2016-01-27_HYDROLOGY - M1979059 (2)

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2.0 MODEL PURPOSE AND DESIGN The objective of this model is to predict ground water levels and flow rates in the alluvium around the Flying W gravel mine. Within that objective, the model will also be used to predict the surface water elevations of the lakes in the vicinity and determine the amount of water flowing between the aquifer and the lakes under different operational scenarios. The model is used to simulate and quantify the recirculation flow of water between the aquifer, the gravel mine, an infiltration pond, alluvial wells and the Big Thompson ponds. The model is used to simulate conditions before, during and after mining operations. To accomplish this we used a steady-state two layer model that is convertible from confined to unconfined conditions where water levels drop below the top of each layer. To simulate the dewatering effect of the mine, the MODFLOW drain package was used. (The model we used in the March 9th report used perimeter wells to simulate dewatering effects. This resulted in dewatering rates which were much higher than local operational experience would suggest). The MODFLOW lake package was used to simulate the twelve ponds occurring in the model domain including the ponds of the Big Thompson Ponds State Wildlife Area (Thompson ponds). The lake package also includes the infiltration/sediment pond, which accepts the gravel dewatering effluent. The MODFLOW well package was used to simulate alluvial wells pumping adjacent to the infiltration/sediment pond. 2.1. MODEL DISCRETIZATION AND METHODS The finite difference grid measures 6200ft in the X direction and 5500ft in the Y direction. The grid is oriented North-South with no rotation. The Grid cell size is set to 50ft. The model consists of two hydro-stratigraphic units consisting of a lower sand and gravel unit and an upper low permeability silty clay. Top and bottom elevations are based on test hole data and resultant TINs developed in Autodesk Land Desktop Development 3. The TINs were sampled at node centers and exported to MODFLOW via the pre-processor Argus One utilizing the MODFLOW GUI developed by the USGS. 2.2. HYDRO-STRATIGRAPHIC UNITS The general physiographic setting can be described as a broad flat valley generally trending east west. The principal aquifer in the valley is composed of alluvial sand and gravel with an average thickness of 10ft. The water table varies seasonally with local irrigation both in the valley and to the north and south of the valley contributing to enhanced recharge and higher water tables during the Flying W Pit Groundwater Modeling Report Revised June 28, 2005 2of13