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! • 0 <br />! 2.4 Nature and Extent of Petroleum Hydrocarbon Contamination <br />Historical investigation results confirm that the soil and groundwater in the saturated zone at the <br />site has been impacted by petroleum hydrocarbons, including benzene, ethyl benzene, toluene, <br />and xylenes (alternatively referred to as BTEX). The bulk of the contamination appears to be <br />within the former UST location near the east end of the property, with elevated TPH-g and <br />BTEX concentrations found at VW -11), VW -21), VW -31), and MW-4RS. Figure 4 shows the <br />distribution of TPH-g and BTEX in the groundwater across the site and the approximate lateral <br />extent of the groundwater plume for treatment. <br />! Most of the hydrocarbon contamination also appears to be between 35 and 60 feet bgs. Although <br />TPH-g and BTEX were detected as deep as 80 feet bgs at MW-4RD, the concentrations were <br />substantially low compared to those found shallower than 60 feet bgs. The notable concentration <br />difference supports the belief that the vertical migration of the hydrocarbon to below 60 feet bgs <br />is limited due in part to the underlying presence of a more confining layer of silt and clay soils <br />(Figure 3). As previously mentioned, additional investigation of the vertical extent of the <br />hydrocarbon contamination in the saturated zone is being conducted at the site. Findings will be <br />incorporated in the final design of the biosparge treatment system. <br />1 <br />i <br />BiospargePilot Test Final. doc 2-4 Shaw Environmental, Inc. <br />