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COLORADO Division of Reclamation, Mining and Safety Department of Natural Resources March 26, 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) Fourth Adequacy Review Dear Mr. Tennyson: The Division has reviewed your adequacy response dated March 25, 2021, and we appreciate Tri-State's efforts related to this Technical Revision. Based on the Division's review, the following comments are associated with Adequacy Item 95 in response to Tri-State's proposed pages and comments in the response dated March 25, 2021. All other Adequacy Items have been satisfied and no longer require any additional comments. Adequacy Item 95 On proposed Exhibit 7, Page 2 of the PAP, Tri-State proposes the removal of some of the currently approved text in Section 1.5. The Division accepts the removal of the term "worst case condition," but believes that additional text should be added to Exhibit 7. Rule 4.05.6(3) states that a pond must contain or treat the runoff from a 10-year, 24-hour precipitation event. It is the Division's interpretation that this is true at all times prior to final bond release, including when the watershed conditions are such that the runoff volume is highest and not just the final post -mining topography. In order to address Rule 4.05.6(3), the Division suggests providing revised text to be included to Exhibit 7. Please see the following text as a recommendation: Hydrologic modeling related to the analysis of channels and/or ponds is based on the largest runoff volume or peak flow for a given return event (e.g. 10 year; 24 hour event) during the life of the structure (conservative estimate). The largest values for runoff volume and peak flow are generally associated with the watershed condition immediately after all reclamation has been completed, however, that may not always be the case. In some situations the highest volume or flow could occur prior to the end of reclamation. As such, a judgment to determine the appropriate condition must be made in the hydrologic modeling. In some cases, it may be appropriate to execute the model under more than one condition and compare results for runoff volume and flow and utilize the more conservative estimate. • On page Exh. 7-ET-3, there is a sentence near the middle of the page that states, "The assumed hydrologic condition for all three channels is the post mine condition when the entire East Taylor watershed is reclaimed and reporting to the East Taylor Pond." Based on the updated SEDCAD models associated with the East Taylor Pond, it appears that this statement may no longer remain accurate. The Division respectfully requests that this statement be revised. OF COlO n O 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 * 1876 *