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Interim Remedial Action Test Report,Former Unocal#0187 Stockton CA Au ust 15,200] occupying the northeast corner of California Street and Lindsay Avenue appears to be a former service station building, indicating that USTs may have been present at that location Based on data collected during investigations at the Unocal site, the contribution from these other UST sites to the dissolved hydrocarbon plume is not known, but the presence of these former sites can limit the extent of subsurface assessment work for the Unocal site Former Vintage Car Wash On July 23, 2001, GR personnel reviewed files at the Regional Water Quality Control Board (RWQCB) offices in Sacramento, California,for the Vintage site, located downgradient of the subject site(Figure 3) The files indicate that approximately 3,200 gallons of gasoline have been released from USTs at this site Current work at the site includes additional subsurface assessment and vapor extraction Soil and groundwater chemical analytical data for wells and borings at the Vintage site are included in Appendix B Recent work at the Vintage site includes the collection of groundwater samples from cone penetration test (CPT) borings at depths comparable to Zone B wells at the Unocal site (up to 100 feet bgs) Dissolved hydrocarbon concentrations up to 10,000 ppb TPHg and 5,200 ppb benzene, and MtBE concentrations up to 180 ppb have been detected a this depth, downgradient of the Vintage site As Figure 4 shows, the vertical and downgradient extent of the dissolved hydrocarbon and oxygenate plume from the Vintage site has not been delineated,but given the concentrations at the base of boring CPT-2,dissolved hydrocarbons from the Vintage site most likely extend to Unocal's Zone C depth Also,concentrations of benzene(7 4 ppb), MtBE (21 ppb), and DIPE (1 4 ppb) were detected in boring CPT-1, suggesting cross-gradient migration has occurred from the Vintage site INTERIM REMEDIAL ACTION System Installation As an interim remedial action measure, GR installed a K-V Associates, Inc (KVA) "C-SpargeT'"" ozone microspargmg system Ozone microsparging is a process where ozone in air is introduced into the groundwater at low flow rates (2-6 cubic feet per minute) through specially designed sparge points to create small "microbubbles " As these microbubbles rise within the column of water, they strip volatile organic compounds (VOCs) from the groundwater Upon entering the microbubbles, the VOCs are rapidly oxidized The system installed at this site was intended to be used as a pilot test Eight ozone sparge points were installed at the Unocal site (GR report #240042 08-1, Monitoring Well Installation report, Vapor Extraction Pilot Test Report, And Interim Remedial Action Plan, dated December 22, 2000) Based on the manufacturer's recommendations, an approximate zone of influence of 20 feet for each point was used as design criteria Four of the points were placed at a depth of approximately 65 feet bgs and four were placed at a depth of approximately 110 feet bgs The sparge point locations were chosen to introduce ozone into the areas of highest petroleum concentrations, near the eastern property boundary of the Unocal site The point locations are shown on Figure 2 The system was placed into operation on December 12, 2000 • The C-Sparge"panel includes an ozone generator,air compressor, and a programmable timer/controller Sparge points are connected to the panel by LDPE tubing Each sparge point has a dedicated line The 24004209-1 6