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2.3 Groundwater Monitoring Groundwater monitoring of the site was performed by AGS and Nations Groundwater Associates (NGA) on seven occasions between June of 1987 and November 1992 AGS and NGA reported that groundwater flowed consistently to the southwest between June 1987 and April 1991 However, in a report dated February 2, 1993, NGA calculated a northeasterly groundwater flow during the November 1992 monitoring This direction may be a one-time anomaly or, more likely, may be based on erroneous data (Mr Harlin Knoll - PHS/EHD, personal communication, 1994) ' Hydrocarbons were detected in groundwater samples from the monitoring wells during each monitoring event (Table 2) Concentrations have varied over time, but no clear pattern has emerged Relatively low levels of benzene, toluene and xylenes were detected during the November 1992 groundwater monitoring event ' 2.4 UST Removal The USTs were removed from the site in March of 1990 Eight samples were analyzed by EPA methods 8240 and 8260 for volatile and semi-volatile hydrocarbons Gasoline-impacted soil was detected in samples collected beneath the tanks According to the San Joaquin County Public Health Services- Environmental Health Division (PHS/EHD), approximately 800 cubic yards of gasoline-contaminated soil were removed from the tank excavation and stockpiled on-site 2.5 Further Investigation ' Between January 11 to 17, 1994, four soil borings were drilled at the site Borings MW-5 and MW-6 were advanced to a depth of 85 feet and completed as groundwater monitoring wells Borings B7 and B8 were advanced to a depth of 65 feet and backfilled the entire depth with Portland cement No petroleum hydrocarbons were detected in any of the 14 samples analyzed from these four borings ' The stratigraphy varies considerably across the site Clayey to sandy silts are present from the surface to 6 to 12 feet The silt overlies moderately-graded to poorly-graded sand, which extends to 25 or 30 feet in all borings A relatively widespread clay or clayey silt bed is ' present from 35 to 40 feet, but below 30 feet the dominant lithology is sand in all borings to a depth of 78 feet Greenish gray silt is present from 78 feet to the bottom of borings MW-5 and MW-6 (85 feet) TPH asg asoline was detected in the water sample from MW-2 at 85 ppb Benzene was detected at 2 4µg/L and 2 7µg/L in samples from MW-2 and MW-6 respectively Toluene ' was detected below 1 µg/L in MW-2 and MW-5 3en6yo1.4adq Seam.I.. A Lodzi-d B--.wp2,9 9416665 `F 1