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ARCADIS <br />Data Adequacy: Two follow -up suggestions associated with the characterization work are proposed, <br />including: <br />• In addition to the alluvial well near the toe of the South Waste Rock Pile, installation of a <br />monitoring well into the alluvium beneath the rock pile is recommended to characterize <br />groundwater quality beneath the rock pile. If the alluvial groundwater beneath the rock pile has <br />elevated uranium, then mitigation measures such as sheet pile or a grout curtain may be <br />needed to prevent groundwater from flowing beneath the rock pile and contacting the alluvial fill. <br />Percolating leachate from the waste rock could also contribute to the impacted groundwater, <br />and the data from the lysimeters can be used to assess this possibility. <br />• Cotter should consider using pan lysimeters instead of suction lysimeters. Suction lysimeters <br />only contact a limited surface area of the surrounding material and percolating water through the <br />coarse - grained waste rock could easily pass by the lysimeter. A pan lysimeter has a much <br />larger contact area to intercept percolating water and is more suitable in coarse- grained <br />material. If the lysimeters reveal production of appreciable leachate with elevated uranium <br />concentrations, then additional work on the cover may be needed. If not, then it can be <br />concluded that the waste rock is not leaching appreciable uranium to groundwater, and no <br />further work on the cover is needed. <br />Recommendations: Cotter should consider installing a monitoring well into the alluvium beneath the South <br />Waste Rock Pile to characterize underlying groundwater quality. The use of pan lysimeters instead of <br />suction lysimeters should also be considered. <br />5.0 MINE POOL MITIGATION MEASURES <br />5.1 Mine Dewatering <br />Overview: Plans are to install a pump into the mine pool and pump 80 gpm to lower the mine pool <br />elevation by 500 feet. This is consistent with the directive from DRMS to pump down the mine pool to 500 <br />feet below the Steve Level, and consistent with recommendations made during the EPP review process. <br />Of the 80 gpm, 56 gpm will be treated and discharged to the creek. The remaining 24 gpm will be mixed <br />with a carbon amendment (molasses) and injected back into the mine for in -situ treatment of the mine <br />pool. After the mine pool is treated, plans are to stop pumping and allow the pool to rebound to equilibrium <br />conditions. <br />Data Adequacy: As stated during the review of the EPP, it is essential to keep the mine pool pumped <br />down below the elevation of the creek to reduce the hydraulic connection between the mine pool and <br />alluvium /creek. As indicated, it will take approximately 1 year to draw down the mine pool to 500 feet. <br />There does not appear to be a contingency plan in place if the mine pool water cannot be drawn down to <br />the desired level at the given extraction rate, and it is unclear what the plan is to maintain the water level <br />after the first year. Also, during that first year, there may be isolated pockets of water at greater depth that <br />may not be treated. Once allowed to rebound, the mine pool could re- introduce elevated uranium into the <br />Page. <br />6/16 <br />