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Minrec, Incorporated Water Quality Assessment CO- 0048233 <br />Regulation 93 — 303(d) List and Total Maximum Daily Loads <br />This stream segment is not listed on the Division's 303(d) list of water quality impacted streams. <br />Regulation 94 — 305(b) Monitoring and Evaluation List <br />This stream segment is on the 305(b) list of potentially impacted streams for total recoverable iron. <br />IV. Receiving Stream Information <br />Low Flow Analysis <br />The Colorado Regulations specify the use of low flow conditions when establishing water quality based <br />effluent limitations, specifically the acute and chronic low flows. The acute low flow, referred to as 1E3, <br />represents the one -day low flow recurring in a three -year interval, and is used in developing limitations <br />based on an acute standard. The chronic low flow, 30E3, represents the 30 -day average low flow <br />recurring in a three -year interval, and is used in developing limitations based on a chronic standard. <br />Because there were no current data upstream or just downstream of the Minrec facility, historical data <br />(1964 through 1979) from USGS Gage Station 09082800 (North Thompson Creek) approximately 2 miles <br />upstream of the facility was used in the previous WQA to determine the low flows. Since no current flow <br />data has been collected in this area, the historical data will continue to be used to develop representative <br />flows for North Thompson Creek in the vicinity of the discharge of the Minrec facility. <br />The annual IE3 and 30E3 low flows were previously calculated using U.S. Environmental Protection <br />Agency (EPA) DFLOW software. The output from DFLOW provides calculated acute and chronic low <br />flows for each month. <br />Table A -5 <br />Low Flows for North Thom son Creek at the Minrec Thompson Creek Mine <br />Low Flow (cfs) <br />Annual <br />Jan <br />Feb <br />Mar <br />Apr <br />May <br />Jun <br />Jul <br />Aug <br />Sep <br />Oct <br />Nov <br />Dec <br />IE3 Acute <br />0.1 <br />0.1 <br />0.2 <br />0.5 <br />1.0 <br />1.2 <br />0.3 <br />0.1 <br />0.1 <br />0.1 <br />0.3 <br />0.2 <br />0.1 <br />30E3 Chronic <br />and 7E3 <br />0.2 <br />0.1 <br />0.2 <br />0.5 <br />1.0 <br />1.2 <br />0.3 <br />0.2 <br />0.2 <br />0.2 <br />0.3 <br />0.2 <br />0.2 <br />The 7E3 was not calculated in the previous WQA. Because the data set used to calculate the 30E3 and <br />OIE3 is no longer accessible, the 30E3 (less conservative) was used in the temperature exclusion analysis. <br />The ratio of the low flow of North Thompson Creek to the Minrec facility design flow is 2:1. <br />As reported in the previous WQA, during the months of February through June and again in October, the <br />acute low flow calculated by DFLOW exceeded the chronic low flow. In accordance with WQCD <br />standard procedures, the acute low flow was thus set equal to the chronic low flow for these months. <br />Appendix A (WQA v 7.0) Page 9 of 25 Last Revised Sept 14, 2009 /ES <br />