2022-04-04_PERMIT FILE - M2022013 (21)

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Dewatering Evaluation Report Varra Pit 124 Weld County,Colorado Page 3 Model Parameters The effects of dewatering on groundwater flow within the study area were evaluated by using the three dimensional groundwater flow model Visual Mod Flow Pro. The general parameters used in the model are presented below. Plate 1 depicts model boundary conditions. The model grid is depicted on Plate 2. • The model boundary is 12800 feet(east-west) by 10050 feet(north-south); • The model grid is 251 rows by 320 columns or 80,320 cells; • Constant head boundaries were assigned for the dewatering line sinks; • River boundaries were assigned for the South Platte and Big Thompson rivers; • Barrier walls were assigned to the excavation limits of central and northeast and northwest pits for shadow and mounding simulations; and, • General head boundaries were assigned to the model limits (north, south, east and west). A uniform flow field was defined in the model with an unconfined aquifer. The natural groundwater flow direction varies from southeast to east, northeast within the model boundary. Water levels obtained from published water level data and existing monitoring well data were used to generate water level contours unaffected by any pumping influences. Monitoring wells within the mine boundaries were used to calibrate the model based on current conditions. Ground surface and bedrock elevations were obtained from site surveys, drill hole data and USGS maps. The ground surface and bedrock elevations were input into the geo-statistical model Surfer', which created surface and bedrock contour maps. These surface and bedrock elevation data were imported into Visual ModFlow to define the ground surface and bedrock elevations within the flow model.The river stage elevations were extrapolated from survey data. The model was calibrated by using model assigned observation wells outside of the proposed excavations. Measured water levels (obtained from monitoring well data) were compared to the model predicted elevations. The model was adjusted using trial and error methods of reassigning river boundary conductance and general head elevations until the model predicted water table elevations closely matched measured water levels. Water table contours generated from measured water levels (VCI monitoring wells) are presented on Plate 3.The calculated head values for the model assigned wells were then used as observed levels so that subsequent model simulations would predict changes to groundwater hydrology compared to the calibrated simulation.The calibration simulation included seepage from the river boundaries. The calibration simulation is depicted on Plate 4. The model calculated head values referenced above are depicted on Plates 5 and 5A.