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The shallow alluvial aquifer was modeled as a single homogeneous aquifer (sand and small gravel) underlain by <br />impervious claystone bedrock. The model parameters such as site topography, bedrock elevations, geologic conditions, <br />recharge and hydraulic conductivities are discussed in the following sections. <br />3.0 MODEL PARAMETERS <br />3.1 Site Toaographv <br />A digital elevation model was built for the Site topography based onten-foot USGS digital quadrangle data using <br />AUTOCAD. The total surface elevation gain from the north to the south is approximately 10 feet. <br />3.2 Regional Topography <br />The surrounding topography for the 10 square miles modeled varies from flat slopes immediately east of the Site to <br />moderate slopes to the west. The surface topography for the area around the Site was taken from USGS digital <br />quadrangle (the Fort Lupton 7.5 minute quadrangle and the Brighton 7.5 minute quadrangle). <br />3.3 Site Geologv <br />Subsurface data from earlier aggregate investigations was used to identify the Site geology. Geotechnical investigations <br />were completed at the Site by the following consultants: <br />• Rocky Mountain Consultants (30 geotechnical borings, July ,1997 and February 1998) <br />• Hall-Irvvin Corporation (10 piezometer installations) <br />The investigations focused on overburden soils and sand, aggregate quality and insitu claystone bedrock samples in <br />order to obtain geotechnical index and engineering properties for a slurry wall design. Available boring location data <br />were approximated along with piezometer locations, as seen in Figures 3 & 3a. <br />The surficial geology at the Site was also mapped as part of Civil Resources' studies. Geologic mapping and exploratory <br />drilling at the Site identified the major natural geologic strata. The units pertinent to this model are: 1) alluvial sand and <br />gravel and 2) claystone bedrock. These units are described more completely as follows: <br />Piney Creek & Post Piney CreekAlluvlum Qpc, Qpp: The alluvial deposit underlying the Site is primarily composed of <br />Piney Creek & Post Piney Creek Alluvium and extends approximately one-hall mile on either side of the South Platte <br />River. The alluvium generally consists of dark-gray sand and gravel, containing organic matter. The alluvium generally <br />underlies stream terraces with surfaces approximately 10 to 20 feet above the nearby floodplain. Locally, the deposit <br />includes some Post-Piney Creek alluvium (Colton, 1978). <br />Denver and Arapahoe Claystone Tkda: Localized outcroppings of the Denver and Arapahoe formations are located a <br />couple of miles southwest of the Site along the Todd Creek Drainage and generally represents a locallized no-flow <br />boundary of the groundwater model. These geological formations are classified as olive gray claystone and siltstone <br />interbedded with tuffaceous sandstone and conglomerate. The formation is approximately 565 feet thick (Colton, 1978). <br />3.4 Repional Geologv <br />The model covers approximately 10 squares miles, around the approximate one-half of a square mile Site as presented <br />in Figure 1. The geologic units surrounding the Site were obtained from geologic mapping (Colton, 1978 & Trimble & <br />Machette, 1979). The units are defined as follows: <br />Groundwater Model Repod -Hall-Irwin Parker•Dersham Site - 2 - April 2006 <br />