2021-03-22_REVISION - C1981019 (7)

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COLORADO Division of Reclamation, Mining and Safety Department of Natural Resources March 22, 2021 Tony Tennyson Colowyo Coal Company L.P. 5731 State Highway 13 Meeker, CO 81641 Re: Colowyo Coal Mine (Permit No. C-1981-019) Technical Revision No.145 (TR-145), Discussion regarding SEDCAD model for East Taylor Pond Dear Mr. Tennyson: After reviewing your correspondence received via email on March 20, 2021 regarding the Division's third adequacy letter for Colowyo Coal Company L.P. (Colowyo) TR-145, the Division has the following comments: • Under Section 1.5 of Exhibit 07 — Methodologies and Assumptions for Sedimentation Pond Design Evaluations of the Colowyo PAP, it states, "[In] many cases determination of the `worst case' condition with respect to volume of runoff has to be determined by examining several stages of the mining operation. The final post- mining topography would usually be expected to produce the largest volume of runoff as the entire watershed is contributing to the runoff by this stage, and is always evaluated but may not control the design." • The Division respects Tri-State's judgment regarding the "worst case" condition for the hydrology model (SEDCAD) for the East Taylor Pond. However, the Division is uncertain that the SEDCAD model provided with the TR-145 submittal represents a worst case scenario condition in terms of runoff volume to the East Taylor Pond. This is based on our experience with SEDCAD. The Division feels that there is the potential for larger runoff volumes to the East Taylor Pond during a different stage of reclamation than what has been provided in the proposed SEDCAD model, and we need more information to make an informed decision regarding the "worst case" condition. • The Division respectfully requests that Colowyo provide a demonstration that would model a reclamation condition when the upper part of the watershed is contributing very little runoff and the parcels directly north of this upper area (reclamation blacks labeled as 2021 and 2020-2022 on Figure Exh. 7-14ET-2) are bare (Curve Number equal to 80). To help illustrate this, please refer to Figure 1 of this document below. • For the area at the top of the watershed, it may be easier to change the curve numbers to a very low value rather than to remove the sub -watersheds from the model. If that is the case, the Division will accept that approach. Sincerely, Zach Trujillo Environmental Protection Specialist (303) 866-3567 ext. 8164 Zach.Trujillogstate.co.us 1313 Sherman Street, Room 215, Denver, CO 80203 P 303.866.3567 F 303.832.8106 http://mining.state.co.us Jared S. Polis, Governor I Dan Gibbs, Executive Director I Virginia Brannon, Director