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Mountain Coal and Beaz Coal Water Quality Assessment CO-0038776 and CO-0044377 <br />III. Water Quantity <br />The Colorado Regulations specify the use of low flow conditions when establishing water quality <br />based effluent limitations, specifically the acute and chronic low flows. The acute low flow, referred <br />to as 1E3, represents the one-day low flow recurring in athree-yeaz interval. The chronic low flow, <br />30E3, represents the 30-day average low flow recurring in a three-year interval. <br />Low Flow Analysis, North Fork of the Gunnison River: To determine the low flows for the <br />North Fork of the Gunnison River available to the Mountain Coal and Beaz Coal WWTFs, USGS <br />gage station 09132500 (North Fork of the Gunnison River neaz Somerset, CO) was used. This flow <br />gage provides a representative measurement of the upstream flow because there aze no diversions or <br />confluences of significance between the flow gage and the discharges. <br />Daily flows from USGS Gage Station 09132500 were obtained and the annual lE3 and 30E31ow <br />flows were calculated using U.S. Environmental Protection Agency (EPA) DFLOW software. The <br />output from DFLOW provides calculated acute. and chronic ]ow flows for each month. <br />Flow data from October 1, 1992 through September 30, 2002, were available from the gage station. <br />The gage station and time frames were deemed the most accurate and representative of current flows <br />and were therefore used in this analysis. <br />Based on the low flow analysis described previously, the upstream low flows available to the <br />Mountain Coal and Beaz CoaI.WWTFs were calculated and,aze.presented in Table A-7. <br />During the months of Mazch, April, May and July, the acute low flow calculated by DFLOW <br />exceeded the chronic low flow. In accordance with WQCD standazd procedures, the acute low flow <br />was thus set equal to the chronic low flow for these months. <br />Low Flow Analysis, Sylvester Gulch, West Sylvester Gulch, the Dry Fork of Minnesota Creek, <br />and Lone Pine Gulch: Although there is periodic flow in Sylvester Gulch, West Sylvester Gulch, <br />the Dry Fork of Minnesota Creek and Lone Pine Gulch upstream of the Mountain Coal W WTFs, the <br />1 E3 and 30E3 monthly low flows aze set at zero based on information provided by the local Water <br />Commissioner. For this analysis, low flows aze summarized in Table A-8. <br />Appendix A Page 10 of 20 Last Revised 3/9/2004 <br />