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1.1 Sources of Data <br />Two types of available published data were collected to summarize the configuration of the <br />South Platte Alluvial Aquifer. These include (1) published interpretive contour maps, and (2) <br />individual values (referred to in the TM as "picks") representing the elevation of the bottom of <br />the alluvium. Additionally a U.S. Geological Survey (USGS) 30-meter National Elevation <br />Dataset (NED) grid is used to represent the ground surface. <br />Published geologic contour data were collected from Bjorklund and Brown (1957), Duke et al. <br />(1966), Erker et al. (1967), Hurr and Schneider (1977, 1972a -1972f), NHPQ (1967), Robson (1996), <br />Robson et al. (2000a - 2000d), Smith et al. (1964), and Willard Owens Associates (1971). All of <br />these reports contain elevation contours for the bottom of alluvium and the groundwater level <br />of the alluvium. These reports present a detailed interpretive analysis of the bottom of the <br />alluvium with contour intervals of 20 feet, except for Willard Owens Associates and Duke et al. <br />in the southern end of their report, which presents contour intervals of 100 feet. The <br />groundwater level contours represent measurements from multiple historical time periods and <br />are reported in contour intervals of 10, 20, and 100 feet. Even though the original data used to <br />develop the contour maps are not available for al reports, these reports provide a basis for <br />understanding the configuration of the alluvium. Figure 2 identifies the extent of the contours <br />used from each source. <br />Point values for the bottom of alluvium elevations were gathered and evaluated from several <br />sources, which include SPDSS Task 35, Smith et al. (1964), Bjorklund and Brown (1957), Weist <br />(1964), DWR well permit images, and HydroBase. Under Phases 1 and 2 of Task 35 of the <br />SPDSS, SPDSS alluvial well drilling used geophysical logging, which has the capacity to <br />identify bedrock although it is not as reliable as a physical basis such as discrete coring or <br />review of drill log cuttings. During SPDSS field work, bedrock was identified when the direct <br />push probe encountered refusal accompanied by an increase in electroconductivity on the <br />geophysical log indicating a higher clay lithology had been encountered. If the refusal was <br />caused by coarse sands and gravels locking the direct push probe and not by a hard shale <br />bedrock layer, then this would be in inaccurate determination of bedrock. Fourteen well drilling <br />sites identified the depth to bedrock as described above in Phase 2, and five identified bedrock <br />in Phase 1 (see Figure 2). Details on these wells and their drilling logs are provided in the Task <br />35 Alluvial Well Construction and Testing TM. Details for one of the Phase 1 wells cannot be <br />found in the Task 35 TM since this well was drilled by the USGS for the SPDSS; the USGS report <br />is pending. <br />Additional point values were obtained from the Smith et al. (1964) report, which provided data <br />for the alluvium of Beebe Draw, Box Elder Creek, and the mainstem of the South Platte River <br />from the northern edge of Denver County to Greeley (see Figure 2). These picks were compiled <br />by Smith et al. (1964) and were used to develop the bottom of alluvium contours presented in <br />their report. The picks are presented on a figure from the report, with approximately 1,072 <br />points labeled with the bottom of alluvium elevations. The paper map was scanned, <br />georeferenced, and the picks were digitized. Each digitized pick was electronically assigned its <br />elevation value and its horizontal coordinates. <br />SPDSS Phase 3 Task 42.3 TM -Final <br />11/30/2006 <br />