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p Gasoline/Diesel UST Removal In April 1991,a 550-gallon gasoline UST was removed from the site.During removal of the UST, a soil sample was collected in the vicinity of the former fuel dispenser at four feet below surface grade(bsg).Laboratory analysis ofthe soil sample detected total petroleum hydrocarbons as gasoline (TPH-g)and total petroleum hydrocarbons as diesel(TPH-d).In January 1994,a 1,000 gallon diesel UST was removed from the site.During UST removal activities,a soil sample was collected beneath the west end of the former UST at a depth of 11 feet bsg. Laboratory analysis of the soil sample detected TPH-g, TPH-d,benzene,toluene, ethylbenzene, and total xylenes(BTEX). Preliminary Soil and Ground Water Assessment In June 1994,soil and ground water assessment activities were conducted at the site.A total of three soil borings were completed, soil samples were collected for analysis, and three monitoring wells (MW-1 through MW-3)were installed in the soil borings(Figure 2).Relatively low concentrations of TPH-d were detected in two of the six soil samples collected from the soil borings. During the period from June 1994 through September 1995, three ground water sampling events were conducted at the site.During the June 1994 sampling event,laboratory analysis of ground water samples collected from monitoring well MW-3 detected relatively low concentrations of benzene (1.4 ppb),total xylenes(1.7 ppb),and TPH-g(55 ppb). There were no other reported detections of petroleum hydrocarbon compounds in any of the monitoring wells during the June 1994, October 1994, or September 1995 sampling events. Soil Excavation and Sampling In April 1995,petroleum hydrocarbon-impacted soil was excavated and removed from the area of the former diesel UST. During the excavation activities, soil samples were not collected due to encountering the ground water table above the base of the former tank excavation. Therefore, a ground water sample was collected from the excavation. Laboratory analysis of the ground water sample detected elevated levels of benzene (1,000 ppb), toluene (2,000 ppb), ethylbenzene (800 ppb),total xylenes (6,400 ppb), and TPH-g(17,000 ppb). In August 1995,additional petroleum hydrocarbon impacted soil was excavated and removed from the former diesel UST location and soil samples collected for analysis. Laboratory analysis of the soil samples collected from the four sidewalls of the excavation detected benzene(5.9 ppm),toluene (46 ppm),ethylbenzene(23 ppm),total xylene(150 ppm),TPH-g(3,800 ppm),and TPH-d(45 ppm). The majority ofthe reported detections ofpetroleum hydrocarbon compounds were from the east and west sidewalls of the excavation. Subsequently,the City of Stockton backfilled and compacted the excavation. Site Characterization In 1996,The Twining Laboratories,Inc.conducted a site investigation to assess the lateral extent of petroleum hydrocarbon impacts to soil and ground water at the site. The site characterization Advanced GeeEnvironmentaL Inc.