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Soil samples collected at the base of the UST over-excavation still contained elevated <br /> concentrations of gasoline compounds Soil samples collected to a total depth of 60 feet during the <br /> drilling of MW-1 contained gasoline compounds throughout the unsaturated vadose zone soils <br />' This continuous interval of impacted soil between the base of the over-excavation and the <br /> groundwater surface at approximately 45 feet bgs suggests that the source of impacted soil and <br /> groundwater is the former UST#3 <br /> Based on the soil sample analytical results the only other significant potential source of <br /> hydrocarbons appears to be the underground product piping between the diesel UST#2 and the <br /> southern dispenser island However, based on the results from two hand-augered soil borings (HA- <br /> 1 and HA-2), the diesel hydrocarbons appear to be defined to shallow soil less than 10 feet deep <br /> 33 Nature and Extent of Impacted Soil and Groundwater <br />' The samples collected from impacted soil during UST removal and drilling activities mostly consist <br /> of TPH-as-gasoline, and lesser concentrations of BTEX and MTBE The highest concentrations of <br /> gasoline compounds in soil were detected beneath the former UST#3, and south at MW-1 <br /> Vertically, the highest concentrations of gasoline compounds were detected in the vadose zone <br /> between approximately 28 and 43 feet bgs, and at the groundwater table interface and capillary <br /> fringe between 43 and 48 feet bgs The concentrations of gasoline compounds in soil declined <br /> significantly below the groundwater table As shown in cross-section (Figure 6), the lateral and <br />' vertical extent of impacted soil remains undefined <br />' Given the presence of sandy soils between the base of the former UST and groundwater table, <br /> much of the mass of gasoline hydrocarbons released from the UST system appears to have <br /> migrated to groundwater Groundwater has been impacted by free product and significant <br /> concentrations of TPH-as-gasoline, BTEX, MTBE, TBA and 1,2-DCA These concentrations <br /> appear to have remained relatively constant since groundwater samples were first collected in April <br />' 1999 As shown on Figure 7, the lateral extent of impacted groundwater is undefined, except at <br /> MW-3 north of the former UST source area The vertical extent of impacted groundwater has not <br /> been investigated <br /> 1 <br /> 9 <br />