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L • Remedial Action Plan and Further Site Assessment 2 7-Eleven Store No 19976 1399 N Main St Manteca CA May 30 2000 20 SUMMARY OF SITE ASSESSMENT DATA AND PROPOSED NEW ASSESSMENT ' 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 underground storage tanks (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-tertiary-butyl-ether (MTBE), total petroleum ' hydrocarbons as gasoline (TPH-G) (EPA Methods 801518020), 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, 200 mg/kg TPH-G, and 0 35 mg/kg MTBE (EPA Method 8260) (IT Corporation, 2000) The tank pit was over excavated slightly to accommodate the new ' underground storage tanks, which were installed in the old tank pit (Figure 3), and post-excavation samples collected on September 20, 1999, were largely non-detectable ' Following receipt of tank replacement findings, the San Joaquin County Public Health Services Department (SJCPHS) requested site assessment to further evaluate soil and groundwater conditions Three direct-push borings (GP-1 - GP-3)(Figure 3)were installed on April 29, 1998 near the hydrocarbon source area (the UST pad and pump island) Nineteen soil samples were collected from depths ranging ' between 5 and 45 feet below grade for analysis for BTEX, MTBE, and TPH-G (EPA Methods 801518020) One sample from each soil boring was also analyzed for MTBE and other oxygenates by EPA Method 8260 Concentrations ranged up to 15 mg/kg benzene, 27 mg/kg TPH-G, and no detectable MTBE (EPA ' Method 8260)(IT Corporation, 2000) Highest concentrations were at a depth of 35 feet below grade Grab groundwater samples collected from the base of each boring were also analyzed for the same ' constituents and contained up to 5,800 micrograms per liter (ug/L) benzene and 290,000 ug/L TPH-G Dissolved MTBE ranged up to 3,700 ug/L (EPA Method 8260) ' Upon receipt of these data, the SJCPHS requested further assessment Three groundwater monitoring wells (well MW-1 is dual-nested to depths of 30 as well as 66 feet below grade)were installed in ' September, 1998 (Figure 3) Twelve soil samples were analyzed for BTEX, MTBE, and TPH-G (EPA Methods 801518020) Three of the samples were also analyzed for MTBE and the other oxygenates using ' EPA Method 8260 The highest benzene and MTBE concentrations were detected in capillary fringe soils (0 0708 mg/kg in MW-2@25'and 0 0829mg/kg in MW-1@20', respectively) The highest TPH-G concentration was detected in sample MW-1@45' at 17,000 mg/kg (IT Corporation, 2000) Upon receipt of these data, the SJCPHS requested additional assessment Soil boring SB-1 and cluster wells MW-4/4A and MW-5/5A were installed in May 1999 (Figure 3) Soil boring SB-1 was installed to