Laserfiche WebLink
Model Dezulopment - Conceptaralization and Calibration <br />The Groundwater Modeling System (GMS Version 5.0) developed by the Department of <br />Defense was used to create the input files for use in the MODFLOW model. Initially, a <br />single layer 40,000-cell grid was created encompassing the model area. The conceptual <br />model was then developed by creating point, polyline, and pol}gon features representing the <br />various real-world features such as rivers, wells and no-flow boundaries. Figure-2 shows the <br />conceptual model of the area that a~as created for the purposes of the various simulations. <br />The conceptual model of the area consisted generally of constant head boundary conditions <br />along Boulder Creek and constant flux boundary conditions along the north, south and east <br />boundaries of the model. <br />The model was initially calibrated to match water level data as reported by the tJSGS in <br />2000. Constant heads along the western border of the model were adjusted, and the model <br />was run in an iterative fashion until the resulting water elevation contotus matched historic <br />measurements and a point of stability was established in the model. This initial calibration, <br />shown in Figure-3, was used as the initial water table elevations for all subsequent <br />simulations. <br />Observation data was taken from nine locations in the vicinity of the site and a~as input into <br />the model. The observation data was split into a summer period (April though September) <br />and a winter period (October through March) and average water levels in the observation <br />wells over the two periods was computed as shown in Table-1. From this information, two <br />separate models were cahbrated to represent the summer and winter periods. The model <br />representing conditions in the summer was calibrated to include recharge from Idaho Creek <br />and the Smith & Emmons Ditch, whereas the model representing conditions in the winter <br />did not include these hydrologic features. <br />The resulting water table elevations in the tcvo models were used as a baseline for evaluating <br />the effects of the dewatering operation at the Duckworth Pit during the summer and winter <br />periods. Figure-4 shows a comparison of the current (baseline) water table elevations during <br />each period. <br />Simulation oflmpacts of Mining Operation <br />During the rnining operation, trenches exposing groundwater will be excavated and water <br />will be pumped out of the trenches in order to dewater the area to be mined. The site will <br />be mined in two phases, each phase mining the respective area associated with the two lakes. <br />It is anticipated that once one lake is completed, it will continue to be dewatered while the <br />other lake is mined. The exact configuration of the dewatering trenches is not known at this <br />time, thus in order to simulate the effects of the dewatering operation, it was assumed that <br />the water levels in the two lakes would be maintained at a level just above bedrock, at an <br />elevation of 4855 feet. This is a highly conser-eative assumption due to the fact that the pits <br />will only be fully dewatered for a short period of time over the two-~~ear life of the mining <br />operation. Also, during the short. period when the pit is fully dewatered, steady-state <br />conditions will take some time to occur, and may not occur prior to the cessation of <br />dewatering at the site. <br />Lafarge West. Inc. J- Applegate Group, Inc. <br />Duckworth Pit Groundwater Study March, 2004 <br />