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Report on Additional Site Assessment 3 7-Eleven Inc.,Store#19976,1399 N.Main St.,Manteca,California March 5,2001 Soil boring SB-1 and cluster wells MW-4/4A and MW-5/5A were installed in May 1999,after receipt of a letter from the SJCPHS requesting additional lateral and vertical plume delineation in April 1999 (Figure 2). Soil boring SBA was installed to about 65 feet.below grade near the hydrocarbon source area to evaluate the vertical extent of the hydrocarbon plume.`The cluster wells were installed to access the saturated zone at depths of 20-,30 feet below grade (MW-4 and MW-5), and 45-50 feet below grade (MW-4A and MW-5A)to provide further information on the lateral and vertical extent of the dissolved hydrocarbon plume. Fourteen soil samples were analyzed for BTEX,MTBE, and TPH-G by EPA methods 8020/8015 modified and for oxygenates MTBE,TBA, DIPE, ETRE, and TAME using EPA method 8260 (Table 2).The base of the benzene,-TPH-G, and MTBE/oxygenates plume in the area of boring SBA was identified at between 40 and 55 feet below grade, as evidenced by non-detectable concentrations of all analytes. Soil samples from wells MW-4 and MW-4A were non-detectable for all constituents, and samples from well MW-5 contained low levels of hydrocarbons from 30-35 feet below grade (below which concentrations were non=detectable for all constituents). Additional wells were requested by the SJCPHS as outlined in their August 16, 1999 correspondence;in response a workplan was submitted on October'18, 1999.'Groundwater monitoring well MW-6 was installed on December 22-1999 (IT Corporation,2000). Soil samples collected during well installation contained no detectable.levels of benzene or MTBE, and up to 15.4 mg/kg TPH-G,detected at 20 feet below grade. Groundwater analytical data trends overtime indicate highest dissolved hydrocarbon levels are consistently present in well MW-2 (Table 3). Benzene,T.PH-G, and MTBE concentrations in this well have ranged up to 6,990 micrograms per liter(Ng/L),68,000 pg/L, and 1,400 pg/L, respectively. Low to non-detectableconcentrations in downgradient wells MW-5, MW-5A, and well MW=6 indicate adequate 0 lateral and vertical plume delineation (Figures 3 and 3A). Well MW-7 data will be included in the first quarter 2001 monitoring and sampling report. 19976ASA