2018-03-01_HYDROLOGY - C1981010 (2)

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COLORADO Water Quality Assessment l�+ .;' Department of Public I Health b Environment Unless a facility has performed a mixing zone study during the course of the previous permit, and a decision has been made regarding the amount of the assimilative capacity that can be used by the facility, the Division assumes that the full assimilative capacity can be allocated. Note that the review of mixing study considerations, exemptions and perhaps performing a new mixing study (due to changes in low flow, change in facility design flow, channel geomorphology or other reason) is evaluated in every permit and permit renewal. If a mixing zone study has been performed and a decision regarding the amount of available assimilative capacity has been made, the Division may calculate the water quality based effluent limitations (WQBELs) based on this available capacity. In addition, the amount of assimilative capacity may be reduced by TFtE implications. Since Lower Yampa River segments 03a and 03b have a zero low flow as indicated above, the WQBELs would be equal to the WQS, and therefore consideration of full or reduced assimilative capacity is inconsequential. The amount of assimilative capacity available in the Lower Yampa River segment 02 is eliminated for aquatic life parameters due to the threatened and endangered habitat designation. Therefore, no dilution is available for aquatic life parameters for that segment. Note that to remove consideration of the Yampa river, the facility can conduct a drainage/flow study to demonstrate that the effluent does not reach the downstream waters. V. Facility Information and Pollutants Evaluated Ambient Water Oualit The Division evaluates ambient water quality based on a variety of statistical methods as prescribed in Section 31.8(2)(a)(i) and 31.8(2)(b)(i)(B) of the Colorado Department of Public Health and Environment Water Quality Control Commission Regulation No. 31, and as outlined in the Division's Policy for Characterizing Ambient Water Quality for Use in Determining Water Quality Standards Based Effluent Limits (WQP-19). The ambient water quality was not assessed for No -Name Gulch, East Pyeatt Gulch, Middle Flume Gulch, East Middle Flume Gulch, East Flume Gulch, Oak Gulch and Deacon Gulch because the background in - stream low flow condition is zero. Ambient water quality for the Yampa River is evaluated in this WQA analysis for use in determining assimilative capacities and in completing antidegradation reviews for pollutants of concern, where applicable. To conduct an assessment of the ambient water quality upstream of the Trapper Mine, data were gathered from WQCD 12804 (Yampa River at HWY 394 South of Craig) located directly upstream from the confluence of the Deep Cut Ditch (where Oak Gulch, Middle Flume, East Middle Flume, East Flume and Deacon Gulch discharge into the Yampa River) and the Yampa River and WQCD 12804C (Yampa River at Dorsey SWA Boat Launch) located approximately 12 miles upstream from the confluence of the Deep Cut Ditch and the Yampa River. Data from these sources were available for a period of record from July 2011 through March 2014. Additional ambient data for dissolved cadmium, dissolved copper, dissolved lead, dissolved silver, dissolved selenium and dissolved zinc were available from USGS 09244490 (Yampa River above Elkhead Creek near Hayden) located approximately 12 miles upstream from the confluence of the Deep Cut Ditch and the Yampa River; the period of record for data from USGS 09244490 is April 2010 through November 2014. Data from these sources were used to reflect upstream water quality. These data are summarized in Error! Reference source not found.7. Table A-7 Ambient Water Quality for the Yampa River Page 18 of 40 4300 Cherry Creek Drive S., Denver, CO 80246-1530 }� 303-692-2000 www.colorado.gov/cdphe/wqcd I �®