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2.0 MODEL PURPOSE AND DESIGN <br />The objective of this model is to predict ground water levels and flow rates in the <br />alluvium around the Gardels gravel mine. The model is used to predict ground <br />water levels before, during and after mining operations. To accomplish this we <br />used asteady-state single layer model that is convertible from confined to <br />unconfined conditions where water levels drop below the top of the aquifer. <br />To simulate the dewatering effect of the mine we used the MODFLOW drain <br />package. (The superseded model in the May 9th report used perimeter wells to <br />simulate dewatering effects. This resulted in dewatering rates which were much <br />higher than operational experience would suggest.) We used the MODFLOW <br />lake package to simulate the pond that will be created during reclamation of the <br />west pit. <br />2.1. Model Discretization and Methods <br />The finite difference grid measures 5150 feet in the X direction and 1950 feet in <br />the Y direction. The grid is oriented generally East-West with the Y axis rotated <br />12.44 degrees west of north. The Grid cell size is set to 50ft. The model <br />simulates ground water movement in the sand and gravel alluvium as a single <br />hydro-stratigraphic unit. Top and bottom elevations are based on test hole data <br />and resultant TINs developed in Autodesk Larid Desktop Developmerit 3. The <br />TINs were sampled at node centers and exported to MODFLOW via the pre- <br />processor Argus One utilizing the MODFLOW GUI developed by the USGS. <br />2.2. Hydro-stratigraphic Units <br />The general physiographic setting can be described as a broad flat valley <br />generally trending east west. The principal aquifer in the valley is composed of <br />alluvial sand and gravel with an average thickness of 10-15ft. The water table <br />varies seasonally with local irrigation both in the valley and to the north and south <br />of the valley contributing to enhanced recharge and higher water tables during <br />the irrigation season. The gravel layer becomes thinner at the north and south <br />margins of the valley (3-4 ft in areas), and extends out of the valley north and <br />south providing a good conduit for valley gravel recharge from the uplands. <br />Gardels Pit <br />Revised Groundwater Modeling Report <br />1 of 9 <br />