2018-03-01_HYDROLOGY - C1981010 (2)

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COLORADO +y��_ ,�� Water Quality Assessment „:9 Department of Public Health b Environment The ratio of the low flow of No -Name Gulch, East Pyeatt Gulch, Middle Flume Gulch, East Middle Flume Gulch, East Flume Gulch, Oak Gulch and Deacon Gulch to the Trapper Mine facility design flow is 0:1.Note that for streams with a low flow of zero, the ambient water quality discussion is unnecessary. To determine the low flows of the Yampa River near the Trapper Mine facility, USGS 09247600 (Yampa River below Craig) located on the Yampa River immediately downstream from the confluence with Johnson Gulch was used. This gage is the nearest gage that provides a recent period of record. Daily flows from the gage station were obtained for a period of record from January 2005 through September 2016, and the annual 1E3 and 30E3 low flows were calculated using US EPA DFLOW software. The output from DFLOW provides calculated acute and chronic low flows for each month and are presented in Table A -6b. This is a downstream gage. Based on the facility DMR for the previous permit term, the outfalls have not discharged at design flow and discharge sporadically. Therefore, it was determined that the facility flow has a negligible effect on the statistics used to calculate low flows. Further, this section of the Yampa river is a Threatened and Endangered Species receiving water, and therefore no dilution is allocated for all aquatic life parameters. Therefore, it was not necessary to subtract the facility flow from the daily flows obtained at this gage station. Table A -6b Table A -6a Low Flows for No -Name Gulch, East Pyeatt Gulch, Middle Flume Gulch, East Middle Flume Gulch, East Flume Gulch, Oak Gulch and Deacon Gulch at the Trapper Mine 7E3 0 0 0 0 0 0 0 0 0 0 0 0 0 Chronic Apr May Jun Jul Aug Sep Oct Nov Dec (cfs) 30E3 0 0 0 0 0 0 0 0 0 0 0 0 0 Chronic 174 296 78 99 73 70 73 98 70 Acute The ratio of the low flow of No -Name Gulch, East Pyeatt Gulch, Middle Flume Gulch, East Middle Flume Gulch, East Flume Gulch, Oak Gulch and Deacon Gulch to the Trapper Mine facility design flow is 0:1.Note that for streams with a low flow of zero, the ambient water quality discussion is unnecessary. To determine the low flows of the Yampa River near the Trapper Mine facility, USGS 09247600 (Yampa River below Craig) located on the Yampa River immediately downstream from the confluence with Johnson Gulch was used. This gage is the nearest gage that provides a recent period of record. Daily flows from the gage station were obtained for a period of record from January 2005 through September 2016, and the annual 1E3 and 30E3 low flows were calculated using US EPA DFLOW software. The output from DFLOW provides calculated acute and chronic low flows for each month and are presented in Table A -6b. This is a downstream gage. Based on the facility DMR for the previous permit term, the outfalls have not discharged at design flow and discharge sporadically. Therefore, it was determined that the facility flow has a negligible effect on the statistics used to calculate low flows. Further, this section of the Yampa river is a Threatened and Endangered Species receiving water, and therefore no dilution is allocated for all aquatic life parameters. Therefore, it was not necessary to subtract the facility flow from the daily flows obtained at this gage station. Table A -6b Low Flows for the Yampa River near the Trapper Mine Low Flow Annual Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec (cfs) 1E3 70 90 95 123 174 296 78 99 73 70 73 98 70 Acute 7E3 84 108 110 123 174 296 93 93 84 84 85 111 96 Chronic 30E3 113 119 120 123 174 296 140 113 113 113 113 115 115 Chronic During the months of March, April and May, the acute low flow calculated by DFLOW exceeded the chronic low flow. In accordance with Division standard procedures, the acute low flow was thus set equal to the chronic low flow for these months. Mixing Zones The amount of the available assimilative capacity (dilution) that may be used by the permittee for the purposes of calculating the WQBELs may be limited in a permitting action based upon a mixing zone analysis or other factor. These other factors that may reduce the amount of assimilative capacity available in a permit are: presence of other dischargers in the vicinity; the presence of a water diversion downstream of the discharge (in the mixing zone); the need to provide a zone of passage for aquatic life; the likelihood of bioaccumulation of toxins in fish or wildlife; habitat considerations such as fish spawning or nursery areas; the presence of threatened and endangered species; potential for human exposure through drinking water or recreation; the possibility that aquatic life will be attracted to the effluent plume; the potential for adverse effects on groundwater; and the toxicity or persistence of the substance discharged. PagQe 17 of 40 � 4300 Cherry Creek Drive S., Denver, CO 80246-1530 p 303-692-2000 www.cotorado.gov/cdphe/wqcd I \ /