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Oxbow Mining WWTF Water Quality Assessment CO-0000132 and Lower Dolores River Basins, section NF, page 10. The stated value of 400 mg/1 was used in TVS standazd calculations, with the results shown in Table A-3. The Dissolved Selenium chronic standard in Table A-3 is specified as existing ambient quality. The existing ambient quality of 10.6 ug/1 was derived from the WQCD Document Rationale for Class cations, Standards and Designations ofSegmenis ofthe Gunnison and LowerpoloresRiver Basins, section NF, page 10. Table A-2 In-stream Standards for Stream Se went COGUNF06 Ph sical and Biola 'cat Dissolved Ox en DO = S m 1, minimum H=6.S-9su Fecal Coliform = 325 colonies/100 ml Inorganic Un-ionized ammonia acute =TVS Un-ionized ammonia chronic = 0.06 m /1 Chlorine acute = 0.019 m 1 Chlorine chronic = 0.011 m I Free C 'de acute = 0.005 m Sulfide chronic = 0.002 m Boron chronic = 0.75 m 1 Nitrite = 0.05 m /l Nitrate = 10 m l Chloride chronic = 2S0 m Sulfate chronic = 2S0 m /1 Metals Total Recoverable Arsenic acute = 50 u Dissolved Cadmium acute and chronic =TVS Total Recoverable Trivalent Chromium awte = 50 u Dissolved Hexavalent Chromium acute and chronic =TVS Dissolved Co acute and chronic =TVS Dissolved Iron chronic = 300 u /1 Total Recoverable Iron chronic = 1000 u Dissolved Man anew chronic = 50 u Dissolved Man anew acute =TVS Total Mer chronic = 0.01 u /1 Dissolved Nickel acute and chronic =TVS Dissolved Selenium acute and chronic =TVS Dissolved Selenium chronic = existin ambient uali tem ora modification throw b 12/31/06 Dissolved Silver acute and chronic =TVS Dissolved Zinc acute and chronic =TVS Ambient Water Quality The WQCD evaluates ambient water quality based on a variety of statistical 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. The ambient water quality was not assessed for Sanbom Creek and Elk Creek because the backgound in-stream low flow condition is zero, and because no ambient water quality data are available for Sanborn Creek and Elk Creek upstream of the Oxbow Mining, LLC WWTF discharges. Appendix A Page 4 of 10 Draft