine discharges is currently listed on the State's 303(d) list for development of TMDLs for aquatic life (provisional). However, a TMDL has not yet been established. The permit may be reopened to include limitations based upon a finalized TMDL. d. 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 further analysis 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 if the ratio of the facility 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 receiving water is a Page 5 of 17 4300 Cherry Creek Drive S.,Denver,CO 80246-1530 P 303-692-2000 www.colorado.gov/cdphe/wgcd * r,7 John W.Hickenlooper,Governor Larry Wolk,MD,MSPH,Executive Director and Chief Medical Of i CO . :pp:et peutreas