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SECTION 5.0 RESULTS AND DISCUSSIONS 1996 sampling event (when groundwater was at a depth of 59 3 feet bgs) (see Table 3-3) The highest concentration of benzene in soil was observed at a depth of 45 feet bgs (see Table 3-2) TPH-g were not detected in soil samples collected from MW-6 and BTEX were not detected in soil samples collected below 60 feet bgs at the time of the well installation in 1995 5.1.2 Quarterly Groundwater Sampling Results The First Quarter 1999 groundwater monitoring at the Stockton Yard was conducted by PSI during April I and 2, 1999 The groundwater elevation data and analytical results for TPH-g, BTEX, and MTBE from the on site wells are provided in Table 3-3 Using PSI's April 1999 data, URSGWC prepared a groundwater elevation map, which is presented as Figure 5-2 The i estimated groundwater gradient is 0 002 foot/foot (0 0006 meter/meter) and the predominant ■ flow direction is to the south-southeast First Quarter 1999 analytical results for well MW-6 were non-detect for TPH-g, BTEX, and MTBE, except for an estimated xylenes concentration of 2 7 µg/L The quarterly groundwater monitoring results for well MW-6 were significantly less than the discrete vertical sampling results The TPH-g results at a depth of 47 feet bgs for well VW-2 were non-detect during the quarterly monitoring, however, the BTEX and MTBE results were greater (by less than one order of magnitude) than, the discrete vertical sampling results for well VW-2 (see Table 5-2) MTBE was detected in a groundwater sample from well VW-2 at a concentration of 83 µg/L during the quarterly monitoring event, MTBE concentrations ranging from 21 to 89 µg/L were detected during the discrete vertical sampling event First Quarter 1999 groundwater monitoring data along with the discrete vertical sampling data are shown on Figure 5-3 for companson purposes 5.1.3 Comparison to Action Levels Groundwater analytical results from April 1999 were compared to potential action Ievels for contaminants in groundwater The EPA maximum contaminant levels (MCLs) for benzene, toluene, ethylbenzene, and total xylenes are 1 µg/L, 100 µg/L, 680 µg/L, and 1,760 µg/L, respectively The Califorrua 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 µg/L based on taste and odor thresholds Based on the discrete vertical sampling results for well VW-2, BTEX, and MTBE action levels have been exceeded for the 47-foot sample, benzene and MTBE action levels have been exceeded for the 61- and 67 5-foot samples For well MW-6, the benzene action level was exceeded in the 59 5- and 71-foot samples Based on the quarterly tsampling results, the benzene action level was exceeded at wells VW-1, VW-2, MW-2, and MW- 3 Toluene, ethylbenzene, xylenes, and MTBE action levels were also exceeded at well VW-2 S 1RROM1040100441SYYARCIMAIMARCSA.DOC 5-2 URS Greiner Woodward Clyde