2017-05-11_PERMIT FILE - C1981012 (18)

Home Browse Search

11MITIM" COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Fact Sheet- Page 15, Permit No. C00000906 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 Middle Fork Purgatoire River is Reviewable, an antidegradation evaluation is required for this segment pursuant to Section 31.8 of The Basic Standards and Methodologies for Surface Water. As set forth in the WQA, an antidegradation evaluation was conducted for Middle Fork Purgatoire River 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. Because this facility is considered a new discharger as of September 20, 2000, NILS were not calculated. b. Antibacksliding - As one of the receiving waters 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 stream segments are not on the State's 303(d) list, and therefore TMDLs do not apply. 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 ratio of the design flow to chronic low flow at outfall 001 is 1.7:1, the permittee must perform additional studies to determine if further requirements apply. The remaining threshold tests require site-specific information that is currently not available and thus a determination cannot be made about how the regulation may affect the setting of effluent limits in this permit. Therefore, a special study is necessary for acquisition of this information, which will be used to complete the testing of exclusion thresholds before the next permit renewal. e. Reasonable Potential Analysis - Using the assimilative capacities contained in the WQA, an analysis must be performed to determine whether to include the calculated assimilative capacities as WQBELs in the permit. This reasonable potential (RP) analysis is based on the Determination of the Requirement to Include Water Quality Standards -Based Limits in CDPS Permits Based on Reasonable Potential, dated December, 2013. This guidance document utilizes both quantitative and qualitative approaches to establish RP depending on the amount of available data.