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gently to the east, exhibiting the asymmetric synclinal shape of the Denver Basin <br />formations. A comparison of the top and bottom elevations of the Denver Aquifer <br />indicates that the thickness of the aquifer ranges up to 1,100 feet. <br />Figure 13 presents the net sand thickness of the Denver Aquifer. There are 852 database <br />picks with additional control picks used to contour the net sand thickness (Table 1). Net <br />sand thickness ranges upward of 400 feet along the western edge of the basin in east- <br />central Douglas County. This area of thickest sands is near the mountain front and the <br />sand thickness lessens to the east. Areas of relatively high net sand thickness of over <br />300 feet also occur in the middle of the basin near where Arapahoe, Douglas, and Elbert <br />Counties meet. <br />3.3 Arapahoe Aquifer Configuration <br />Figure 14 presents the top elevation contours of the Arapahoe Aquifer in its divided and <br />undivided portions. The entire Arapahoe Aquifer extends from Jefferson County in the <br />west to Arapahoe County in the east and from Weld County in the north to El Paso <br />County in the south, covering a total area of approximately 4,670 mil. A total of 1,589 <br />database picks with 220 additional control picks for the top elevation of the Arapahoe <br />Aquifer were used to contour this surface (Table 1). As shown in the figure, an area of <br />localized highs in this surface exist in the southeast portion of the contoured area, which <br />is caused by control points used to constrain the thickness of the overlying confining <br />unit to values consistent with the rest of the confining unit. The elevation of the top of <br />the Arapahoe Aquifer ranges from 4,600 to 6,300 feet, with the lowest point in the basin <br />occurring in the same location as the low point of the bottom of the Denver, in Township <br />6S, Range 66W. The geophysical log picks are distributed throughout the Arapahoe <br />Aquifer extent, with the majority of picks residing in the northern half of the basin. <br />There are areas of steep dips along the western edge of the basin, representing the <br />asymmetrical shape of the Denver Basin. <br />The bottom elevation of the Upper Arapahoe Aquifer, which is also the top of the <br />Arapahoe confining layer, is shown in Figure 15. The divided region of the Arapahoe <br />Aquifer represents an area of approximately 1,195 mil and is located primarily within <br />Denver, Adams, and Arapahoe Counties. There were 635 database picks with no <br />additional control picks used to contour the bottom of the Upper Arapahoe Aquifer <br />Table 1). Bottom elevations range from 4,500 to 5,500 feet with the highest elevation <br />areas occurring near the northwest corner of the aquifer. Several picks are plotted just <br />outside the aquifer extent in Township 5S and Range 68W, which may indicate that the <br />confining unit between the Upper and Lower Arapahoe Aquifers extends beyond the <br />outline shown. A comparison of the top of the Arapahoe and bottom elevations of the <br />Upper Arapahoe Aquifer indicates that the Upper Arapahoe's thickness extends to 200 <br />feet. <br />The top elevation contours of the Lower Arapahoe Aquifer, which is also the bottom of <br />the Arapahoe confining layer, are shown in Figure 16. A total of 618 database picks with <br />one additional control pick were used to contour this surface. Elevations range from <br />4,400 feet to 5,400 feet with the lowest elevations occurring along the south-central <br />boundary and the highest elevations occurring along the northwestern boundary. <br />SPDSS Phase 2 Task 42.2 TM -Final 16 <br />2/13/06 <br />