Final Environmental Impact Statement - New Federal Courthouse

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Geology, Soils, and Seismic Conditions Affected Environment Information presented in this section is based primarily on data, maps, and other information obtained from the Oregon Department of Geology and Mineral Industries, the U. ~S. Geological Survey, and the Oregon Department of Environmental Quality. Project Location and Topography Alternative site 1, the Riverfront site, is located west of downtown Springfield, Oregon in section 35, township 17 south, range 3 west. Alternative site 2, the Chiquita (formerly Agripac) site, is located in downtown Eugene, Oregon in section 32, township 17 south, range 3 west. The cities of Eugene and Springfield lie in the southern end of the Willamette Valley physiographic region of Oregon. The two alternative sites are located on broad valley terraces within t he alluvial flood lain of th e Willamette River. Each of the sites is located on a relativ P el Y flat surface with slopes ranging from zero to 3 percent. The Riverfront site in Springfield (Alternative 1) is located at an elevation of about 450 feet above mean sea level; the Eugene site (Alternative 2) is located between 420 and 430 feet above mean sea level. Although the Riverfront site has a slight slope, neither of the alternative building sites is situated on steep slopes, and therefore neither site is subject to landslides or severe erosion. Geology and Soils The Malabon-urban land soil complex is the dominant soil mapping unit for the Alternative 1 and 2 sites (USDA SCS 1982). The Malabon unit consists of silty clay loam, which is a deep and well-drained soil formed in silty and clayey alluvium. In some areas the soil has been covered with fill materials, pavement, and buildings, making it difficult to determine the specific soil unit. In these areas the soil is designated as urban land. The Chapman-urban land soil complex is also present along the railroad tracks at the Alternative 2 site (the Chiquita site). Typically, the Chapman unit consists of loam, which is a deep and well-drained soil formed in mixed alluvium. Strata of coarse sand and gravel are common below a depth of 40 inches. In some areas the soil is designated urban land, as previously discussed for the Malabon unit. The surficial geology of both alternative sites consists of unconsolidated alluvial deposits, subdivided into older and younger alluvium of late Pleistocene and Holocene ages, originating from the Willamette River system. The younger alluvium consists largely of sand, gravel, and cobbles with lesser amounts of silt and clay (LTSGS 1973). These deposits range in thickness from a thin veneer along the margins of the floodplain to about 35 to 40 feet near the river. The older alluvial deposits consist of lenses of coarse volcanic sand and gravel with fine sand and silt New Federal Courthouse 39 Final EIS