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Pittsburg and Midway Coal Company Edna Mine Water Quality Assessment CO-0032638 <br />Trout Creek. However, Division procedure has changed and therefore standazds must now be <br />applied based on discharges to the unnamed tributary and thus the upstream low flows of the <br />unnamed tributary must be determined. <br />Although there is periodic flow in the unnanred tributary to Trout Creek upstream ofOutfa11004, the <br />lE3 and 30E3 monthly low flows are set at zero consistent with finding of other neazby unnamed <br />tributaries in the Routt County area. For this analysis, low flows aze summarized as follows: <br />• Chronic low flow (30E3) = 0 cfs <br />• Acute low flow (lE3) = 0 cfs <br />Low Flow Analysis 007 <br />At the request of the pennittee as part ofprevious permits, ratios of effluent flow to receiving stream <br />flows were established in lieu of specific low flows. At that time, the facility had the capacity, via <br />sedimentation ponds, to control the flows released to the receiving stream in order to maintain the <br />ratio. However, because of cun•ent reclamation efforts, mining areas have been contoured to original <br />topography and many sedimentation ponds have been eliminated, with the exception ofMoffat Pond, <br />which is the source of Outfa11004. Thus, the ability to control the flows released via Outfa11007 to <br />the receiving stream has been eliminated and thus the facility's ability to comply with the ratios has <br />diminished. Furthermore, due to changes in the regulations and Division procedures, particularly <br />those that require the establishment of baseline water quality concentrations and antidegradation- <br />basedaverage concentrations, it is impossible to continue to use ratios. Therefore, an in-stream low <br />flow must be determined. <br />Since 1988, the P&M Coal Company has conducted daily monitoring of flows in the receiving <br />stream at sampling location TR-a, which is approximately one mile upstream of Outfall 007. <br />However, these data reflect in-stream flows collected seasonally, usually from April through <br />October. Due to the limited flow record, the available period ofrecord from May 1, 1988 through <br />October 14, 2002, were used. <br />Because the data were not adequate to input to DFLOW, a manual determination of the acute and <br />chronic low flow was determined. Because the recurrence interval of once every three yeazs is <br />equivalent to 0.000913, the percentiles based on this value considering the daily flow record and the <br />30-day average flow record were determined, resulting in the following annual low flows. <br />• Chronic low flow (30E3) = 0.25 cfs <br />• Acute low flow (lE3) = 0.046 cfs. <br />IV. Technical Analysis <br />In-stream background data and low flows evaluated in Sections II and Ill are ultimately used to <br />determine the assimilative capacity of Trout Creek and tributaries near the Edna Mine for pollutants <br />of concern. For all pazameters, it is the Division's approach to conduct a technical analysis of stream <br />assimilation capacity using the lowest of the monthly low flows (referred to as the annual low flow) <br />as calculated in the low flow analysis. <br />Appendix A twQw er ve,~;o~ a-s FneL> Page 8 of 19 Last Revised September 2, 2005 <br />