2020-06-08_REVISION - C1982057 (31)

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Table 20 includes the analytical results for the spring samples collected from 2015 — 2019. No Agricultural Use surface water quality standard was exceeded at the Spoil Springs. The Manganese Agricultural Use Standard in CDPHE Regulation 31 were recently updated to specify that this standard is only applicable to plants that are grown in areas with acidic soils (<6.0 pH). In alkaline soils, as are found in the Seneca II-W region, a more appropriate standard would be 10 mg/L (EPA, 1976). None of samples collected from the spoil springs exhibited manganese above 10 mg/L. There were also no excursions of the CDPHE Yampa Segment 13d acute or chronic standard for manganese at the spoil springs or at surface points WSH7 (SW-S2W-SG7), WSHF1 (SW-S2W-FG1), WSD5 (SW-S2W-SG5) downstream of outfalls 005, 006, and 016. Sage Creek Segment 13e There were no excursions of the CDPHE Yampa Segment l3e Sage Creek water quality standards at downstream monitoring point WSSF3 (SW-S2W-FG4) during the last five years (Table 6). As discussed above in the Dry Creek Mercury section the lab used by SCC has a method detection limit for mercury (0.02 ug/L)that is above the 0.01 ug/L aquatic life standard. None of the samples collected during the last five years exceed the labs method detection limit. The CDPHE performed a reasonable potential analysis for the Sage Creek NPDES outfalls to exceed the mercury limits during the last NPDES renewal and determined that there was no reasonable potential for the mines outfalls to exceed the mercury limit. As discussed in greater detail in the Dry Creek Sulfide section the sulfide detection method used by SSC's lab exceeds the instream water quality standard. The analytical method used by SCC's lab detects both dissolved sulfides and acid-soluble metallic sulfides that are present in suspended matter and provides a single cumulative concentration that includes both the ionized (HS-) and un-ionized forms of hydrogen sulfide (H2S). The un-ionized hydrogen sulfide is the potentially toxic form that the instream water quality standard was established for. Calculations of the un-ionized H2S indicate that the alkaline surface water at this location will not result in un-ionized H2S above the water quality standard when the sulfide concentrations are less than or equal to 0.02 mg/L. None of the samples collected from the stream points exceed the 0.02 mg/L concentration. There have also been no excursions of the NPDES discharge limits during this period at Outfalls 009 and 015 which discharge to Sage Creek. C. Permit Requirements of the Colorado Department of Public Health &Environment The six Seneca II West Mine NPDES outfalls located within the Phase-III request area are permitted under CPDS Permit No. CO-0000221. Outfalls 006, 016, and 017 discharge to unnamed tributaries to the Hubberson Gulch, Outfall 005 discharges to an unnamed tributary to Dry Creek, and Outfalls 009 and 015 discharge to unnamed tributaries to Sage Creek. Between January 1, 2015 and December 31, 2019 there was only a single exceedance of the NPDES limits at these six outfalls (Tables 7— 12). Total recoverable iron exceeded the monthly average limit at Outfall 005 in April 2016. The NPDES permits monthly average limit is based on the 1 mg/L chronic total recoverable iron table value standard. However, a temporary modification of the chronic total recoverable iron standard is in place for Dry Creek segment 13d and was recently extended by the Colorado Water Quality Control Commission until 6/30/2023 (see CDOH Regulation 33). The temporary modification includes a chronic total recoverable iron standard of 1.11 mg/L for May — February and a standard of 3.04 mg/L for March — April. Although the Water Quality Control Division acknowledges that this modification is in place they have been unable to incorporate it into permit CO-0000221 while the permit is under Administrative Extension. The total 19