2025-01-07_GENERAL DOCUMENTS - C1981019

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Proposed Decision and Findings of Compliance Partial Phase I, II and III Bond Release SL-23 Page 9 of 16 applicable are contributing suspended solids to streamflow or runoff outside the permit area in excess of pre-mining levels as determined by baseline data. To evaluate for compliance with the applicable Rules, the Division reviewed SEDCAD sedimentology demonstrations provided by Colowyo in Exhibit 07 Item 14 Appendix Exh7-14SP as approved in 2021 under TR- 145. Exhibit 07 Item 14 Appendix Exh7-14SP provides demonstrations for a 10-year 24-hour, 25-year 24-hour and 100-year 24-hour storm events for the Streeter Ditch and Pond. All calculations demonstrated that under the design specified, the Streeter Ditch and Pond effectively manage peak flow, runoff volume, and sediment loads during all required storm test events. This determination was further corroborated by utilizing design criteria specified in the SEDCAD demonstration for comparison with on the ground measured conditions. This comparison revealed that measured design criteria, such as channel width, depth, and material (Table 1), was consistent with the design criteria provided in the approved SEDCAD demonstration (Table 2). The Streeter Ditch and Pond are constructed and functioning in line with design requirements and are effectivity managing peak stormwater flows, runoff volumes, and sediment loads. This conclusion is supported by visual inspections of the ditch and pond which showed overall stability in structure design, with no signs of flow outside the approved channel, overtopping at the pond, or erosional features. PHASE III Post-mining Land Use The post-mining land use at the Colowyo mine is rangeland and wildlife habitat. The Phase I, II and III Bond Release areas requested with the SL-23 application were walked during the inspection. Streeter Pond has been retained as a permanent structure in support of the post-mining land use, providing a water source for livestock and wildlife. While Colowyo has not used the reclaimed lands for domestic livestock grazing, there is a large resident elk herd that uses the reclaimed lands extensively. There are also several smaller herds of deer and pronghorn that use the reclaimed East and West Pit. The wildlife use includes grazing (including winter grazing) and calving areas. Although not specifically addressed in Rule 3.03.1, the evaluation by the Division must also comply with the considerations outlined in Rule 3.03.2(2). These include: “results of inspections and monitoring conducted pursuant to 4.05.13 and 5.02, the degree of difficulty to complete any remaining reclamation, and whether pollution of surface or subsurface water is occurring, the probability of future occurrence of such pollution, and the estimated cost of abating such pollution”. Colowyo is not requesting release of groundwater or surface water monitoring requirements through SL-23. Therefore, Colowyo remains in compliance with Rule 4.05.13. The Division conducted a review of Colowyo’s Annual Hydrology Reports (AHR) from the past three water years (2021, 2022, 2023). This review broke up discussion of surface and groundwater trends by drainage. The SL-23 release area is contained within the Good Spring Creek watershed. The review found that while the Good Spring Creek Point of Compliance (POC) well LGSW-1 was in compliance with NPL standards at the end of 2023, there had been spikes in Total Dissolved Solids (TDS) and Sulfate (SO4) concentrations throughout the last few years. However, aside from well LGSW-1, concentrations in all other surface and groundwater wells along Good Spring Creek show a consistent pattern of increase from upstream/upgradient to downstream/downgradient and generally do not exceed NPL standards, outside of some noted anomalies. Therefore, based on the Division’s AHR review and on-the-ground inspections, the Division has made the determination that Streeter Ditch and Streeter Pond are not contributing to the pollution of surface or subsurface water and have a low probability of enacting a future cause of such pollution. If the trend of exceedances does not decrease as is predicted over time in Good Spring Creek or there is a marked impact on water quality, a mitigation strategy may need to be developed. However, this is not the case at this time and any future mitigation developments would not likely involve re-disturbing Streeter Pond or Ditch. Therefore, Streeter Pond and Ditch are in compliance with the requirements of Rule 3.03.1 and 3.03.2.