2009-03-06_PERMIT FILE - C1981012A (6)

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CO-0000906 New Elk Mine Water Quality Assessment as 1 E3, represents the one-day low flow recurring in a three-year interval t rvhe chronic low flow, • to 30E3, represents the 30-day average low flow recurring in a three-year Low Flow Anal sis for the Middle rom of UIS rui RIV Ma=? t 1 1 USGS Gage Station determine the low flows available to the New Elk wall, CO) wassused. This flow gage 07124050 (Middle Fork of the Purgatoire River at Stone provides a representative measurement of upstream flow because it is located just upstream of the New Elk Mine Outfall 001. the Daily flows from the USGS Gage Station 07124050 (M3 dole flows were calc gated atoire River U.St Stonewall, CO) were obtained and the annual IE3 and 30E provides Environmental Protection Agency (EPA) DFLOW software. The output from DFLOW p calculated acute and chronic low flows for each month. Flow data from May 1978 through September 1981 were available from t of gage station flows and wage station and time frames were deemed the most accurate and therefore used in this analysis. These same data were used in previous analyses. Based on the low flow analysis described previously, the in stye Al w flows available to the New Elk Mine Outfall 001 were calculated and are presented i is Table A-5a Low Flows for the Middle Fork of the Pur atoire River at the New Elk Mine 001 Low.Flow Jun Jul. Aug Sep Oct Nov Dec Annual Jan Feb Mar Apr May (CA) IS 3.0 3.0 3.5 3.9 3.7 4.8 9.5 14 9.6 5.6 4.0 5.1 43 Acute 300 4.3 4.3 43 4.3 4.4 4.8 9.5 17 9.6 5.6 5.5 5.2 4.3 Chronic During the months of May, June, August, and September, the acute lowLO w exceeded the chronic low flow. In accordance with Division standard procedures, was thus set equal to the chronic low flow for these months. Low Flow Analysis for the Left Fork of Apache Canyon Creek of the New Although there is periodic flow in the Left Fork of Apache Canyon set Creek zero based on available and Mine Outfall 009, the IE3 and 30E3 monthly low flows are historical information. For this analysis, low flows are summarized in Table A-5b. • Pa e 10 of 29 Last Revised August 26, 2008 Appendix A (WQA ve,sio„ 5-4(FASL)) g