2020-06-08_REVISION - C1982057 (20)

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collected from the Sage Creek stream point during the last five years (2015—2019) and a comparison against the Segment 13e water quality standards. Additional discussion of the water quality in each stream segment follows. Upper Dry Creek Segment 13d Total Recoverable Iron Total recoverable iron exceeded the Yampa Segment 13d surface water quality standard once (n: 15) at WSH9 (SW-S2W-SG9), 13 times (n: 15) at WSH7 (SW-S2W-SG7), 12 times (n: 22) at WSHF1 (SW-S2W- FG1), and zero times (n: 21) at WSD5 (SW-S2W-SG5) between the 2015 and 2019. Table 17 provides a statistical summary of the pre-mine and post mine total recoverable iron for the Dry Creek/ Hubberson Gulch stream points. Although the total recoverable chronic water quality standard was exceeded frequently at WSH7 and WSHF1,the iron in the samples collected during the last five years did not exceed the pre-mine concentrations measured at WSHF1 and only exceeded the pre-mine concentrations measured at WSH7 once. Total recoverable iron was monitored at Outfalls 016 and 017 during 10 of the 15 WSH7 (SW-S2W-SG7) monitoring events (Table 18).This included nine of the 13 events that total recoverable iron exceeded the water quality standard at WSH7 (SW-S2W-SG7). During all nine of those events the total recoverable iron measured at Outfalls 016 and 017 met the Yampa Segment 13d surface water quality standard and was an order of magnitude less than the concentration measured downstream at WSH7 (SW-S2W-SG7). Similarly,total recoverable iron was measured at Outfall 006 during eleven of the 22 WSHF1 (SW-S2W- FG1) monitoring events (Table 19). During six of these events the total recoverable iron exceeded the water quality standard at WSHF1(SW-S2W-FG1).Total recoverable iron at Outfall 006 ranged from < 0.1 —0.16 mg/L during the eleven events, never exceeding the Yampa Segment 13d surface water quality standard.This indicates that the elevated iron at WSH7 (SW-S2W-SG7) and WSHF1 (SW-S2W-FG1) is unrelated to runoff from the mine Total recoverable iron is strongly correlated with suspended solids at stream points WSH7 (r2: 0.92) and WSHF1 (r2: 0.97) (Figure 2).As described in the Phase II Suspended Solids Evaluation,TSS in the effluent of the NPDES outfalls is typically one to two orders of magnitude less than the concentrations observed at the stream points. Furthermore,the post mine effluent TSS concentrations do not exceed the stream points pre-mine concentrations.This indicates that the iron observed at WSH7 and WSHF1 is unrelated to runoff from the reclaimed mine and is more likely the result of natural erosion that is occurring from the unaffected portion of the watershed. Ammonia, Nitrogen The CDPHE Yampa Segment 13d ammonia, nitrogen water quality standard of 0.05 mg/L, which is the laboratories detection limit, was exceeded once at WSD5 (SW-S2W-SG5).The aquatic life ammonia standard, as established by USEPA, is dependent on both the pH and temperature of the surface water body.The sample was compared to the temperature and pH dependent values of the chronic criterion found in Table 6 of USEPA's 2013 Aquatic Life Ambient Water Quality Criteria for Ammonia—Freshwater (USEPA 2013).The measured value of 0.13 mg/L did not exceed the 1.25 mg/L chronic aquatic life criteria for surface waters with pH of 8.2 standard units and temperature of 7.6 degrees Celsius. Sulfide