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COLORADO <br />Division of Reclamation, <br />Mining and Safety <br />Department of Natural Resources <br />December 31, 2020 <br />Tony Tennyson <br />Colowyo Coal Company L.P. <br />5731 State Highway 13 <br />Meeker, CO 81641 <br />Re: Colowyo Coal Mine (Permit No. C-1981-019) <br />Technical Revision No.145 (TR-145) <br />Adequacy Review <br />Dear Mr. Tennyson: <br />After reviewing TR-145 from Colowyo Coal Company L.P. (Colowyo), the Division has the following comments: <br />1. When reviewing all proposed List of Maps for each associated Colowyo PAP Volume, it is observed that Map 35A — <br />Bond Calculation Map Worst Case Topography has not been included. Map 35A was last updated with Colowyo TR- <br />141 and has not be requested for removal. Please update the proposed List of Maps with the TR-145 revision <br />application. <br />Volume 2D <br />2. There appears to be a wording error on Exhibit 7, page 1. The text "the Colowyo's sedimentation" should be <br />changed to "Colowyo's sedimentation." <br />3. When reviewing the Streeter Pond figures: <br />a. Comparing the proposed Streeter Pond As -Built drawing (Figure Exh. 7-14SP-1) to the currently <br />approved drawing (Exhibit 7-SP, Attachment 4), the stage/storage curve is the same but the table with <br />Storage Volume Computations has changed significantly. Please explain this apparent discrepancy (the <br />Division acknowledges that the graph and table match in the proposed drawing). <br />b. The as -built drawing (Figure Exh. 7-14SP-1) does not have a scale that functions with electronic copies <br />(it only works for hard copies at proper scale). Please add a scale such as the one on Figure Exh. 7-14SP- <br />2. <br />4. When reviewing the Streeter Pond SEDCAD: <br />c. Please provide an explanation to why the stock ponds in the proposed SEDCAD model are modeled as <br />empty prior to storm runoff. In the currently approved SEDCAD model for the Streeter Pond, the stock <br />ponds are full of water before the runoff begins. Full stock ponds are consistent with the term "worst - <br />case hydrologic conditions," which is used in the introductory text (page Exh. 7-14SP-1). This change <br />potentially has significant consequences; for example, the 10-year flow into Streeter Pond decreases from <br />78 cfs (currently approved model) to 3 cfs (proposed model) in the respective SEDCAD pages. <br />OF COlO <br />n O <br />1313 Sherman Street, Room 215, Denver, CO 80203 P 303.866.3567 F 303.832.8106 http://mining.state.co.us * <br />* <br />Jared S. Polis, Governor I Dan Gibbs, Executive Director I Virginia Brannon, Director 1876 <br />