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Mining and Water Storage Analysis <br />Raptor Pit 125 <br />Weld County, Colorado <br />Page 3 <br /> <br />Water levels obtained from published and measured water level data were used to generate <br />water level contours. Ground surface and bedrock elevations were obtained from site surveys, drill <br />hole data and USGS maps. The ground surface and bedrock elevations were input into the geo- <br />statistical model Surfer®, which created surface and bedrock contour maps. These maps were <br />imported into VMOD to define the ground surface and bedrock elevations within the flow model. <br />The river stage elevations (Saint Vrain Creek and ditches) were estimated from survey data and <br />estimated groundwater elevations. <br />The model was calibrated by adjusting the general head boundary heads and gradients to <br />simulate available groundwater level data. Measured water levels (obtained from Raptor <br />monitoring well data ) were compared to the model predicted elevations. The model was adjusted <br />using trial and error methods of reassigning river boundary conductance and general head <br />elevations until the model predicted water table elevations closely matched measured water <br />levels. Once the model was calibrated four model assigned observation wells were placed up <br />and down gradient of tracts A, B and C. The calculated head values for the model assigned wells <br />were then used as observed levels so that subsequent model simulations would predict changes to <br />groundwater hydrology compared to the calibrated simulation. As mine dewatering is occurring <br />to the east and south of the model area an elevation of 4755 was assigned for mine dewatering <br />elevations for all mines. Model generated groundwater contours and calibration graph for the <br />calibration run are presented in Attachment A. <br /> <br />To evaluate the effects of dry mining (dewatering) and water storage (lining the three cells) on the local <br />groundwater hydrology, constant head boundaries were used to simulate dewatering and lake <br />boundaries were assigned to simulate water storage. To simulate filled ponds, the interior of the <br />reservoirs were assigned a head value of 4795, which is slightly below ground surface. An <br />elevation of 4775 was assigned to pit constant head boundaries to simulate the ultimate <br />dewatering depth. Model boundary conditions are depicted in Attachment B. <br />Five simulations were ran and included the following. <br />1. Calibration Simulation <br />2. Cell A Dewater Simulation <br />3. Cell A Lake – Cell B Dewater Simulation <br />4. Cell A Lake – Cell B Lake – Cell C Dewater Simulation <br />5. Cells A, B and C – Lake Simulation