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Mr. Bill Roberts Adequacy Review No. 2, Permit Application No. M-2006-046 October 4, 2006 e. The Martin & Wood Consultants groundwater model predicts significant, adverse hydrologic impacts to a number of resources (wells, wetlands, riparian vegetation, irrigation ditches) most of which are dependent upon groundwater. Please clarify the time of yeaz that the model predictions account for (spring, summer, fall, winter), the maximum pumping rates that will be needed for each cell or phase of mining, the length of time each cell or phase will be in a de-watered state, and specific mitigation measures that the applicant will commit to for each cell or phase of mining to minimize the potential impacts. Please respond. f. The Martin & Wood Consultants ground-water model and assessment predicts a ntunber of significant, adverse hydrologic impacts to water and vegetative resources in the immediate and sutrounding areas based on the proposed de-watering operations. The authors suggest installation of a slurry wall prior to de-watering and excavation as a method to eliminate and/or minimize the various predicted impacts. What is not clearly defined is the maximum pumping rate required specifically for each cell or phase to maintain a de-watered mining condition; and the allocation of where and how pit de-watering flows will be returned back to the Colorado River in such a way so as to minimize potential adverse impacts to ground-water wells, imgation ditches, wetlands, riparian vegetation, and any other resources dependent on surface or ground-water resources. In response, please confirm whether a slurry wall will or will not be installed prior to excavation and de-watering activities. If a slurry wall will not be installed, the Division requests that the applicant provide a comprehensive "Water Management Plan" and schematic which describes how pit inflows and pit outflows from de-watering operations will be managed during each phase of the operations, i.e.: Phases la, Ib, 2, 3a, & 3b. The Water Management Plan must include a schematic diagram for each phase of the operation showing how water inflows and outflows will be managed and routed so as to minimize impacts to water dependent sources. g. The application references two (2) irrigation ditches that cross through the proposed permit boundary. Please provide a discussion of the physical condition of the portions of the Rising Sun Ditch and Last Chance Ditch that pass through the proposed permit area and the potential for them to be adversely affected by pit de- watering operations. For example, are the ditches lined or unlined? What type of leakage rates do they exhibit? What times of year aze they typically in use? Do either of the ditches have a direct hydrologic connection with the shallow groundwater surface in the area? What is the potental for an adverse impact to either ditch due to de-watering? Please respond. 24. Review of the Pre-Mining and Mining Plan Maps indicates that most of the proposed mining operation will take place within the 100-year floodplain of the Colorado River. In order to minimize the potential for flood related impacts, i.e.: a significant flood event capturing the pit and causing off site damage, the applicant has proposed monitoring the forecast flood stage, turning off de-watering pumps in anticipation of the flood stage, installing an armored (8-inch or larger riprap) spillway in each mined cell, and installing a spillway between the clarification basin and the production area of Cell I. In order to fully approve these measures, the Division requests that the applicant provide the following information: a. Plan view and cross sectional diagrams for each spillway to be installed*; b. discussion of the flow capacity and maximum permissible velocity for each design spillway; c. revised Mine Plan & Reclamation Plan Maps to show the location of all five (5) spillways; d. discussion of when (during the mining sequence) that each spillway will be installed; e. characterization of the soil types on site (sand, silt, loam, sandy loam, etc) for all exposed surfaces; f. characterization of the 25-year and 100-year peak flow velocities & flaw volumes expected in the main channel of the Colorado River and surrounding excavation areas and; g. cross-sectional schematic depicting the elevations of the invert and top of righUlefl banks for the Colorado River, 200'setback zone, 4 excavation cells, base flood elevation for the 25-year flood, base flood elevation for the 100-year flood, significant wetland features, and significant riparian features (cottonwoods, willows, etc.).