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' 26 <br /> ' complete Field work reported herem yielded data that indicate that the assessment of vertical <br /> extent of sof impacts is also complete <br /> ' 6.3.2 Groundwater Impacts <br /> Previous investigations have documented the lateral impacts of petroleum <br /> hydrocarbons to groundwater in the vicinity of the Site The Site monitoring well array <br /> provides data which allows interpretation of the impacts to groundwater above the MCLs <br /> The February 4, 1999 quarterly monitoring data indicate that the downgradient extent of <br /> impacts to groundwater by TPHg is less than 100 ft (Figure 6-1) The same data indicate that <br /> the downgradient extent of impacts to groundwater by benzene at concentrations greater than <br /> 1 ug/L are less than 100 ft, and the downgradient extent of any benzene detections is less than <br /> r � <br /> 300 ft(Figure 6-2) <br /> I <br /> The SJCPHS concluded in a letter received by UNOCAL on January 9, 1997 that the <br /> Massessment of the lateral extent of groundwater contamination at the Site was complete <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 /> The Site meets the criteria for a "low risk" groundwater site set forward by Mr Walt Petit, <br /> Executive Director of the State Water Resources Control Board, an a December, 1995 letter to <br /> the California Regional Water Quality Control Board Chairpersons and Executive Officers <br /> ' Based on the Site-specific data regarding the lateral extent of groundwater impact, and on the <br /> consistency of the findings of this study with those of two comprehensive scientific studies, this <br /> site is characterized as a low risk groundwater site <br />