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Report on Additional Site Assessment 1 7-Eleven Inc.,Store#19976, 1399 N.Main St.,Manteca,California March 5,2001 1.0 INTRODUCTION This report is submitted by IT Corporation to summarize the results of site assessment conducted at the 7-Eleven Inc., Store No. 19976, located at 1399 N. Main Street,in Manteca, California (Figures 1 through 3/3A). The scope of work for the site assessment included the installation of eleven soil gas survey points, one vertical delineation boring, and one additional (offsite)groundwater monitoring well, including soil vapor,soil, and groundwater sample collection. The work was completed as outlined in IT Corporation's August 1, 2000,workplan prepared as requested and approved by the San Joaquin County Public.Health Services(SJCPHS). The work scope described in this report was completed at the site between September 18, 2000 and January5, 2001. Results indicate that the soil source area is defined at depths of less than 40 feet below grade, and is localized to the immediate tank pit area. 2.0 BACKGROUND Underground storage tank replacement and associated soil sampling activities were conducted in September 1996. Five underground storage tank pit samples were collected on September 19, 1999, one from beneath each end of the three USTs at 15 feet below grade (except for the north end of the central tank,which was under an excavation stockpile). Sample analysis for benzene,toluene, ethylbenzene, and total xylenes(BTEX), methyl-tert-butyl-ether(MTBE),total petroleum hydrocarbons as gasoline (TPH-G) (EPA methods 8020/modified 8015),total lead (EPA method 6010), and confirmation sampling for MTBE by EPA method 8260 indicated the presence of hydrocarbons in levels of up to 0.93 milligrams per kilogram (mg/kg) benzene,220 mg/kg TPH-G, and 0.35 mg/kg MTBE (EPA Method 8260) (Table 2). The tank pit was overexcavated slightly to accomodate the new underground storage tanks,which were installed in the existingtank pit(Figure 2 and samples collected on September 20 1999 at the lateral P ( 9 ), P P � , and vertical edges of the over-excavation were largely non-detectable (Table 2). i 19976.4SA i