2012-07-24_REVISION - M1977300 - Laserfiche WebLink

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ARCADIS alluvium and eventually to the creek. Even if the mine pool was treated to reduce uranium concentrations to approximately 3 mg/L, the mine pool water could re- contaminate the alluvial groundwater. Recommendations: Pump down the mine pool to 500 feet below the Steve Level and maintain the mine pool below the elevation of the creek to reduce the hydraulic connection between the mine pool and alluvium /creek. Performance criteria should be established to determine if/when pumping should be discontinued. 5.2 Mine Dewatering with Ex -situ Water Treatment Overview: The existing water treatment plant (WTP) will be moved to a new location in order to remediate the alluvial fill under the existing WTP location. The configuration of the WTP infrastructure will be modified to accommodate RO treatment systems to treat the mine pool water. First, the mine pool water will be pumped at a rate of 80 gpm to the pre - treatment system. The flow will progress through the 5 micron cartridge pre - filter and then be dosed with an anti - scalant at 4 mg /L before passing through the RO system. Seventy percent of the mine water will pass through the RO treatment membranes and will be discharged into Ralston Creek via the existing water treatment discharge line located within the WTP. The RO concentrate stream (i.e., residuals), which will consist of approximately 30 percent of the mine water that will not pass through the RO treatment membranes, will be returned to the mine pool ( #1 Shaft) at an elevation 500 feet below the Steve Level using a small booster pump and new pipeline alignment. Data Adequacy: The following comments are provided for the ex -situ RO treatment of the mine pool water during the dewatering process: • As described in the in -situ treatment section, a carbon source will be added to the residuals line going back to the mine. Over time, if the carbon source is not consumed and the organic concentrations begin to rise, the RO membranes may be subject to biofouling. • The basis for the RO system design and overall recovery of 70 percent is not explained. • It is not clear whether the RO system been designed to provide redundancy. Consequently, • there is no discussion of the impact to the plan if the RO system shuts down for an extended period. • The pretreatment process for the RO system is not described. • COCs are not explicitly identified to allow for accurate ex -situ treatment design. It is unclear which constituents were considered during the RO system design, the specific parameters that will be removed, and to what levels. • It is not clear whether the RO model run took into account the potential for changes in RO feed water concentration over time and the ability to maintain a 70 percent recovery rate. A plan needs to be put in place for disposal of the sand filter (used to treat the alluvial water) backwash waste and the RO chemical cleaning waste. Other RO brine treatment technologies, other than placement in the mine pool or evaporation ponds, should be considered for use at this site. Page 7/16