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Cotter Corporation Water Quality Assessment CO-0036251 The hardness values used in calculating the appropriate metal standard should be based on the lower 95% confidence limit of the mean hazdness value at the periodic low flow criteria as determined from a regression analysis of site-specific data. Where insufficient site-specific data exists to define the mean hardness value at the periodic low flow criteria, representative regional data shall be used to perform the regression analysis. Where a regression analysis is not appropriate, asite-specific method should be used. Because the immediate receiving stream for the Cotter Corporation discharger has no in-stream metals standazds, there was no reason to evaluate hazdness for the unnamed tributaries of East Paradox Creek and Bull Canyon, and East Paradox Creek and Bull Canyon. It was necessary, however, to assess hardness values and the resulting metal standards for the downstream receiving waters, the Dolores River, because metals standazds are included for stream segment COGULD02. Hazdness data for the Dolores River were insufficient to conduct a regression analysis based on the low flow. Therefore, the WQCD's alternative approach to calculating hardness was used, which involves computing a mean hazdness. The mean hazdness was computed to be 223 mg/1 as CaCO3 based on sampling data from USGS water quality station 09169500 (Dolores River at Bedrock) located approximately three miles upstream of the Dolores River and East Pazadox Creek confluence. This hazdness value and the formulas contained in the TVS were used to calculate the in-stream water quality standards for metals with the results shown in Table A-4. Because the Regulation Classification and Numeric Standards for Gunnison and Lower Dolores River Basins is expected to incorporate changes to its TV S equations within the next several months, this WQA analysis has also been developed to reflect the water quality standards based on the new TVS equations. These are set forth in Table A-5. In addition to the segment and site-specific standazds discussed above, Section 31.11 of the Regulations also establishes basic standazds applicable to all waters of the State. Forpurposes of the Cotter Corporation WQA, pollutants of concern and their respective standards follow: • Uranium shall not exceed 40 pCi/1 • Radium 226 and 228 shall not exceed 5 pCi/1 Ambient Water Quality The WQCD evaluates ambient water quality based on a varietyofstatistical methods as prescribed in Section 31.8(2)(a)(i) and 31.8(2)(b)(i)(B) of the Colorado Department of Public Health and Environment Water Quality Control Commission Regulation No. 31. Ambient water quality is evaluated in this WQA to determine assimilative capacities for pollutants of concern. Appendix A Page 5 of 13 Final