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Groundwater Evaluation For Proposed Heit Aggregate Operation <br />Weld County, Colorado <br />The model grid is bounded by the extent of the alluvium to the west, the 4,860-foot water table <br />contour to the south, the centerline of the South Platte River to the east, and the 4,840-foot water <br />table contour to the north. A uniform cell size of 50 feet in both horizontal directions results in a <br />grid with 280 rows, 259 columns, and approximately 24,000 active cells. <br />The bottom of the aquifer was set to the bottom of the alluvial material (See Figure 1). A <br />number of points along the contours defined by Hurr, et al (1972) were used as a data set, which <br />was then interpolated to the MODFLOW grid. <br />The north, east, and south boundaries of the model are represented as specified-head boundaries. <br />The north and south boundaries were assigned elevations of 4,840 and 4,860 feet, respectively, <br />along their entire lengths. The head along the east boundary varied linearly between the 4,840- <br />and 4,850-foot contours and between the 4,850 and 4,860-foot contours on the west side. The <br />shape of the water table contours on the west side of the model indicates that there is some <br />recharge occurring from the western boundary. This boundary was therefore represented in the <br />numeric model using the General-Head Boundary Package; the head was set ] 0 feet above the <br />water table and the conductance was varied until the simulated water table elevations roughly <br />matched the observed elevations. Also, it was assumed in all runs that 10 percent of the average <br />annual precipitation of I S inches recharges the alluvial aquifer. <br />Lastly, it is assumed that the slurry wall that will be installed prior to removal of the aggregate <br />will form an impermeable barrier surrounding the pit. Thus, the proposed pit and the existing pit <br />to the south were represented as no-flow boundary conditions. <br />4.2 Model Results <br />Two different conditions were considered in this study. First, a baseline run was performed to <br />match the water table elevations of Hurr, et al (1972). Once the model adequately represented <br />the conditions of Hun, et al, the water well to the north of the Heit Pit (Permit No. 50354-F) was <br />included in the model and a base run was conducted. The well was inserted at column 167 of <br />row 151 and was assigned an extraction rate of 125 gpm, which assumes that the well produces <br />021-110.030 Wright Water Engineers, Inc. Page 4 <br />March 2003 <br />