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0 0 An iridescent fuel sheen was observed at two intervals within DP -9: (1) from 47.5 to 48 feet bgs, and (2) from 52.5 to approximately 54 feet bgs. The fuel sheen suggests the potential presence of separate -phase hydrocarbons. A Third Quarter 2007 groundwater monitoring event was conducted in September 2007. In addition to water level measurement, groundwater samples were collected from nine wells on and off site. These wells are MW -1R, MW -2R, MW -3R, MW-4RS, MW-4RD, VW -11), VW -21), VW -31), and VW -4. The furthest offsite well is MW -3R and is located upgradient of the site. Sampling results were presented in the quarterly monitoring report submitted to CVRWQCB in October 2007 (Shaw, 2007). Historical site investigation information also suggests the presence of residual and dissolved phase hydrocarbons in the groundwater beneath the facilities on the other three corners of the intersection of Lincoln and West Charter Way (Shaw 2006a). As of September 2007, no active groundwater remediation is known to have been occurring at those facilities. 2.4 Nature and Extent of Petroleum Hydrocarbon Contamination Historical investigation results confirm that the soil and groundwater in the saturated zone at the site has been impacted by petroleum hydrocarbons, including benzene, ethyl benzene, toluene, and xylenes (alternatively referred to as BTEX). The bulk of the contamination appears to be within the former UST location near the east end of the property, with elevated TPH-g and BTEX concentrations found at VW -11), VW -21), VW -31), and MW-4RS. Based on the Third Quarter 2007 sampling results, the highest TPH-g concentration was reported at 85,000 micrograms per liter (µg/L), while the highest benzene concentration was reported at 4,900 gg/L. Both were found at VW -21), right in the former UST location (Shaw 2007). TPH and BTEX contamination levels decline away from the former UST location, with no detections at MW -1R, MW -2R, MW -3R, and VW -4. Figure 4 shows the distribution of TPH-g and BTEX in the groundwater across the site and the approximate lateral extent of the groundwater plume for treatment. ' Most of the hydrocarbon contamination also appears to be within 25 and 60 feet bgs (see Figure 3). Although TPH-g and BTEX were detected as deep as 80 feet bgs at MW-4RD, the concentrations were substantially low compared to those found shallower than 60 feet bgs. The notable concentration difference supports the belief that the vertical migration of the hydrocarbon to below 60 feet bgs is limited due in part to the underlying presence of a more confining layer of silt and clay soils (Figure 3). A work plan has been prepared separately to further verify the vertical extent of the hydrocarbon contamination in the saturated zone at the ' site. Findings from the delineation activity will be incorporated in the final design of the treatment system. FS Onsite Petroleum Hydrocarbon Remediation.doc 2-4 Shaw Environmental, Inc.