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

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Dewatering Evaluation Report Asphalt Specialties Company,Inc. Weld County,Colorado Page 4 measured water levels (ASCI wells) are presented on Plate 3. The model boundary was superimposed over a topographic map and the author generated hand drawn contours that best reflected measured groundwater and surface water elevations. General head boundaries were assigned to the model perimeter and head elevations were assigned where hand drawn elevations intersected the model boundary. The model was then run and model predicted heads were compared to observed heads.This process took three iterations as water table gradients were fairly uniform throughout the model boundary. After the calibration process was completed the "cell inspector"function was used to determine the predicted head elevation for model assigned wells. The head elevations at the model assigned wells prior to any mining activities were used as a baseline to measure the effects of dewatering and mine slurry wall installations. The calibration simulation included seepage from the river boundaries which included the South Platte River and Godfrey Ditch. The calibration simulation is depicted on Plate 4. The model calculated head values referenced above are depicted on Plate 5. Plate 6 depicts calibration boundary conditions. To evaluate the effects of the slurry walls on the local groundwater hydrology, barrier walls were assigned near the excavation limits of the two pits.Two simulations were then run to predict the effects of the slurry walls on the groundwater hydrology. Plates 7 depicts predicted groundwater contours for post slurry wall conditions for Phase I mining. Groundwater conditions for Phase II mining are depicted on Plate 8. Plates 9 and 10 present model predicted water level graphs for post slurry wall installations. To simulate the effects of mine dewatering constant head line sinks were assigned to cells within the pit areas with head values approximately four feet above the bedrock elevation, which accounts for the predicted seepage face. To account for significant differences in bedrock elevations linear gradients were assigned to the line sinks where appropriate. Predicted groundwater contours resulting from mine dewatering are presented on Plates 11 and 12 for Phase I and Phase II mining. Calibration graphs for the dewatering simulations are presented on Plates 13 and 14. Tables 1 and 2, presents the predicted water level changes resulting from the slurry wall installations and dewatering for each of the two mining phases. Table 1—Phase I Post Slurry Wall Water Levels MW-1 250 4665.06 4664.74 4666.09* -1.35 1.03 MW-2 162 4659.00 4658.82 4658.93* -0.11 -0.07 MW-3 50 4663.04 4663.08 4663.50* -0.42 0.46 MW-4 488 4660.47 4659.83 4660.11* -0.28 -0.36 Note- * Denotes water level without dewatering