I-10 Twin Peaks Traffic Interchange, Environmental Assessment

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Affected Environment and Environmental Impacts October 2005 Grabe series soils consist of well-drained loams, gravelly loams, and silty clay loams. These soils form in recent alluvium deposited in flood plains, alluvial fans, and valley slopes. Slopes range from zero to three percent. Permeability of these soils is rapid and the hazard of erosion is slight. Grabe gravelly sandy loams are generally found in the alluvial fans of the Santa Cruz River Valley. Impacts No Build Alternative The no build alternative would have no effects on soils in the project area. Preferred Alternative During construction, disturbed soils in the project area would be vulnerable to erosion. This is especially true for soils that are highly susceptible to erosion by water or wind. According to the soil survey, soils present in the project area have a slight to moderate hazard of erosion. Prior to construction, a Storm Water Pollution Prevention Plan (SWPPP) and Dust Control Plan would be developed and implemented. Management practices contained in these plans would minimize soil erosion from stormwater runoff and wind as a result of the preferred action. Mitigation The SWPPP, which is discussed in detail in the National Pollutant Discharge Elimination System (NPDES)/Arizona Pollution Discharge Elimination System (AZPDES) Section entitled NPDES/AZPDES/SWPPP (page 4-21), would outline the implementation sequence of erosion and sediment control measures. These may include stabilization practices, structural controls, storm water management measures, and best management practices to mitigate the water erosion of soils. In addition, an activity permit from the Pima County Department of Environmental Quality and a grading permit from the Town of Marana would be obtained to limit the amount of dust generated from construction activities (see Air Quality Section, page 4-45). Geologic Setting and Mineral Resources Existing Conditions Regional Geology The project site is located within the Tucson basin, which is a sub-area of the Upper Santa Cruz River drainage basin (Davidson 1973, Anderson 1987). The Tucson basin is a structural depression within the Basin and Range physiographic province. The basin is filled with sediments and generally trends north to northwest. The Town of Marana, and this project, is located in the northwestern part of the basin. The primary formations of interest for this project are, in descending order, the Fort Lowell Formation and the Upper Tinaja Beds. Both the Fort Lowell Formation and the Tinaja beds were developed as a result of sedimentation in a Interstate 10 Traffic Interchange at Twin Peaks/Linda Vista 4-6 Project No.: NH-010-D (AIW) TRACS No.: 10 PM 236 H5838 01D
Affected Environment and Environmental Impacts October 2005 closed basin of internal drainage. The Fort Lowell Formation, which is generally 300 to 400 feet thick, grades from silty gravel near the edges of the basin to silty sand and clayey silt in the central part of the basin. In most of the basin, the Fort Lowell Formation was deposited in fans by streams that spread out from the canyons in the surrounding mountains. The Tinaja beds, which vary from less than one foot to more than 2,000 feet thick and have up to three subunits, consist of gravel and sand (upper bed) that grade into a very thick sequence of gypsiferous clayey silt and mudstone in the center of the basin (lower bed). The Fort Lowell formation is early and middle Pleistocene in age, while the Tinaja beds range in age from Miocene to Pliocene. Tilting, accompanied by minor faulting, ended the sedimentation of the Fort Lowell Formation and initiated erosion and the early stages of the present drainage system, including the deposition of young Quaternary alluvium along the Santa Cruz River. Localized Surficial Geology Since the project site is located within or close to the range of influence of the meander migration of the Santa Cruz River, it can be expected that the near surface soil deposits would exhibit gradational characteristics ranging from fluvialdeposited silty and clayey soils to sandy and gravelly deposits. Below these surficial deposits, dense layers of sand and gravel of the Fort Lowell Formation with varying quantities of silt and clay would be encountered. The base of the Fort Lowell Formation is expected to be at approximately 2,000 feet, or approximately 130 feet below existing grade, and so the very dense Upper Tinaja unit is unlikely to be encountered on this project. Seismic Conditions Based on ADOT seismic acceleration maps (Euge, Kenneth, and Schell 1992) and AASHTO Standard Specifications for Highway Bridges, seismic loading is not incorporated in the design of the bridges. Furthermore, there are no special seismic design requirements for the foundations and abutments of bridges in this category. Mineral Resources Sand and gravel mining operations have operated and continue to operate within the study area. An existing sand and gravel mining operation (I-10 Avra Valley Mining and Development) is located on the west side of the Santa Cruz River south of Avra Valley Road, which is north of the project area. Although no longer a mining operation, a former sand and gravel mine was located in the northern portion of the project area between the Santa Cruz River and I-10. This area is now used for industrial operations. An active sand and gravel mining operation (Rinker Materials) is located in the southern project area immediately south of the south access road. In addition to sand and gravel mining, Arizona Block and Brick, in the southern portion of the project area mines and processes raw material on-site into adobe block. Interstate 10 Traffic Interchange at Twin Peaks/Linda Vista 4-7 Project No.: NH-010-D (AIW) TRACS No.: 10 PM 236 H5838 01D
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Interstate 10 Traffic Interchange a
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Draft Environmental Assessment Octo
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Mitigation Measures October 2005
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Conclusion October 2005 Interstate
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Bibliography October 2005 CHAPTER 8
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Bibliography October 2005 Tetra Tec
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I-10 Twin Peaks Traffic Interchange, Environmental Assessment

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