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I SECTION 5.0 RESULTS AND DISCUSSIONS concentration of benzene in soil in VM-2 was detected in soil samples collected from 50 feet bgs at 410 µg/kg(Table 3-1) TPH-g, BTEX, or MTBE concentrations were not detected in the groundwater samples collected from well MW-14 5.1.2 Quarterly Groundwater Sampling Results The First Quarter 1999 groundwater monitoring at Shop 10 was conducted by PSI between March 30 and April 2, 1999 (see Table 3-2) The groundwater elevation data and analytical results for TPH-g, BTEX, and MTBE from the site wells are provided in Table 3-2 Using PSI's April 1999 data, URSGWC prepared a groundwater elevation contour map, presented as Figure 5-2 The estimated groundwater gradient is 0 005 foot/foot (0 0016 meter/meter), and the predominant flow direction is to the east-southeast The concentration of TPH-g in the groundwater sample collected from MW-13 during the quarterly monitoring event was significantly less than the concentrations detected in the discrete samples, and concentrations of BTEX in samples collected from MW-13 during the quarterly ie monitoring event were greater than those of the discrete samples lThe TPH-g concentration in well VM-2 from the quarterly groundwater monitoring event was greater than those in the samples collected from the discrete sampler Concentrations of benzene were similar between the two events, although concentrations were slightly higher in the samples collected with the discrete sampler ' Concentrations of TPH-g, BTEX, or MTBE were not detected in samples collected from MW-14 during the quarterly monitoring event or collected from the discrete sampler A graphic comparison of the analytical laboratory results from both sampling events is depicted on Figure 5-3 ' 5.1.3 Comparison to Action Levels Groundwater analytical results from April 1999 were compared to potential action levels for contaminants in groundwater The EPA maximum contaminant levels (MCLs) for benzene, toluene, ethylbenzene, and total xylenes are 1 gg/L, 100 µg/L, 680 pg/L, and 1,760 µg/L, respectively The California Department of Health Services (CDHS), in association with EPA, has established a primary action level for MTBE of 13 µg/L for the protection of human health and the environment In addition, CDHS has established a secondary MCL for MTBE of 5 pg/L ' based on taste and odor thresholds Based on the discrete vertical sampling results for well VW-2, the action levels for benzene, toluene, ethylbenzene, and xylenes have been exceeded in ' the 46 5- and 59 5-foot samples, with the exception of xylenes in the 59 5-foot sample For Well S 1PROJIAlUO100041SHOP101SHOPIOSADOC 5-2 URS Greiner Woodward Clyde