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Bulk Transportation Page 3 Site Conceptual Model& Closure Report GPE Project No. 606.3 August 28, 2009 The formation of soils in San Joaquin County is dependent on the sedimentary '- transport path from the primary sediment sources. These sediment sources are the Coastal Range to the west and the Sierra Nevada to the east of the County. Generally, the longer the transport path the greater the working of the sediments prior to deposition as soils. Along the San Joaquin River and in the delta regions west of Stockton, the low energy river and flood plain environments deposit layers of silt and clays during seasonal floods. These ` fine-grained sediments flocculate out of slack water as floodwaters recede. The erosion of the Coast Range forms portions of far western San Joaquin County soils and transport of these sediments is facilitated through seasonal run-off of the minor streams of the Coast Range. These soils are formed in environments similar to those of eastern San Joaquin county, i.e., alluvial fans, but tend to be less well worked in comparison to their Sierran derived counterparts due to the shorter transport path. +� The USDA Soils Conservation Service Soil Survey of Sari Joaquin County, California, October 1992 soil survey report classifies soil in San Joaquin County. The Survey generally classifies the soils in this area of Stockton as part of the Jacktone-Hollenbeck-Stockton association; a nearly level soil in basins or basin rims. Somewhat poorly drained to moderately well drained fine textured soils, moderately deep to deep to a cemented hardpan. The specific on site soil is classified as Urban Land Complex, meaning it is so severely disturbed by human activity as to be unidentifiable as to type. It is, however mostly Jacktone clay, a heavy, dense, clay soil with rapid drainage and poor permeability. Observations taken in 2003 by Hygienetics Environmental during the over excavation of the tank pit indicated that from about 4 to 16 feet below grade, the soil consists principally of yellow-brown silt. At the 16-foot level, a fine- grained silty sand was observed. 2.2 Site History A former 12,000-gallon diesel UST was operated on the site until it was removed in 2002. Confirmation soil samples collected from the tank pit indicated that a leak had impacted the surrounding soil. It appears the ` source(s) of the diesel release were related to dispenser piping rather than a defective tank. The San Joaquin County EHD issued a Notice of Unauthorized Release on November 8, 2002. In 2003, the pit was over-excavated by Hygienetics Environmental, and the diesel-impacted soil was removed and stockpiled on the site. The report of the excavation estimated that 110 cubic yards of soil was removed. Seven �. 2937 Veneman Ave.,#B240 Geo-Phase Environmental Inc. Phone (209) 569-0293 Modesto, CA 95356 Fax (209) 569-0293