2022-10-27_PERMIT FILE - M2022001

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CIVII RFS-VURCFS, LLI ENGINEERS 6 PLANNERS October 14,2022 Mr. Peter Hays Division of Reclamation, Mining, and Safety 1313 Sherman Street, Room 215 Denver, Colorado 80203 RE: Red Tierra Equities, L.L.C.,Section 20 Mine, File No. M-2022.001, 112c Permit Application Groundwater Model Review; Response Dear Mr. Hays: This letter addresses the Adequacy Review letter dated September 12, 2022 regarding the Section 20 Gravel Mine 112 Construction Materials Reclamation Permit Application Package. Responses to your comments follow: 1. On the acknowledgement page the signature and seal are missing, please provide the missing information. Signature and Seal have been included. 2.The area is modelled as one slurry wall but there are going to be up to 8 individual walled cells. The Division is concerned that during construction of the individual cells, not just cell 5S,there could be mounding and shadowing effects that are overlooked or obscured by modelling the site as one cell, please comment. The site was modelled as individual cells from Cell 3 to Cell 6 to determine the phasing of the mitigation measures proposed in the August 2022 report. The tables and figures 8 through 10 from the individual model runs have been included in the modified Report. The Mining Plan report discusses the mounding resulting from the slung walls being interconnected resulting in full obstruction of groundwater flow which represents the"worst case scenario"for mounding and shadowing effects. 3.The Division is aware there is an approved slurry wall structure that will be located immediately to the west of this site, Monarch Mountain (M2022-009).The installation of this structure needs to be included into the model and the model re-run to determine how groundwater is affected by both slung walled structures. The inclusion of the Monarch DENM site in the model simulations results in CR recommending additional mitigation measures to include: 1)dewatering wells or an underdrain in the southeastern corner;and 2)an underdrain starting west of the Western Mutual Ditch and CR 44 intersection extending west finally daylighting into the South Platte River valley. The dewatering infrastructure(wells/underdrain)in the southeast will be designed to remove a minimum of 3.25 cfs and the underdrain to the west will remove at least 3.26 cfs. An underdrain is also needed on the Monarch DENM Site between Cell 3 and WCR 42 to offset this mine's contribution to the regional groundwater level impacts and this same alignment would be easily combined with the Section 20 discharge in a single outfall(Refer Figure 6). 4.The report states the shadowing effects to the north will be minimized by the unnamed slough and infiltration ponds. Please provide modelling that demonstrates this will be the case. Additionally, if the infiltration ponds were to go dry as a result of the shadowing the Applicant needs to provide a mitigation plan. Included in the mitigation plan for shadowing the Applicant needs to propose groundwater levels that will begin the mitigation efforts. Mitigation measures proposed to address the potentially negative impacts of constructing the subject slurry wall(s)include: 830E C❑LORADO BLVD • SUITE 200 • FIRESTONE, COLORADO B0504 • PHONE: 303.833.1 41 6 • FAX: 303.833.28SO