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• A more detailed description of general site physiography and geology, data collection, database <br />structure, and data analysis conducted as part of this investigation is presented in the following sections. <br />2.0 GENERAL SITE PHYSIOGRAPHY AND GEOLOGY <br />The Tamarack SWA property comprises approximately 10,500 acres extending over 16 miles along the <br />South Platte River in northeast Colorado. Highway 55 divides the property near the center into two <br />areas. West of Highway 55 the DOW property consists of narrow ownership centered on the South <br />Platte River. East of the highway includes both a river corridor and an extensive amount of upland area <br />south of the river. This eastern property is the area where recharge activities are centered and is the focal <br />point for this study. <br />Two primary physiographic features are evident in the study area. <br />The South Platte River Valley: The river occupies a broad valley with a width of 1 to 4 miles. <br />Immediately adjacent to the South Platte River the ground surface is fairly flat and is typical of river <br />flood plain topography. <br />Sand Hills: Approximately 500 to 4000 feet southeast of the river, the topography rises and forms a <br />terrace of sand dune deposits approximately 50 to 100 feet above the river floodplain. <br />The area along the north side of the valley, outside the study area, is a nearly level terrace deposit of <br />alluvium overlain with loamy eolian materials. These well- drained terrace areas have some of the most <br />productive soils for irrigated agriculture in the region. <br />A summary of the key hydrogeologic features of the South Platte River Valley alluvium and dune sand <br />deposits were presented by Bjorklund and Brown (195 7) and are outlined below: <br />South Platte River Valley Alluvium <br />• The South Platte River Valley alluvium consists mainly of heterogeneous mixtures of clay, sand <br />and gravel or lenses of these materials. In addition, extensive lenses of clay are present within <br />the alluvium. Clay lenses occur more frequently in the tributary valleys and probably represent <br />shallow -lake deposits. The lenses of silt, sand, and gravel were deposited by braided streams as <br />they aggraded their channels. <br />• Alluvium thickness can range from less than 1 foot at the valley edge to 293 feet in USGS test <br />hole B10-48-11 ac. This test hole is located at the Tamarack site near DOW Hill well WH -9. <br />• Alluvium ranges in age from early Pleistocene through Recent and includes both Pleistocene and <br />Recent terrace deposits and Recent flood plain deposits. <br />• Lateral and vertical gradation in the alluvial material causes yields from wells in these deposits to <br />range widely. <br />3 Bjorklund, L.J., and R. F. Brown, 1957 Geology and Ground -Water Resources of the Lower South Platte River Valley <br />Between Hardin Colorado, and Paxton Nebraska United States Geologic Survey (USGS) Water Supply Paper 1378. <br />2 <br />