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le Coal Company Water Quality Assessment 000042161 <br />using hardness and fish species specific to each site. The regulations include a specification for <br />• appropriate hardness values to be used. Specifically, the regulations state that: <br />The hardness values used in calculating the appropriate metal standard should be <br />based on the lower 95 percent confidence limit of the mean hardness value at the <br />periodic low flow criteria as determined from a regression analysis ofsite-specific <br />data. Where insufficient site-specific data exist to define the mean hardness value at <br />the periodic low flow criteria, representative regional data shall be used to perform <br />the regression analysis. Where a regression analysis is not appropriate, asite-specific <br />method should be used. <br />Hazdness and flow data for Fish Creek near the point of discharge of Twentymile Coal Company <br />were insufficient to conduct a regression analysis based on the low flow. in the absence of a <br />regression analysis, the WQCD's approach to calculating hardness involves computing a mean <br />hardness. <br />The mean hazdness value determined for Fish Creek is 407 mg/1 as CaCO3 based on sampling at Site <br />3021ocated approximately 1 %: miles downstream of the Twentymile Coal Company Outfa11001. The <br />Basic Standards and Methodologies for Surface Water (5 CCR 1002-31) state that hardness must be <br />capped at 400 mg/1 when determining in-stream metal water quality standards using the equations in <br />the TVS. Thus, the hardness was capped at a concentration of 400 mg/1 for Fish Creek. <br />This hardness value and the formulas contained in the TVS were used to calculate the in-stream <br />• water quality standards for metals. Table A-3 provides the in-stream metals water quality standards <br />for Fish Creek calculated based on the approach summarized above. <br />Ambient Water Quality <br />The WQCD evaluates ambient water quality based on a variety of statistical methods as prescribed in <br />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. Ambient water quality was <br />assessed to determine assimilative capacities available to the Twentymile Coal Company in Fish <br />Creek. <br />Ambient water quality data for Fish Creek from Site 161ocated approximately 1 %x miles upstream <br />from Outfall 001 and chronic water quality standards are presented in Table A-4. <br />III. Water Quantity <br />Section 31.9 of the Colorado Regulations requires compliance with water quality and specifies the <br />use of low flow conditions when establishing water quality based effluent limitations. Specifically, <br />the acute (one-day low flow recurring in a three-year interval or 1 E3) and chronic (30-day average <br />low flow recurring in a three-year interval or 30E3) low flows must be used. <br />Low Flow Analysis: <br />• <br />Appendix A Page 4 of 12 <br />