2021-03-19_REVISION - C1981019 (4)

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5. Please explain how the CN values were chosen for the post -mining conditions to model in SEDCAD. In Figure Exh. 7-14ET-2 the large majority of the drainage area has a CN of 62 (925 acres out of 1049 acres). Please explain why that is the worst -case hydrologic condition. Colowyo: Curve numbers are selected in accordance with Table I in Exhibit 7 in Volume 2D in the approved permit as required, and the model methodology is further descried in Volume 2D, Exhibit 7 Methodologies and Assumptions for Sedimentation Pond Evaluations, Section 1.5. The East Taylor Pond watershed in its current condition, a large portion of which the southern portion of the watershed is not reporting to the pond due mostly to the existence of the final cut of the West Pit, and other areas that have not been backfilled and graded to date. Therefore, a large volume of surface water flows are being contained within the final cut of the West Pit and other mining related disturbances in the southern portion of the watershed, and are not reporting to the East Taylor Pond. West Pit reclamation areas currently reporting to the East Taylor Pond are well established and the majority are Phase II released further indicating surface water flows from these reclamation parcels in the West Pit reclamation are being reduced by successful revegetation. Once the southern portion of the East Taylor watershed is reclaimed, the post mine condition, a much larger area that was not previously reporting to the pond will be contributing within the watershed to the East Taylor Pond that at this time is not due to the West Pit and other disturbed areas limiting flows. Division: The Tri-State response does not completely address the Division's comment. Please explain why the worst -case condition, soon after the West Pit has been entirely reclaimed and a large part of the watershed is bare soil, is not modeled for the East Taylor Pond analysis. The Division refers Tri-State to the following language on page Exh. 7-ET-2: "The following pages present the results of the SEDCADTm models for the worst case hydrologic conditions under the post mining condition. At this stage the oldest reclamation is on the northern extent of the reclaimed West Pit, and the younger (topsoil and seeded) reclamation is the southern reaches of the East Taylor Pond watershed. " It is our opinion that this language suggests that the southern parcels of the watershed should by modeled with a curve number higher than 62. Colowyo: First, it appears the Division is not reviewing the proper materials submitted under the second adequacy response for TR-145 for Appendix Exh. 7-14ET. Please refer to page Exh. 7-ET-2, dated 1120121 on the footer, first paragraph under East Taylor Pond. There are not any references to a worst -case scenario only the post mining condition. Second, all SECADTmmodels for all watersheds at Colowyo, in the post mining condition, are setup exactly the same manner in accordance with Table I in Exhibit 7, and Exhibit 7 Methodologies and Assumptions for Sedimentation Pond Evaluations, Section 1.5. Tri-State believes it might be helpful for the Division to refer to Map 29. The spoil grading timing shown on Map 29 is where the applicable timing corresponds to the curve numbers selected for this pond model and other pond models whose watersheds correspond to the timing on Map 29. This approach is also outlined in the last sentence in Section 1.5 in Exhibit 7 Methodologies and Assumptions for Sediment Pond Evaluations. Finally, the Division is suggesting with this adequacy comment on changing the approved post mining model methodology that is currently approved in the permit and has been utilized at Colowyo for some time. A change in this methodology has drastic impacts to most of the pond models at Colowyo, and Tri-State does not agree there is a need to change a model methodology that has been successfully implemented, nor revising a model that is compliance with the Rules and complies with the model methodology outlined in the permit. After reviewing the all associated pages submitted for TR-145 dated 1/20/21, it does not appear Colowyo has provided page Exh. 7-ET-2. Please provide the referenced page. Regarding curve numbers and stage of reclamation, the PAP shows under Table 1, SCS Curve Number (page 4 of Exhibit 7) for lands with less than three years of vegetation growth, the CN value should be greater than 62. Please provide and explanation to why most of the areas in the East Taylor Pond watershed have a CN value of 62 despite the fact that Map 29 shows significant reclamation after year 2020. 6. This Item has been resolved. 7. This Item has been resolved.