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Dewatering Evaluation Report <br />Varra Pit 122 <br />Weld County, Colorado <br />Page 3 <br />Model Parameters <br />The effects of dewatering on groundwater flow within the study area were evaluated by using the <br />three dimensional groundwater flow model Visual Mod Flow. The general parameters used in the <br />model are presented below. Plates 1 through 3, depict model boundary conditions. <br />e The model boundary is 8640 feet (east -west) by 9900 feet (north -south); <br />e The model grid is 248 rows by 216 columns or 53,568 cells; <br />e Constant head boundaries were assigned for the dewatering line sinks and for the pit <br />interiors subsequent to lining; <br />e River boundaries were assigned for the unlined ditches; <br />e Barrier walls were assigned near the excavation limits of cells A and B in Pits 116 and 122 <br />to simulate the lining of mined excavations; and, <br />e General head boundaries were assigned to the model limits (north, south, east and <br />west). <br />A uniform flow field was defined in the model with an unconfined aquifer. The natural <br />groundwater flow direction is to the northwest within the model boundaries. Water levels <br />obtained from published water level data was used to generate water level contours unaffected <br />by any pumping influences (pre -mining). Monitoring wells within the Pit 116 and Pit 122 mine <br />boundaries were used to calibrate the model based on current conditions. Ground surface and <br />bedrock elevations were obtained from site surveys, drill hole data and USGS maps. Ground <br />surface and bedrock elevations were input into the geo-statistical model Surfer®, which created <br />surface and bedrock contour maps. These maps were imported into Visual Mod Flow to define the <br />ground surface and bedrock elevations within the flow model. The river stage elevations (ditches) <br />were estimated from survey and map data. <br />As mentioned, constant head boundaries were used to simulate pumping from the proposed Pit <br />122 and the existing Pit 116. The model was calibrated by using model assigned observation wells <br />inside and outside of the proposed excavations. A pumping simulation for Cell A of Pit 116 was <br />ran using current elevations for the drainage ditches within the excavation. Measured water