2024-11-04_PERMIT FILE - M2024056 (6)

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Dewatering Evaluation Report Asphalt Specialties Company,Inc. Weld County,Colorado Page 6 Table 4—Phase II Post Slurry Wall Water Levels With Drain&Gallery MW-1 250 4665.06 4665.42 4666.22 -0.80 4665.43 MW-2 162 4659.00 4658.72 4658.71 0.01 4658.72 MW-3 50 4663.04 4664.34 4664.24 0.10 4664.31 MW-4 488 4660.47 4659.64 4659.56 0.08 4659.64 Note—Column 6 is the difference between column 4 minus column 5 Water captured by the drain will be diverted to an infiltration gallery which will be located on the north side of the Phase I and Phase II pits.The approximate location is depicted on Plate 18. The infiltration rate was determined by trial and error until the gallery inflow equaled the drain outflow. A flow calibration graph is presented as Plate 19. Tables 3 and 4 present the predicted water levels with the infiltration galley recharging recovered groundwater. Plates 20 through Plate 21A show the model calibration graphs. A review of these graphs show the infiltration gallery will not adversely affect the Aurora recharge ponds. Results A review of Tables 1 and 2 show that the groundwater hydrology exhibits minor shadow and mounding effects from the slurry walls with very minimal influences from dewatering.Wells MW- 1 and MW-3 are located on the upstream side of the barrier walls and MW-2 and MW-4 are on the downstream side of the walls.The model predicts mine dewatering will result in a drawdown of less than one foot in all directions from the mine faces.A review of Tables 3 and 4 show that groundwater mounding,as minimal as it is,will be mitigated by installing a drain as described.In all cases the predicted head levels immediately up and downgradient of the slurry walls are within the range of normal seasonal water table elevation changes.The infiltration of water captured by the French drain will not affect water levels within the Aurora recharge zones. Conclusions The results of numerical solutions indicate that the proposed mine dewatering activities will not adversely affect the regional groundwater hydrology. In the author's opinion one cannot reasonably differentiate the head differences of seasonal groundwater fluctuations and head differences created by mining activities—they are both within expected seasonal head variations. The placement of slurry walls substantially mitigates the effects of dewatering and will substantially reduce the pumping rate required for dry mining. If required, groundwater mounding resulting from the placement of slurry walls can be mitigated by installing a single drain and associated infiltration gallery.