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ENSR ' 2.0 Site Status Soil and groundwater investigations conducted at the site provide significant information for evaluating the extent and distribution of petroleum hydrocarbon concentrations in soil and groundwater at the site It is not the intention of this report to document all activities that have ever been performed on the Site, rather, the following sections focus on the site conditions Important to evaluate ongoing remedial activities and potential future activities at the site 2.1 Site Location and Description Unocal operated the former service station at 1665 Pacific Avenue in Stockton, California from 1947 through 1988 The former station is located on the southwestern corner of the Intersection of Walnut Street and Pacific Avenue In 1967, the site was remodeled and three previously installed 4,000-gallon underground storage tanks (USTs), two pump Islands, and six dispensers were replaced with two 10,000-gallon USTs, two dispenser Islands, and one 550-gallon waste oil UST In 1988, the former station budding, dispenser Islands, and three USTs were demolished and removed from the site The site is currently a fiat-lying, vacant lot and the Immediate vicinity has been developed for commercial use Figure 2-1 depicts the site location and Figure 2-2 depicts the site features 2.2 Site History Ag eneral chronology of site activities that occurred between 1947 and 2006 is presented in Table 2-1 In summary, during removal of the two 10,000-gallon gasoline USTs as part of station closure in 1988, petroleum hydrocarbons were detected in soil beneath the USTs at a depth of approximately 14 feet During March and April, 1988, a remedial response action was performed involving the excavation of approximately 1,100 to 1,350 cubic yards of soil from the UST area resulting In an excavation approximately 25 feet x 35 feet x 30 feet deep (groundwater table) The results of post-excavation soil samples from the former gasoline USTs area Indicated residual levels of TPH (maximum of 18 ppm), benzene (maximum of 1 72 ppm), toluene (maximum of 5 01 ppm), ethylbenzene (maximum of 0 58 ppm), and total xylenes (maximum of 3 83 ppm) These maximum concentrations were detected in sidewall soil samples Concentrations in the soil samples from the bottom were less Subsequently, environmental site investigations were conducted between 1988 and 2002 which included the installation of soil borings and monitoring wells to assess site soil and groundwater conditions Impacted soil was detected in Sall borings near each of the former pump Islands Quarterly groundwater mnnitoring has been performed at the site since 1988 The most recent event was conducted during January 2006 An ozone microsparging remediation system was installed at the site to mitigate contaminants in groundwater and was In operation from August 2002 to December 2005 The system was shut off because the groundwater analytical data suggested that contaminant concentrations were not decreasing as the current system efficiency is minimal Table 2-1 Summary of Previous Environmental Activities January 1947 Union Oil Company of California (Unocal) began leasing the site 1967 Station was completely remodeled PHR Consultants (1992) reported that during the remodel, three previously installed 4,000-gallon underground storage tanks (USTs), two pump islands, and six dispensers were replaced with two 10,000-gallon USTs and four dispensers on two pump Islands (ARCADIS, 1999) It should be noted that + during the time of the remodeling, the Stockton, CA area was recovering from a groundwater depression (due to over pumping of groundwater in the area for 2-1 48 o 0 .r April nr' ';)Ob 1