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amec disposed of at an off-site facility. In December 2003, following a significant rain event, about 22,000 gallons of oil and water mixture were removed using vacuum trucks until the oil and water stopped flowing onto ground surface. An underground storage tank and associated piping was discovered in the area; the underground storage tank and piping were removed and about 100 tons of impacted fill material and native soil was excavated and disposed of at an off-site facility. • Initial Groundwater Investigation. Four groundwater monitoring wells were installed at the water table on site in 2005 by UniFirst. Well locations are shown on Figure 1 and construction information for the wells is summarized in Table 1. • Additional Groundwater Investigation. Additional groundwater investigation was conducted in 2007 and 2008, including cone penetrometer testing to delineate the hydrostratigraphy beneath the site and vicinity, discrete-depth groundwater sampling, soil sampling, and on-site soil vapor sampling. Based on the results of the soil vapor sampling, sub-slab vapor samples were collected inside the UniFirst building and an evaluation of the potential migration of CVOCs into indoor air was conducted. Seven off-site groundwater monitoring wells were installed. Well locations are shown on Figure 1 and construction information for the wells is summarized in Table 1. • Groundwater Monitoring. Regular groundwater monitoring has been conducted at the site by UniFirst since the installation of the off-site monitoring wells in March 2008. The groundwater monitoring network consists of four on-site groundwater monitoring wells and seven off-site groundwater monitoring wells. Groundwater monitoring was performed quarterly during 2008 and semiannually during 2009 and 2010. The results of monitoring are presented in reports submitted by UniFirst to the RWQCB following each monitoring event. • Soil Vapor Remediation. As a result of vapor sampling from the UniFirst building sub-slab and inside the UniFirst building, in 2009 a sub-slab depressurization system was installed to reduce the potential for vapor-phase tetrachloroethene (PCE) migration from the sub-slab to indoor air in the UniFirst building. 4.0 GROUNDWATER CONDITIONS Groundwater conditions beneath the UniFirst site and vicinity are described in this section, including the geologic setting, site-specific hydrogeology, and groundwater occurrence. Also described in this section is the native groundwater quality beneath the site and vicinity. 4.1 GEOLOGIC SETTING The City of Stockton is located on the lower flood plain of the San Joaquin River at the eastern edge of the California Delta, a large delta formed by the confluence of the San Joaquin and Sacramento Rivers. The area is underlain by marine and continental sedimentary deposits. The shallow sediments beneath the Stockton area are a mix of predominantly fine-grained alluvial fan deposits and delta deposits that consist of unconsolidated, interbedded clays, silts, sands, and gravels designated as Quaternary Flood Basin Deposits (Geomatrix, 2007a). AMEG Geomatrix, Inc. IM 3000s1132901Archive113290-49.DOC 3