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Ozone Microsparge Well and System Installation Report—Former Tosco(Unocal)Service Station No 6981 February 1,2002 sediments beneath the site consist of Ouaternary alluvial deposits of the Modesto Formation, which were deposited by the Mokelumne River Previous subsurface studies indicate that the site is underlain by alluvium to the maximum explored depth of 86 feet below ground surface (bgs) The alluvium consists primarily of silts and clays with lenses of poorly graded sand and silty sand Historically, depth to groundwater has risen from approximately 51 to 53 feet bgs in 1993 to 30 to 32 feet bgs in September 2001 Groundwater flow has been to the northeast with the hydraulic gradient ranging from'0 002 to 0 013 feet/feet PREVIOUS ENVIRONMENTAL WORK In May and June 1988, RESNA Industries collected soil samples during the replacement of the USTs and from test pits excavated adjacent to the product dispenser islands Petroleum hydrocarbons were detected in soil samples collected from the UST pit,product lines, and test pits Approximately 1,600 cubic yards of hydrocarbon- impacted soils were excavated and removed from the site Due to the proximity of roads and structures, overexcavation was terminated before all hydrocarbon-impacted soils could be removed A soil vapor survey was performed to assess the extent of residual hydrocarbons in the soil beneath the site iThirteen groundwater monitoring wells (MW-1 through MW-13), six vapor extraction wells (VW-1 through VW-6), and one groundwater extraction well (PTW-1) were installed on or near the site between January 1990 and August 1992 A vapor extraction test (VET) was performed in March 7992 to evaluate the feasibility of vapor extraction as a remedial option for soils beneath the site An aquifer pump test was performed in August 1992 to evaluate the feasibility of utilizing groundwater extraction to remediate hydrocarbon-impacted groundwater A VES utilizing a catox unit has been in operation at the site since September 1994 As of the end of the second quarter of 2001, approximately 14,794 pounds of total petroleum hydrocarbons calculated as gasoline (TPHg) and 183 pounds of benzene has been extracted and treated The VES was being operated in a pulse mode During the second quarter of 2001, the VES extracted and treated <0 10 pounds of TPHg and <0 001 pounds of benzene The data appears to indicate that asympotic levels were reached in February 2000 No petroleum hydrocarbon concentrations have been detected in the influent since December 2000 The system was shut down in the third quarter of 2001 due to equipment failure Because of the planned remedial system modifications, the catox unit was not repaired In February 1996,wells MW-3, MW-10, MW-12, and MW-13 were abandoned at the request of the city of Stockton This work was performed to facilitate reconstruction of the intersection and widening of Pacific Avenue 14012412 2