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The method detection limit for the sulfide analysis (MDL: 0.02 mg/L) conducted by SCC's lab <br /> exceeds the Yampa Segment 13i water quality standard for un-ionized sulfide (0.002 mg/L). The <br /> analytical method employed detects both dissolved sulfides and acid-soluble metallic sulfides <br /> that are present in suspended matter and provides a single cumulative concentration that includes <br /> both the ionized (HS-) and un-ionized forms of hydrogen sulfide (H2S). The un-ionized <br /> hydrogen sulfide is the potentially toxic form which the standard is based on. The distribution of <br /> sulfide between the un-ionized hydrogen sulfide and ionized form is dependent on the <br /> temperature and pH. At low pH most of the total dissolved sulfide exists as un-ionized hydrogen <br /> sulfide. In alkaline waters,like those present at Yoast Mine,most of the total dissolved sulfide <br /> exists as non-toxic ionized sulfide. A procedure for calculating the un-ionized form from the <br /> sulfide data can be found in the American Public Health Standard Methods for the Examination <br /> of Water and Wastewater. The results of this calculation indicate that un-ionized hydrogen <br /> sulfide will not exceed the water quality standard when the non-detect sulfide concentration is <br /> equal to 0.02 mg/L.None of the samples collected from YSGF5 and YSG5 during the last five <br /> years had detection limits above the non-detect value of 0.02 mg/L (Table 1 and 2). <br /> Manganese <br /> CWQCC Regulation 31 specifies that the manganese agricultural use standard of 0.2 mg/L <br /> standard is only applicable when irrigation water is applied to soils with pH lower than 6.0. The <br /> soils at Yoast Mine are alkaline and the 0.2 mg/L standard is therefore not applicable. Dissolved <br /> manganese at YSGF5 and YSG5 are significantly lower than the CHPHE Yampa Segment 13i <br /> acute and chronic manganese standards. <br /> Mercury <br /> The method detection limit for mercury (0.02 ug/L)used by SCC's lab is above the 0.01 ug/L <br /> aquatic life standard. None of the samples collected during the last five years exceeded the labs <br /> method detection limit. The CDPHE previously performed a reasonable potential analysis for <br /> Outfall 010 and determined that there was no reasonable potential for discharges from this outfall <br /> to exceed the mercury limit and the monitoring requirement was dropped from the NPDES <br /> permit. Total mercury is monitored quarterly at Outfall 011. Between 2014 and 2018,none of the <br /> mercury samples collected(range: 0.0004— 0.002 µg/L; n: 6) exceeded the mercury aquatic life <br /> SL-7 - 37 <br />