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Laramie-Fox Hills were used in the final contouring, due to the presence of steeply <br />dipping, faulted, and overturned beds in other areas included in the HA-742 report. <br />These digitized points provided additional detail for the top and bottom elevations <br />along the northwest corner of the Laramie-Fox Hills. <br />The August 2005 edition of HydroBase containing the data discussed above has <br />approximately 21,187 picks for the aquifer top, bottom, and net sand thickness for the <br />following aquifers: <br />^ Upper Dawson Aquifer <br />^ Lower Dawson Aquifer <br />^ Undivided Dawson Aquifer <br />^ Denver Aquifer <br />^ Upper Arapahoe Aquifer <br />^ Lower Arapahoe Aquifer <br />^ Undivided Arapahoe Aquifer <br />^ Laramie-Fox Hills Aquifer <br />1.2 Data Analysis and Processing <br />The aquifer configuration data found in HydroBase were processed and analyzed to <br />develop structural contour maps of the tops and bottoms for each bedrock aquifer and <br />also to develop net sand thickness (isopach) maps. <br />1.2.1 Denver Basin Bedrock Aquifer Nomenclature <br />The current designation of the Denver Basin Bedrock Aquifers was established in 1985 <br />during the promulgation of the Denver Basin Rules, 2 CCR 402-6 (DWR 1985a). The four <br />principal aquifers of the Denver Basin are, from bottom to top, the Laramie-Fox Hills, <br />Arapahoe, Denver, and Dawson Aquifers. The Arapahoe and the Dawson Aquifers are <br />further subdivided into upper and lower units over portions of their extents, resulting in <br />a total of six bedrock aquifers in the Denver Basin. The bottom of each aquifer is <br />generally defined by the presence of relatively thick series of sand-rich layers underlain <br />by a layer of shale or mudstone that can be identified over large areas on geophysical <br />logs. A plan view of the footprint of the six aquifers is shown in Figure 2. The footprints <br />for the divided Dawson and Arapahoe Aquifers approximate the extent of the confining <br />units that divide the respective aquifers into upper and lower units. <br />For administrative purposes in the Denver Basin Rules, the undivided portions of the <br />Dawson and Arapahoe Aquifers were assigned to the Upper Dawson and Upper <br />Arapahoe Aquifers, respectively. However, for the purposes of the SPDSS study, a <br />physical definition for the Dawson and Arapahoe is used, and the undivided portion of <br />each of these aquifers is referred to as the Undivided Dawson or the Undivided <br />Arapahoe. This nomenclature was adopted because in the northern half of these aquifers <br />exists a continuous confining unit that divides the aquifer forming an upper and lower <br />unit for each aquifer. However, to the south, this confining unit pinches out leaving one <br />undivided aquifer. Figure 3 illustrates a generalized cross-section of the Arapahoe <br />Aquifer and the difference between the 19851ega1 definition and the SPDSS physical <br />definition. <br />SPDSS Phase 2 Task 42.2 TM -Final 4 <br />2/13/06 <br />