2018-01-12_REVISION - M2000041 (32)

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Mr. Elliott R. Russell January 4,2018 Page 4 Plan,profile, cross sections, and details of the bi-directional spillway/rundown have been added to Sheet 11 of the revised plan set attached in Appendix A. A plan and profile of the swale tie-in is shown on Sheet 10. 21. Within the September 2017 Flood Analysis and Flood Control Plan (Appendix D), the Operator states armoring the reservoir slope upstream of the spillway is not required because any inflow to the reservoir through the spillway that results in head cutting and slope failure would only cause damage to property owned by the landowner. In accordance with 3.1.6(3), all surface areas of the affected land shall be stabilized and protected as to effectively control erosion. Additionally, a head cut would compromise the spillway structure and the slurry wall. Please provide a demonstration that the reservoir side slope will remain stable as flood waters flow into the reservoir from the spillway or revise the plan to include appropriate rip-rap, a concrete apron, or other erosion control measures on the reservoir side slope below the spillway. Erosion control measures should be depicted on the cross-section discussed in Adequacy Review Item #20. The RCC spillway channel described in D&A's response to Comment No. 18 will extend down to the toe of the reservoir slope. A horizontal concrete apron at the bottom of the rundown will provide protection for the toe. A stilling basin is not necessary because the reservoir bottom is in hard Pierre Shale bedrock, so erosion downstream of the apron will be minimal and will not compromise the apron's stability. This RCC spillway/rundown has been added to Sheet 11 of the revised plan set shown in Appendix A. 22.In response to Adequacy Review Item#10, the Operator states that if groundwater mounding exceed two feet, a perforated drain pipe would be installed to restore groundwater levels to less than two feet of mounding. To demonstrate the mitigation measure is feasibly and can be installed entirely within the affected lands,please provide the conceptual plans for a drain pipe and commit to, in the event groundwater mounding exceeds two feet, submitting the detailed mitigation plans for review and approval by the Division through the Technical Revision process.Alternatively, the Operator may submit these details for review and approval during the TR-04 process whereby, in the event groundwater mounding exceeds two feet, the Operator shall notify the Division and immediately implement the approved plan. See revised Appendix E. 23. The proposed groundwater mounding monitoring plan utilizes Well I (Permit No. 279124), which is located 200 feet west of the permit boundary on the neighboring property. This well is not owned by the Operator nor the mine site landowner, and therefore is not safeguarded through the life of mine. To ensure there are no adverse impacts to off-site groundwater users, the Division will require an addition monitoring