2025-10-07_REVISION - C1982057 (16)

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Permit Number: C1982057 Page 18 of 20 Phase III Given the findings in this document and the documented use of the reclaimed area for livestock grazing and wildlife, the post mine land use has been achieved. Evaluation of Hydrologic Impacts Rule 3.03.2(2) requires the Division to evaluate hydrologic impacts prior to releasing reclamation liability. This evaluation considers whether pollution of surface or subsurface water is occurring and the probability of future occurrence of such pollution. The Division's evaluation based on Annual Hydrology Reports, Discharge Monitoring Reports and Sediment Pond Inspection Reports is summarized below. The SPL monitoring plan can be referenced in Tab 15 and Probable Hydrologic Consequences are discussed in Tabs 16, 17, and 18. Ground Water Impacts The ground water point of compliance well (GWPOC), monitors well DCAL02 screened in Dry Creek alluvium and located north of the permit boundary. Standards for this well were established in 2009 through TR63. For the years 2019 through 2024 water quality at DCAL02 has consistently met water quality standards, with the exception of excursions of manganese and dissolved iron. The manganese standard was established at 0.2 mg/L. High manganese values may, in part, be due to recharge from the spoil aquifer and natural geochemical conditions. This standard is used to protect crops grown in soils with a pH value lower than 6.0, however, soil at Seneca IIW exhibit alkaline pH. Pre-mining manganese values exceeded the standard. The iron standard was established at 8.06 mg/L. Samples collected over the previous five years indicate no statistically significant change in the iron concentrations when compared to the baseline range (6.96 – 11.1mg/L, a mean of 8.59 mg/L), established in 1998. As dissolved iron and manganese comprise the only ground water quality exceedances, this suggests that Dry Creeks’ alluvium ambient concentrations are above the standard, and that elevated iron and manganese are not indicative of an offsite impact to the groundwater system. DRMS notes that groundwater manganese and dissolved iron measurements fall within the range established with baseline data. DRMS finds that overall groundwater quality at DCAL‐02 consistently meets water quality standards. TDS Predictions of TDS loading were made in the Probable Hydrologic Consequences (PHC, Tab 17) section of the Seneca IIW Permit Application Package (PAP) for anticipated TDS increases to be observed at several of the mines monitoring wells. TDS predictions intend to demonstrate the potential average increase in postmining groundwater quality adjacent to the mine pits. Over the past five years of the seven wells, two wells tend to exceed the predicted TDS values while the others indicate a decreasing trend. Overall, TDS concentrations remain within the historical range for the seven wells. Pre-mine TDS values trend greater than the predicted values. For further discussion please reference the Seneca IIW historic Annual Hydrology Reports. DRMS finds that overall TDS is generally withing the permit predicted values. Surface Water Instream numeric standards for Dry Creek reference the Colorado Water Quality Control Commission’s (CWQCC), Yampa River Segment 13d for upper Dry Creek and Yampa River Segment 13e for Sage Creek. Historically for the Upper Dry Creek segment total recoverable iron excursions have occurred numerous times at points WSH7 and WSHF. Over the last five years there were no exceedances of pre-mine conditions. Monitoring results at outfalls 016 and 017 upstream of points WSH7 and WSHF1, exhibited values significantly