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Additional Site Assessment and Second Quarter 1999 Monitoring and Sampling Report 7 7-Eleven Inc Store#19976 1399 N Main St Manteca California July 15 1999 1 (Figure 3) Capillary fringe soils are generally low-permeability,while the vadose-zone contains relatively ' transmissive sand beds 42 Results of Soil Sample Analyses ' Table 1 summarizes the laboratoryanalytical s an ytica results for soul samples collected between May 11 and May ' 17, 1999 The highest benzene and TPH-G concentrations were detected in sample SB-1@25' at 0 39 mg/kg and 2 1 mg/kg, respectively The highest MTBE concentration was detected in sample SB-1@15' at 0 37 mg/kg Hydrocarbon and MTBE concentrations are very low to non-detectable in sod samples taken from the three borings In SB-1, located near the source area, MTBE and TPH-G were not detected in soil samples analyzed below 25 feet BGS BTEX was not detected in any soil samples ' analyzed from SBA below 40 feet BGS No hydrocarbons or oxygenates were detected in samples analyzed from MW-4A, located offsite and crossgradient of the source area In MW-5 (downgradient) hydrocarbon and oxygenates are confined to a zone between 30 and 35 feet BGS From current and ' historical sod sample analytical results sod impacted with very low hydrocarbon concentrations appears confined to a limited area beneath the former UST pit and pump island Copies of the analytical results and chain-of-custody documents are included in Appendix D 4,3 Results of Groundwater Monitoring[Analyses ' Table 2 summarizes the laboratory analytical results of groundwater samples collected from the boring for MW-5 on May 11, 1999 and the eight monitoring wells on June 6, 1999 During the monitoring and sampling on June 6, 1999, the equilibrated depth to water was approximately 17 feet BGS Figure 3 shows the lateral distribution of dissolved hydrocarbons (benzene, TPH-G) and MTBE in groundwater samples collected on June 6, 1999 Water sample MW-5@W, collected from soil boring MW-5A pnor to well Installation on May 11, 1999, contained benzene,TPH-G, and MTBE (EPA method 8260) concentrations of 200 pg/L, 1,500 Ng/L, and 75 pg/L, respectively In water samples taken from the eight monitoring wells on June 6, 1999, benzene ranged from below ' method detection limit in wells MW-4 and MW-4A to 6,120 pg/L in well MW-2 TPH-G ranged from below method detection limit in wells MW-4, MW-4A and MW-5 to 55,400 pg/L in well MW-2 MTBE ranged from below method detection limit in wells MW-3, MW-4 and MW-4A to 885 pg/L in well MW-2 No other oxygenates were detected above detection limits in any of the wells sampled The lateral and vertical extent of dissolved hydrocarbons and MTBE has been delineated upgradient(south) and �i ' 19970Q2 sar