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Soil and Groundwater Investigation Report ARCO Facility No 2168 Stockton, Califomia Page 3 Local Groundwater Conditions Since 1995, groundwater levels at this site have risen approximately 20 feet (from approximately 37 feet bgs to 20 feet bgs) The increase in groundwater elevation has not corresponded with a significant increase or decrease in petroleum hydrocarbon concentrations in sampled wells Historically, concentrations of TPHG decrease across the site from upgradient, off-site well MW-6 (1998 and 1999 average 49,300 ug/1) to MW-3 (1998 and 1999 average 24,600 ug/l) to MW-1 (1998 and 1999 average 18,200 ppb) The presence of significantly impacted upgradient groundwater (MW-3, MW-5) entering the site confirms the presence of a regional co-mingled plume Soil Remediation Soil vapor extraction was performed by ARCO to remediate vadose zone soil impacts above 25 feet bgs from January 1997 through February 1998 Vapor extraction wells are screened at intervals ranging from 5 feet to 40 feet bgs Soil vapor extraction was terminated after an estimated mass of nearly 10,000 pounds (approximately 1,600 gallons) of TPHG was removed and influent benzene concentrations were less than �y laboratory reporting limits of<0 5 mg/m3 SOIL AND GROUNDWATER INVESTIGATION Soil Sampling and Monitoring Well Installation Prior to the initiation of field activities, well permits were obtained, the proposed locations were marked, and Underground Service Alert and San Joaquin County PHS/EHD were notified of the work schedule The initial five feet of the borings were advanced with a posthole digger and hand auger to reduce the possibility of damaging underground utilities Between July 31 and August 8, 2000, a Delta representative, under the direction of a California Registered Geologist, was on site to oversee the drilling of five soil borings and installation of five groundwater momtonng wells A representative of PHS/EHD was onsite at various times throughout the drilling and well installation activities The wells were installed in clusters of three monitoring wells, each well approximately four feet apart The purpose of well clusters is to provide vertical characterization of groundwater by collecting samples from screen intervals placed at specific depths Monitoring wells MW-1B and MW-1C were installed adjacent to existing monitoring well MW-1, which is downgradient of the former USTs Monitoring wells MW-8, MW-8B, and MW-8C were installed upgradient of the former USTs, near the southwest corner of the property(Figure 2)