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17 September 2001 AGE-NC Project No. 98-0468 Page 3 of 7 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),ethylbenzeile(23 ppm),total xylene(150 ppm),TPH-g(3,800 ppm), and TPH-d(45 ppm). The majority of the reported detections of petroleum 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 activities consisted of the advancement of seven soil probes (SB--1'through SB-7) and the collection of soil and in-situ ground water samples for laboratory analysis (Figure 2). Laboratory analysis of soil samples detected BTEX, TPH-d, TPH-g and methyl tertiary-butyl ether (MTBE) for samples .collected from soil probe boring SB-5: In addition,' relatively low concentrations of petroleum hydrocarbon compounds were detected for soil samples collected from 913-4 and SB-7. The analytical results of ground water samples collected from the soil probe borings suggested that ground water has been extensively impacted by BTEX, TPH-d, TPH-g and MTBE. The greatest petroleum hydrocarbon impacts,to ground water were reported for soil probe boring locations SB-5 and SB-7 located north of the former diesel UST location. Laboratory analysis of ground water samples collected from soil probe SB-5 detected benzene (6,500 ppb), toluene (35,000 ppb), ethylbenzene (4,500'ppb), total xylenes (25,000 ppb), TPH-d (320,000 ppb), TPH-g (93,000 ppb), and MTBE (560 ppb). On 27 and 28 April 1999, five soil borings (B-1 through B-5) were advanced, B-4 and B-5 were subsequently completed as monitoring wells MW-4°and MW-5, respectively, at locations down gradient (north) of previous soil probe boring SB-7 and the former UST locations, and soil boring.. . B-1 was advanced'at a location up gradient(south)of the former UST locations. Soil boring B-2 was Advanced GeoEnvironmental,Inc_