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s <br />Seneca Coal Company Water Quality Assessment CO-0000221 <br />insufficient to conduct a regression analysis based on the low flow. Therefore, the Division's <br />alternative approach to calculating hardness was used, which involves computing a mean hardness. <br />In the absence of downstream data for most discharge locations, the discharge of each outfall is <br />expected to compose the receiving stream at times of low flow. For this reason, the effluent <br />discharge from each outfall was evaluated. With the exception of Outfall 011, which discharges to <br />Grassy Creek, available discharge data indicate that the mean hardness of each outfall is well above <br />400 mg/l. For Outfall 011, the mean hardness was computed to be 258 mg/1 based on sampling data <br />for the period of record of 1998 through 2003. However, in-stream hardness data were available for <br />Grassy Creek based on data collected from Seneca Coal Company sampling location YSGF5, which <br />is located immediately below Outfall 011. The data, which includes 47 samples collected from 1980 <br />through 2004, reflect a downstream mean hardness of 617 mg/1. <br />The Basic Standards and Methodologies for Surface Water indicates that hardness must be capped at <br />400 mg/1 when determining in-stream metal water quality standards using the TVS equations. This <br />maximum hardness value and the formulas contained in the TVS were used to calculate the in-stream <br />water quality standards for metals, with the results shown in Table A-6. <br />Note that although effluent data are not available for all outfalls, in the absence of data, a hardness of <br />400 mg/l is assumed. Note that the metals standards for all stream segments are virtually identical <br />with the exception of the temporary chronic standard for selenium at stream segment COUCYA13E. <br />Ambient Water Quality <br />The Division evaluates ambient water quality based on a variety of statistical methods as prescribed <br />in Section 31.8(2)(a)(i) and 31.8(2)(b)(i)(B) of the Colorado Department of Public Health and <br />Environment Water Quality Control Commission Regulation No. 31. The ambient water quality was <br />not assessed for Grassy Creek and Sage Creek, as well as various tributaries to Dry Creek, Sage <br />Creek, Fish Creek and Grassy Creek because the background in-stream low flow condition is zero, <br />and because no ambient water quality data are available. <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 1 E3, represents the one-day low flow recurring in a three-year interval. The chronic low flow, <br />30E3, represents the 30-day average low flow recurring in a three-year interval. <br />Low Flow Analysis <br />Although there is periodic flow in Grassy Creek and Sage Creek, as well as various tributaries to Dry <br />Creek, Sage Creek, Fish Creek and Grassy Creek upstream of the Seneca Coal Company discharges, <br />the IE3 and 30E3 monthly low flows are set at zero based on information provided by the local <br />Water Commissioner. The permittee has also requested that the alternative low flow analysis be <br />conducted using the harmonic mean approach outlined in Regulation 31.9. The applicable low flows <br />are summarized in Tables A-7a and A-7b. <br />Appendix A Page 8 of 19 Last Revised 8/30/06 AN