2020-06-08_REVISION - C1982057 (20)

Home Browse Search

Seneca II-W Bond Release—Hydrology Demonstration Phase II—Suspended Solids Evaluation The Seneca II West Mine lies within the Dry Creek and Sage Creek Watersheds.The majority of the permit area drains to the west towards Hubberson Gulch (a tributary to Dry Creek) and Dry Creek, which flows north to the Yampa River.The remainder of the permit area drains northeast towards Sage Creek, which flows north-northeast to the Yampa River. Runoff from the reclaimed mine discharges from six NPDES outfalls (Figure 1 Monitoring Site Locations). Outfall 005 discharges to an unnamed tributary to Dry Creek, Outfalls 006, 016, and 017 discharge to the west to unnamed tributaries to the Hubberson Gulch and Outfalls 009 and 015 discharge to unnamed tributaries to Sage Creek. There are five stream monitoring points;four within the Dry Creek Watershed and one within the Sage Creek Watershed. Stream monitoring points within the Dry Creek Watershed include Hubberson Gulch points WSH9 (SW-S2W-SG9),WSH7 (SW-S2W-SG7), and WSHF1 (SW-S2W-FG1) and Dry Creek point (WSD5) SW-S2W-SG5 (WSD5). Monitoring point WSH9 (SW-S2W-SG9) is located upstream of all the outfalls within the Dry Creek watershed,WSH7 (SW-S2W-SG7) is located downstream of Outfalls 016 and 017, WSHF1 (SW-S2W-FG1) is located downstream of Outfalls 006, 016, and 017, and WSD5 (W- S2W-SG5) is located downstream of Outfalls 005, 006, 016, and 017. Stream points WSSF3 (SW-S2W- FG4) is located within Sage Creek downstream of Outfall 009 and 015. Analytical results for the Hubberson Gulch (WSH9, WSH7, WSHF1), Dry Creek (WSD5), and Sage Creek (WSSF3) stream points between 2013—2019 are included in Tables 1—5.Table 6 provides a statistical summary of the pre-mine and post mine total suspended solids (TSS)for all of these locations except for WSH9. WSH9 is located upstream of all mine drainage and sampling at this location was not initiated prior to mining in the watershed.The range of post mine TSS measured between 2015—2019 at stream points WSHF1, WSD5, and WSSF3 are less than or comparable to the pre-mine range.The post mine TSS measured at stream point WSH7, however, indicates that the TSS concentrations have increased at this location. Analytical results for the mines NPDES outfalls between 2015 and 2019 are provided in Tables 7 - 12.TSS was removed as a monitoring requirement from the outfalls when their drainage status was changed to reclamation. Outfalls 005, 006, 009, and 015 were reclassified to reclamation in November 2006 and Outfalls 016 and 017 were reclassified in June 2010. Between 2015 and 2019 only 15 TSS samples were collected, all from outfalls within the Dry Creek Watershed. Table 13 includes a statistical summary of the 2015 -2019 TSS data collected from these outfalls.The TSS concentration measured at Outfalls 005, 006, 016, and 017 ranged <5 -75 mg/L. Both the average and maximum TSS concentrations measured at Outfall 016 (mean: 5.3 mg/L; max: 6 mg/L) and 017 (mean: 5.3 mg/L; max: 6)were less than the average and maximum pre-mine concentrations observed at WSH7 (mean: 73 mg/L; max: 214 mg/L), Outfall 006 (mean: 5.3 mg/L; max: 6 mg/L)was less than the pre-mine concentrations at WSHF1 (mean: 314 mg/L, max: 14000 mg/L), and Outfall 005 (mean: 27.8 mg/L; max: 75 mg/L)was less than the pre- mine concentration at WSD5 (mean: 260 mg/L; max: 5190 mg/Q. Table 14 includes a summary of the six outfalls TSS analytical data during the five-year period prior to their change to reclamation drainage status. During this time the average TSS at the four outfalls in the Dry Creek Watershed (005, 006, 016, 017) ranged from 6.4— 17.8 mg/L and the maximum TSS ranged from 14—50 mg/L.This is significantly less than the pre-mine TSS measured at the three stream points (WSH7,WSHF1, WSD5), where the average pre-mine TSS ranged from 73—260 mg/L and the maximum