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the site This data, coupled with the lustorically high range of groundwater elevations beneath the site (6 to 10 feet bgs), suggest that the bulk of the subsurface impact beneath the site is primarily in the dissolved phase In the vicinity of the former northern UST complex, the vertical extent of petroleum hydrocarbon impact has not been adequately assessed Few soil samples from the bottoms of the soil borings have been analyzed However, detectible concentrations of MTBE were reported in samples collected from the depth of approximately 20 feet bgs in soil borings GP-2 (collected January 23, 2001) and MWO-1X Although the sample collected from MWO-1X at the depth of approximately 30 feet bgs exhibited no detectable hydrocarbons, the vertical extent of hydrocarbon impact to soil has not been adequately characterized Based on soil analytical data collected to date, approximately 0 039 kilograms (Kg) of MTBE remain in the soil in the vicinity of the former southern UST pit Approximately 0 088 Kg remain in the soil near the former northern UST pit, and approximately 0 093 Kg of MTBE remains in the soil north of the site near borings GP-7 and GP-9 Extent of Petroleum Hydrocarbon Impact in Groundwater Groundwater beneath the site is impacted with TPHD, TPHG, benzene, and MTBE During the most recent quarterly groundwater monitoring and sampling event (second quarter, 2003), TPHD, TPHG, or benzene were not reported in any of the groundwater samples collected MTBE was detected in wells MWO-1X, MWO-2, and MW-1 through MW-4 with the highest concentration of 780 µg/L reported in the sample collected from well MWO-1X Historically, the dissolved MTBE values reported in samples collected in MWO-1X, screened into the hard clay layer and the sand unit below it, are of the same order of magnitude of the values measured in MW-2, the adjacent well screened into the sandy units above the hard clay (see historic groundwater analytic data in Appendix B) This would suggest that the sandy units above and below the hard clay are hydraulically connected and that vertical extent of groundwater contamination has not been completely assessed at this site Figure 6 illustrates the distribution of TPHG, benzene, and MTBE in the water-bearing zone at the time of the second quarter 2003 sampling event Based on these data and the predominant groundwater flow direction (north to northeast), the lateral extent of the dissolved hydrocarbon plume remains uncharacterized to the west and east Dissolved MTBE iso-concentration contours for the second quarter 2003 are shown in Figure 8 Based on the second quarter 2003 groundwater analytical data, approximately 0 483 Kg of MTBE as dissolved in the groundwater within the monitoring well network A graph showing the variation of dissolved MTBE over time is shown in Figure 9 MTBE concentrations have remained stable in the wells where MTBE has been reported over time except for wells MW-1 and MW-3 MTBE concentrations have decreased with time in samples collected from well MW-1, while samples collected from well MW-3 have increasing concentrations of MTBE over the same time period Historically, MTBE concentrations reported P Voe's Truck Plaza\ReportslSCM_09-2003Voes truck plaza SCM doe Page 9 5TRAATU5