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indicated in the table. The 90th-percentile residual values generally are low compared to <br />the range of values for the surfaces and the contour interval selected, indicating that the <br />resulting contours can be viewed with confidence. The net sands grids typically have the <br />highest 90-percentile residual indicating the high variability of values observed in those <br />datasets. <br />Table 3: Summary of Absolute Value of Residuals for Aquifer Tops and Bottoms and <br />Net Sand Thickness Grids in Feet <br /> <br />Surface Median Residual <br />(feet) 10th Percentile <br />Residual (feet) 90th Percentile <br />Residual (feet) <br />Dawson top 7.1 1.4 17.7 <br />Upper Dawson bottom 7.8 1.2 28.7 <br />Lower Dawson top 8.8 1.7 25.9 <br />Dawson bottom 6.7 1.0 27.0 <br />Upper Dawson sands 32.6 9.3 87.9 <br />Lower Dawson sands 18.2 3.4 45.2 <br />Dawson sands 27.0 5.7 81.1 <br />Denver top 7.9 1.0 28.0 <br />Denver bottom 5.7 0.8 23.0 <br />Denver sands 35.2 6.5 90.8 <br />Arapahoe top 5.3 0.8 19.7 <br />Upper Arapahoe bottom 3.7 0.6 14.4 <br />Lower Arapahoe top 4.2 0.6 15.8 <br />Arapahoe bottom 4.2 0.6 14.7 <br />Upper Arapahoe sands 13.9 3.2 38.6 <br />Lower Arapahoe sands 10.5 2.1 30.6 <br />Arapahoe Net sands 21.8 2.8 63.6 <br />Laramie-Fox Hills top 2.4 0.4 11.0 <br />Laramie-Fox Hills bottom 2.4 0.4 12.4 <br />Laramie-Fox Hills sands 9.8 1.7 26.1 <br />1.2.9 Construction of Aquifer Configuration Cross-sections <br />A series of cross-sections were constructed to aid in the visualization of the bedrock <br />aquifer configuration and overlying alluvial deposits. These profiles were developed <br />using the top and bottom structural contour maps presented in Section 3 and using a <br />USGS National Elevation Dataset 30 meter DEM. Grids for each aquifer surface were <br />imported into the Groundwater Modeling System (GMS) software package, and they <br />were then interpolated onto an approximately 0.6-mile by 0.6-mile grid (Brigham Young <br />University -Environmental Modeling Research Laboratory, 2000). GMS is a graphical <br />pre- and post-processing groundwater modeling package with extensive visualization <br />capabilities. <br />Twelve layers were used to define the hydrostratigraphic units of the Denver Basin <br />Region for the purpose of generating cross-sections, as shown in Table 4 below. The <br />confining units associated with the Dawson and Arapahoe Aquifers (layers 3 and 9) are <br />the aquitard layers that separate the upper and lower portions of these aquifers where <br />SPDSS Phase 2 Task 42.2 TM -Final 11 <br />2/13/06 <br />