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magnitude less than the concentration measured at the downstream points indicating <br /> the elevated iron in Dry Creek is unrelated to runoff from the mine. <br /> Total recoverable iron is strongly correlated with suspended solids at stream points <br /> WSH7 (r2: 0.92) and WSHF1 (r2: 0.97) (Appendix D Figure D.1). SIIW has been <br /> vegetated and stable for over a decade and TSS in the mine discharges is typically an <br /> order of magnitude less than the concentrations observed at the stream points. This <br /> further supports the conclusion that the iron measured at WSH7 and WSHF1 is <br /> unrelated to runoff from the reclaimed mine and is the result of natural erosion that <br /> is occurring in unaffected areas of the watershed. <br /> The method detection limit for the sulfide analysis (MDL: 0.02 mg/L) conducted by <br /> SCC's lab exceeds the CDPHE Yampa Segment 13d water quality standard for un- <br /> ionized sulfide (H2S) of 0.002 mg/L. All but one of the four sulfide samples analyzed <br /> were non-detect. This analytical method detects both dissolved sulfides and acid- <br /> soluble metallic sulfides that are present in suspended matter and provides a single <br /> cumulative concentration. The single detection occurred in the WSHF1 the 6/2 <br /> sample which also had very high TSS (180 mg/L). Its expected that the detection was <br /> resultant of the acid-soluble metallic sulfides present on suspended matter and not <br /> dissolved sulfide as any dissolved sulfide present in oxygenated surface waters is <br /> expected to be oxidized to sulfate quickly. Furthermore, dissolved sulfide includes <br /> both the ionized (HS-) and un-ionized forms of hydrogen sulfide (H2S). The distribution <br /> of sulfide between the un-ionized hydrogen sulfide and ionized form is dependent on <br /> the temperature and pH. At low pH most of the dissolved sulfide exists as the toxic <br /> un-ionized hydrogen sulfide. In alkaline waters, like those present at SIIW, most of <br /> the dissolved sulfide is present as non-toxic ionized sulfide. The pH at WSHF1 during <br /> the 6/2 event was 8.22 standard units. <br /> The method detection limit for mercury (0.2 ug/L) used by SCC's lab for stream <br /> points WSHF1 and WSD5 is above the 0.01 ug/L aquatic life standard for mercury. <br /> None of the samples collected during 2020 exceeded the labs method detection limit. <br /> The CDPHE performed a reasonable potential analysis for the Seneca NPDES outfalls <br /> and mercury monitoring was dropped from all outfalls except Outfall 005, which did <br /> not have enough sample data for CDPHE to complete the analysis. Based on historic <br /> data its not expected that there were true exceedances of the mercury standard. <br /> Page 12 of 17 <br />