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COLORADO <br />Division of Reclamation, <br />Mining and Safety <br />Department of Natural Resources <br />December 17, 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.144 (TR-144) <br />Adequacy Review <br />Dear Mr. Tennyson: <br />After reviewing TR-144 from Colowyo Coal Company L.P. (Colowyo), the Division has the following comments: <br />1. Based on previous Colowyo Mine Quarterly Fill Monitoring Reports (Reports) for the West Taylor Fill (Fill), global <br />stability for the Fill has been done based on piezometer measurements and there are no past or current surface elevations <br />evaluated in the Reports. Will elevation information be provided in the Reports that show a comparative analysis to <br />baseline elevations? If so, please provide baseline elevation information that the Division can use to evaluate continued <br />global stability of the Fill. <br />2. Will the proposed monitoring plan be using elevation measurements at key points along the Fill to determine potential <br />instability using surveying mirrors? If so, please provide the location of each surveying mirror to be installed along the <br />Fill <br />3. Please provide additional discussion and details regarding the accuracy of the drone aerials that will ensure accurate <br />and consistent measurements are being provided with the Reports. <br />This concludes the Division's adequacy review for the Colowyo TR-144. If you have any questions, feel free to contact me. <br />Sincerely, <br />Zach Trujillo <br />Environmental Protection Specialist <br />(303) 866-3567 ext. 8164 <br />Zach.Trujillogstate.co.us <br />1313 Sherman Street, Room 215, Denver, CO 80203 P 303.866.3567 F 303.832.8106 http://mining.state.co.us <br />Jared S. Polis, Governor I Dan Gibbs, Executive Director I Virginia Brannon, Director <br />