2018-03-01_HYDROLOGY - C1981010 (2)

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COLORADO Department of Public Health b Environment Benzo(g,h,i)perylene (PAH) chronic =0.0038 pg/l Chrysene (PAH) chronic =0.0038 pg/l Dibenzo(a,h)anthracene (PAH) chronic = 0.0038 pg/l Fluoranthene (PAH) chronic =130 pg/l Fluorene (PAH) chronic = 280 pg/l Indeno(1,2,3-cd)pyrene (PAH) chronic = 0.0038 pg/l Naphthalene (PAH) chronic = 140 pg/l Pyrene (PAH) chronic = 210 pg/l Table Value Standards and Hardness Calculations Water Quality Assessment Standards for metals are generally shown in the regulations as Table Value Standards (TVS), and these often must be derived from equations that depend on the receiving stream hardness or species of fish present; for ammonia, standards are discussed further in Section IV of this WQA. The Classification and Numeric Standards documents for each basin include a specification for appropriate hardness values to be used. Specifically, the regulations state that: The hardness values used in calculating the appropriate metal standard should be based on the lower 95% confidence limit of the mean hardness value at the periodic low flow criteria as determined from a regression analysis of site-specific data. Where insufficient site-specific data exists to define the mean hardness value at the periodic low flow criteria, representative regional data shall be used to perform the regression analysis. Where a regression analysis is not appropriate, a site-specific method should be used. Segment COLCLY03a does not include standards for TVS-based metals. Segment COLCLY03b is zero low flow as discussed in Section IV; therefore, hardness data of the effluent rather than the receiving stream was used to calculate TVS equations. The mean hardness of outfalls 002, 011, 017, 020 and 021 discharging to segment COLCLY03b was computed to be 1098 mg/l based on sampling data from 2011 through 2015 provided by the facility. The Basic Standards and Methodologies for Surface Water indicates that hardness must be capped at 400 mg/l when determining in -stream metal water quality standards using the equations in the TVS. This maximum hardness value and the formulas contained in the TVS were used to calculate the in -stream water quality standards for metals, with the results shown in Table A -5a. Hardness data for the Yampa River segment COLCLY02 near the Trapper Mine were insufficient to conduct a regression analysis based on the low flow. Therefore, the Division's alternative approach to calculating hardness was used, which involves computing a mean hardness. The mean hardness was computed to be 124 mg/l based on sampling data from USGS 09247600 (Yampa River below Craig) located on the Yampa River immediately downstream from the confluence with Johnson Gulch. This downstream location includes the influence of the facility's outfalls. Hardness data were available from September 2005 through November 2017. This hardness value and the formulas contained in the TVS were used to calculate the in -stream water quality standards for metals discharging to the Yampa River; the results shown in Table A -5b. Table A -5a TVS-Based Metals Water Quality Standards for COLCLY03b Outfalls 002, 011, 017, 020, 021, 023 Based on the Table Value Standards Contained in the Colorado Department of Public Health and Environment Water Quality Control Commission Regulation 37 Parameter In -Stream Water TVS Formula: Quality Standard Hardness (mgll) as CaCO3 400 Pagge 14 of 40 4300 Cherry Creek Drive S., Denver, CO 80246-1530 p 303-692-2000 www.cotorado.gov/cdphe/wqcd I ��