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 8 of 16 To further ensure accuracy, the Division recorded the elevation of a basepoint at the main office area of Colowyo and compared this point to the elevational reading on the iPhone 13. The office basepoint was at 7240 ft. The surveying device recorded an elevation of 7270 at the same location. This provided a correction factor of -30 ft for all data points recollected on November 20th. These corrected values are shown below in Tables 1 – 3. Comparative analysis of this data reveals that the accuracy of elevational measurements was still slightly compromised but was more accurate than initial readings. When compared to 2023 USGS LiDAR elevational readings taken at the same GPS locations, elevational readings taken by the Division were on average only 28 ft off from on the ground conditions (Table 3). The largest discrepancies were recorded along the mid-section of Streeter Ditch, however the elevation point taken at Streeter Pond matched the approved structure design. The Division considers the elevation reading discrepancies along the mid-section of the Streeter Ditch anomalies due the surrounding steepness in terrain leading to limited GPS reception. Given the expanse of the mine site and the steepness of the terrain at Colowyo, as well as the error contributed by limited GPS reception, the Division has determined that the difference in elevation is acceptable for determination that Streeter Ditch has met all requirements of Phase I release. Streeter Ditch and Streeter Pond were also inspected to ensure that these areas were graded and blended in with the adjacent reclaimed areas while retaining stability. During visual inspection of the Streeter Ditch, no erosional issues were noted, and no areas were identified that would suggest stormwater would flow outside of the approved channel area. Rip rap was in place and stable along the channel areas approved for such design (Stations 0+00 to 45+00). From Stations 45+00 to 68+41, the channel shifts into a vegetated channel. Around Station 40+00, the channel widens and has taken on a braided effect with smaller meandering channels which have established within the larger channel itself (Figure 2, from DRMS inspection report dated 10/30/2024). These meandering channels are not very deep or wide, have been at a stable size for many years based on previous inspection reports, and are well within the width of the constructed ditch channel (Photos 9 and 10, from DRMS inspection report dated 10/30/2024). The same pattern of steady state meander repeats from approximately Station 62+00 to 67+00. No erosional issues were noted during the visual inspection and grading and channel establishment has been constructed to encourage proper storm drainage as required through Streeter Ditch. At Streeter Pond, the primary spillway, riser, and trash rack were measured against the as-built design and found to be constructed accordingly. The emergency spillway, at the southern end of the pond, was inspected and found to be free of obstructions. The primary discharge point was found to be flowing and was also free of obstructions. Although water discharges from this point, the Colorado Department of Public Health and Environment, Water Quality Control Division, released Colowyo of monitoring requirements at the discharge from Streeter Pond in 2017. Streeter Pond was found to be in compliance with the permanent impoundment requirements and water quality standards specified under Rule 4.05.9(13)(a-f). At the request of the landowner, the pond is approved as a permanent structure in support of the post-mining land use. The pond is stable with vegetated embankments and remains functional. The Division reviewed previous inspections of the Streeter Pond as part of the SL-23 process and the pond has had no items of concerns or previous issues. For Streeter Ditch and Streeter Pond, the Division finds that the Colowyo Mine has achieved a post-mining surface that conforms to the approved post-mining topography and resembles the general surface configuration of the land prior to mining. All reclaimed lands blend in well with the adjacent reclaimed slopes. Both the pond and ditch were observed to have achieved long term stability and are functioning effectively as a post- mining drainage channel and permanent impoundment. PHASE II As per Rule 4.15.1(2)(b), vegetation cover shall be capable of stabilizing the soil surface to achieve erosion control equal to pre-mining levels. As discussed earlier in this document, there are no topsoil or revegetation requirements for permanent structures or ponds. Rule 3.03.1(3)(b) states that lands which to which the release would be