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V <br />' AR <br /> CAD1S GERAGHTY&MILLER <br /> extension of the plume to the south as shown The southerly portion of the plume has <br />' historically been in either the cross or upgradient position with respect to the source area and <br /> has therefore not merited the density of sampling points that is present in the source area <br /> The same data indicate that the downgradient extent of impacts to shallow <br />' groundwater by benzene at concentrations greater than 1 ug/L are approximately 165 ft <br /> (Figure 6-2) Log-smear interpretation of the data was also used to generate this contour map <br /> These Site data are consistent with the findings of Rice et al (1995), in their statewide <br /> investigation of petroleum hydrocarbon impacts to alluvial environments, in which it was <br /> concluded that migration of petroleum hydrocarbons from lealang underground fuel tank sites <br />' is limited Rice et al (1995) found that benzene plume concentrations greater than 10 ppb <br /> extended no more than about 250 feet in 90% of the cases studied The findings of Rice et al <br /> (1995)were confirmed in a separate and independent study conducted by Mace et al (1997) <br />' 6.3.3 Deep Groundwater Impacts <br />' As indicated by the Simulprobe water sampling data and the February 5, 1999 quarterly <br />' monitoring data, deeper zones beneath the Site have also been inpacted by petroleum <br /> hydrocarbons <br />' The data indicate that benzene concentrations in 'oundwater at the shallow and mid- <br />' level depths sampled beneath the Site decrease in the downgradient direction As indicated by <br /> the data from NP-1 and NP-2, the magnitude of decrease is by a factor of 62 in the shallow <br />' groundwater and by a factor of over 7 in the mid-level zone Benzene concentration increased <br /> in the downgradient direction m the deep-level zone between NP-1 and NP-2 by a factor of <br />' 45 <br />' 36 <br />