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GROUNDWATER GRADIENT <br />' Previously, static groundwater elevations were measured in each monitoring well on December 5, <br /> 1996 Groundwater was measured at approximately 12 feet below grade The gradient map, <br />' shown on Figure 3, depicts a southwesterly groundwater gradient averaging 0 004 ft/ft or <br /> approximately 23 feet/mile Fluid level measurements indicated groundwater flow direction <br /> beneath the site during the fourth quarter 1996 was generally consistent with recent data but <br /> fluctuating gradients have been observed in the past In November 1992, the groundwater flow <br /> direction was to the north, in July 1994, the groundwater flow direction was to the northwest, and <br /> in August 1995, the groundwater flow direction was to the west (Appendix III) The <br />' groundwater flow direction appears to be influenced by the San Joaquin River which is located <br /> less than 2,000 feet southwest of the site Groundwater monitoring data are summarized in Table <br />' 3 <br /> 1 DISTRIBUTION OF PETROLEUM HYDROCARBONS <br /> M sfliI <br /> The distribution of TPHg and benzene is generally limited to depths between 10 and 16 feet bgs in <br /> the vicinity of the former UST excavation and in the vicinities of monitoring wells MW-2 and <br /> 1 MW-3 In addition, low concentrations of these constituents extend to boring SB-8 to the north <br />' and to monitoring wells MW-4 and MW-6 to the south The isoconcentration maps on Figures 4 <br /> and 5 and the cross-sections on Figures 9 and 10 present the lateral and vertical distribution of <br /> TPHg and benzene in soil These patterns of aereal extent of TPHg and benzene are indicative of <br /> groundwater impact and transportation Soil analytical results are summarized in Table 1 <br /> Groundwater <br />' The estimated lateral extents of TPHg and benzene concentrations in groundwater during this <br /> subsurface investigation and the third and fourth quarters of 1996 are presented on Figures 6 and <br /> 7 As indicated on Figures 6 and 7, the plume extends north of the former UST excavation, <br /> possibly as a result of a reversed gradient The boundary of the plume has been defined in the <br /> south, east and west direction and is inferred in the north (upgradnent) direction Even though the <br /> w\94334Wportslsumryrptdoc 6 <br /> 1 <br /> SMTH <br />