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4C togica t TeC�iAtcs Inc. ' 1101 7'h Street Modesto,Califomia 95354 (209)522-4119/Fax(209)522-4227 Work Plan Risk Evaluation & Appendix B Closure Checklist Morita Brothers ' 814 E. Charter Way Stockton, CA ' Project No 724 2 March 18, 2002 1.0 SITE BACKGROUND In December 1991, four underground storage tanks (USTs) were removed from the site (Figure 1) Gasoline range petroleum hydrocarbons were detected in shallow soils beneath the tanks and seven soil borings were installed in March, 1992, to further define the extent 1 of the contamination The borings revealed low to moderate concentrations of benzene, toluene, ethylbenzene and xylene (BTEX), and gasoline range petroleum hydrocarbons (TPH-Gasoline) in soils up to 50 feet below grade surface (bgs) In March 1993, shallow ' soils from beneath the former USTs and product piping areas were excavated for disposal The resulting excavations were advanced to depths of 20-28 feet bgs, which generated approximately 750 cubic yards of soil for disposal ' Three monitoring wells (MW-1 through MW-3) were installed in September 1992, to determine if groundwater had been impacted by the release—see Figure 2 for well locations Groundwater samples obtained from the wells confinrned that BTEX and TPH-G were ' present and the contaminant concentrations exhibited minor fluctuations during 18 monitoring events conducted between 1992 and 1999 Two additional monitoring wells (MW-4 and MW-5) were installed in September 1999, and incorporated into the monitoring ' schedule Samples obtained in November 1999 and March 2000, from well MW-4 contained elevated levels of benzene and TPH-G for the two events On November 20 —22, 2000, five additional groundwater monitoring wells were installed to investigate the lateral and vertical extent of gasoline-range petroleum hydrocarbons in the groundwater MW-6, MW-7, MW-104, MW-106 and MW-107 (Figure 2, Site Map) Wells MW-6 and MW-106 were placed on the adjacent parcel to the east of the site, ' approximately 60 feet south of MW-5 Wells MW-7 and MW-107 were placed approximately 15 feet east of the former UST field A discretely screened deep well, MW- 104, was placed adjacent to the former dispenser island on the western portion of the site The installation of these wells completed the definition of the soil plume, as soil samples from these borings did not contain detectable contamination