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Mountain Coal and Bear Coal Water Quality Assessment CO-0038776 and CO-0044377 III. Water Quantity The Colorado Regulations specify the use of low flow conditions when establishing water quality based effluent limitations, specifically the acute and chronic low flows. The acute low flow, referred to as 1 E3, represents the one-day low flow recurring in athree-year interval. The chronic low flow, 30E3, represents the 30-day average low flow recurring in a three-year interval. Low Flow Analysis, North Fork of the Gunnison River: To determine the low flows for the North Fork of the Gunnison River available to the Mountain Coal and Beaz Coal WWTFs, USGS gage station 09132500 (North Fork of the Gunnison River near Somerset, CO) was used. This flow gage provides a representative measurement of the upstream flow because there are no diversions or confluences of significance between the flow gage and the dischazges. Daily flows from USGS Gage Station 09132500 were obtained and the annual lE3 and 30E3 low flows were calculated using U.S. Environmental Protection Agency (EPA) DFLOW software. The cutput from DFLOW provides calculated acute and chronic low flows for each month. Flow data from October 1, 1992 through September 30, 2002, were available from the gage station. The gage station and time frames were deemed the most accurate and representative of current flows and were therefore used iri this analysis. Based on the low flow analysis described previously, the upstream low flows available to the Mountain Coal and Bear Coal W WTFs were calculated and are presented in Table A-7. Table A-'S Low Flows for the North Fork of the Gunnison River at the Mountain Coal and Bear Coal WWTFs Low Flow Annual Jan " Feb Mar Apr May Jun- Jul Aug Sep ~ Oct Nov Dec ~F~s) IE3 32 33 38 47 70 234 197 57 41 32 47 43 33 Acute 30E3 43 43 44 47 70 234 223 57 43 43 )2 4~ 43 Chronic During the months of March, April, May and July, the acute low flow calculated by DFLOW exceeded the chronic low flow. In accordance with WQCD standard procedures, the acute low flow was thus set equal to the chronic low flow for these months. Low Flow Analysis, Sylvester Gulch, West Sylvester Gulch, the Drv Fork of Minnesota Creek, and Lone Pine Gulch: Although there is periodic flow in Sylvester Gulch, West Sylvester Gulch, the Dry Fork of Minnesota Creek and Lone Pine Gulch upstream of the Mountain Coal W WTFs, the 1 E3 and 30E3 monthly low flows are set at zero based on information provided by the local Water Commissioner. For this analysis, low flows are summarized in Table A-8. Appendix A Page 10 of 20 Last Revised 3/9/2004