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October 30, 1997 Page 4 • perimeter wells continued to remain below or near the-'detection limit (see graphs in Attachment F) The latest groundwater monitoring report contairung historic analytical data is presented in Attachment F On May 3, 1996, perimeter Wells MW-10, MW-12, and MW-13 along with Well MW-3 were abandoned at the request of the City of Stockton to accommodate widening of Pacific Avenue Efforts are currently underway to located Wells MW-8 and MW-11 which recently were lost during sidewalk and street modifications Before their abandonment, methyl-tert butyl ether (MtBE) was found to be below the detection lin ut iii all perimeter wells The maximum MtBE concentration detected to date in interior wells is 720 micrograms per liter (gg/L) SITE REMEDIATION Soil Excavation (Le.). In May and June 1988, approximately 1,600 cubic yards of hydrocarbon-impacted soil were removed from near the dispenser islands and beneath the three original USTs The excavation beneath the USTs extend to depths of 18 to 22 feet bgs The soil was aerated on site and transported to Forward Landfill in Stockton, California for disposal Soils with TPH-g concentrations ranging from below detection to as tngh as 1,900 ppm remained at the base of the tank excavation The highest TPH-g concentrations were detected in soil from the center to the northeast corner of the tank excavation In July 1994, dispensers and piping were removed and upgraded Approximately 150 cubic yards ofthydrocarbon impacted soil were excavated during the upgrade process and disposed of off-site Soil Vapor Extraction (SVE). In March 1992, a vapor extraction test was performed at the site Results of thus test indicated that by using a wellhead flow rate of 80 cubic feet per minute, an effective radius of influence of 50 feet could be achieved per well (Resna, June 29, 1992) A vapor extraction system (VES) was constructed and began operation on September 22, 1994 The VES consisted of six vapor extraction wells (VW-1 through VW-6) connected to a thermal/catalytic oxidizer system As of June 24, 1997, the VES had removed 11,709 pounds of TPH-g and 168 pounds of benzene The VES is currently operating (recently shutdown for repairs), although data contained in Attachment G, show that the effectiveness of the VES has diminished over time Concentration peaks in influent vapors are the result of system pulsing after shutdowns for repair Current Site Status. In a letter dated August 13, 1997, PHS/EHD requested • replacement of perimeter groundwater Wells MW-10, MW-12, and MW-13 along with Well MW-3, 3111657A1CL6981 DOC