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1.0 GROUNDWATER MODEL <br />1.1. INTRODUCTION <br />This study has been developed to address the potential effects of the File Pit 112 <br />mining operation on the local surface and groundwater. <br />The File Pit 112 will mine gravel below the natural water table. The operation will <br />utilize dry mining techniques and will trench the perimeter of the pit and pump <br />from the trenches to dewater the gravel within the pit area. Dry mined pits <br />typically create a cone of depression locally during operation. The reclamation <br />plan will line the excavations with compacted clay of low permeability (0.03 <br />ft/day). <br />This study utilizes the digital program MODFLOW, which is a three dimensional <br />digital finite difference approximation to the governing equation of groundwater <br />flow. MODFLOW is used as a predictive tool to approximate the potential <br />influence the dewatering operation and the reclamation land configuration may <br />have on the local water table. <br />Formatted MODFLOW input files are available upon request. <br />1.2. MODEL DISCRITI2ATION AND METHODS <br />The finite difference grid measures 5000 ft in the X direction and 4500 ft in the Y <br />direction. The grid is oriented Northwesterly with rotation of 28 degrees west of <br />north. The Grid cell size is set to 50ft. The model consists of three hydro- <br />stratigraphic units consisting of a lower unit of sand and gravel, a middle unit of <br />low permeability clay and an upper unit of silty sand. Top and bottom elevations <br />are based on test hole data and resultant TINS developed in Autodesk Land <br />Desktop Development 3. The TINS were sampled at node centers and exported <br />to MODFLOW via the pre-processor Argus One utilizing the MODFLOW GUI <br />developed by the USGS. <br />1.3. HYDRO-STRATIGRAPHIC UNITS <br />The general physiographic setting can be described as an alluvial valley that <br />generally trends northeast. The project site occurs on the south eastern side of <br />the valley where the flat alluvial terrace meets the steeper flank of the valley. The <br />principal aquifer in the valley is composed of alluvial gravel with an average <br />thickness of 10 ft. Sheet-1 shows a typical cross section and the layering of water <br />bearing units. The section line for Sheet-1 is shown in Sheet-2 <br />1.3.1. Upper Sandy Silt Laver <br />The upper silty sand layer (SH-SM) is 5 to 10 ft thick in the area of the proposed <br />pit. The layer is 6 to 10 ft thick under the area where the Boulder-Weld Canal <br />File Pit 112 <br />Groundwater Modeling Report <br />1 of 6 <br />