Laserfiche WebLink
New Elk Mine Water Quality Assessment CO-0000906 <br />• Regulation 31. The Classification and Numeric Standards documents for each basin include a <br />specification for appropriate hardness values to be used. Specifically, the regulations state that: <br />The hardness values used in calculating the appropriate metal standard should be based <br />on the lower 95% confidence limit of the mean hardness value at the periodic low flow <br />criteria as determined from a regression analysis of site-specific data. Where <br />insufficient site-specific data exists to define the mean hardness value at the periodic <br />low flow criteria, representative regional data shall be used to perform the regression <br />analysis. Where a regression analysis is not appropriate, a site-specific method should <br />be used. <br />Hardness data for the Middle Fork of the Purgatoire River near the point of discharge of the New Elk <br />Mine Outfall 001 were insufficient to conduct a regression analysis based on the low flow. <br />Therefore, the Division's alternative approach to calculating hardness was used, which involves <br />computing a mean hardness. <br />The mean hardness was computed to be 142 mg/l based on sampling data from Riverwatch Station <br />468 (the Middle Fork of the Purgatoire River at Stonewall) located on the Middle Fork of the <br />Purgatoire River approximately four miles upstream from the New Elk Mine Outfall 001. This <br />hardness value and the formulas contained in the TVS were used to calculate the in-stream water <br />quality standards for metals, with the results shown in Table A-3. <br />• Ambient Water Quality for the Middle Fork of the Purgatoire River <br />The Division evaluates ambient water quality based on a variety of statistical methods as prescribed <br />in Section 31.8(2)(a)(1) 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 is <br />evaluated in this WQA analysis for use in determining assimilative capacities and in completing <br />antidegradation reviews for pollutants of concern, where applicable. <br />To conduct an assessment of the ambient water quality upstream of the New Elk Mine Outfall 001 <br />data were primarily gathered from the previously mentioned Riverwatch Station 468 located <br />approximately four miles upstream from the facility. Data from this location were available for a <br />period of record from December 1998 through November 2003. For Boron, in the absence of recent <br />upstream data, data from Division Station 7545 (Purgatoire River Above Weston at Hwy 12A), <br />located approximately eight miles downstream, were used. These data were available for a period of <br />record from August 2004 through June 2006. Finally, for calcium, magnesium, and sodium, very <br />limited recent data were available and therefore data from USGS Gage Station 07124200 (Purgatoire <br />River at Madrid, CO), which had data mainly from June 1978 through March 1981 with one data set <br />from August 2002, were used. Although some of these data were collected at downstream locations, <br />the data were determined to be commensurate with upstream conditions. A summary of the data <br />from these sources is presented in Table A-4 <br />n <br />1. J <br />Appendix A (WQn version s-a(FABL)) Page 7 of 29 Last Revised August 26, 2008