2018-03-01_HYDROLOGY - C1981010 (2)

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COLORADO Water Quality Assessment a- Department of Public Health b Environment TDS: A TDS limit of 3500 pg/l is used for the protection of livestock watering. Information provided by the facility indicates that livestock watering is active downstream of outfalls 002, 011, 017, 020, 021, 023 and 025 on No -Name Gulch, East Pyeatt Gulch, Oak Gulch, Middle Flume Gulch, East Middle Flume Gulch, East Flume Gulch and Deacon Gulch. The TDS limit of 3,500 mg/t is applicable to these outfalls to protect the livestock watering use. VII. Antidegradation Evaluation As set out in The Basic Standards and Methodologies for Surface Water, Section 31.8(2)(b), an antidegradation analysis is required except in cases where the receiving water is designated as "Use Protected." Note that "Use Protected" waters are waters "that the Commission has determined do not warrant the special protection provided by the outstanding waters designation or the antidegradation review process" as set out in Section 31.8(2)(b). The antidegradation section of the regulation became effective in December 2000, and therefore antidegradation considerations are applicable to this WQA analysis. According to the Classifications and Numeric Standards for Lower Colorado River Basin, stream segments COLCLY03a and COLCLY03b are use protected and segment COLCLY02 is reviewable. For receiving waters that are designated as use protected, no antidegradation review is necessary in accordance with the regulations. For segment COLCLY02 an antidegradation review is required for this segment if new or increased impacts are found to occur. The Water Quality Control Commission has recently completed a final action for The Basic Standards and Methodologies for Surface Water, Regulation 31. The final action exempts dissolved iron, dissolved manganese and sulfate from antidegradation consideration on the basis that this level of protection extends to standards that protect "fishable/swimmable" uses, and not water supply uses. Dissolved iron, dissolved manganese and sulfate are based on secondary Safe Drinking Water Act criteria and are not surrogates for any swimmable criteria. Therefore, they are exempt from further antidegradation review. Introduction to the Antidegradation Process The antidegradation process conducted as part of this water quality assessment is designed to determine if an antidegradation review is necessary and if necessary, to complete the required calculations to determine the limits that can be selected as the antidegradation-based effluent limit (ADBEL), absent further analyses that must be conducted by the facility. As outlined in the Antidegradation Significance Determination for New or Increased Water QualitV Impacts Procedural Guidance (AD Guidance), the first consideration of an antidegradation evaluation is to determine if new or increased impacts are expected to occur. This is determined by a comparison of the newly calculated WQBELs verses the existing permit limitations in place as of September 30, 2000, and is described in more detail in the analysis. Note that the AD Guidance refers to the permit limitations as of September 30, 2000 as the existing limits. If a new or increased impact is found to occur, then the next step of the antidegradation process is to go through the significance determination tests. These tests include: 1) bioaccumulative toxic pollutant test; 2) temporary impacts test; 3) dilution test (100:1 dilution at low flow) and; 4) a concentration test. As the determination of new or increased impacts, and the bioaccumulative and concentration significance determination tests require more extensive calculations, the Division will begin the antidegradation evaluation with the dilution and temporary impact significance determination tests. These two significance tests may exempt a facility from further AD review without the additional calculations. Note that the antidegradation requirements outlined in The Basic Standards and Methodologies for Surface Water specify that chronic numeric standards should be used in the antidegradation review; however, where PagQe 28 of 40 77 . 4300 Cherry Creek Drive S., Denver, CO 80246-1530 P 303-692-2000 www.cotorado.gov/cdphe/wgcd