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-e <br /> Reconnaissance push boring groundwater data support the previously mapped and <br /> monitored plume location. The deeper groundwater samples showed reduced TPHG and <br /> greatly reduced Benzene concentrations. The April 2000 data showed higher <br /> concentrations overall, however, these samples were collected somewhat shallower <br /> k around and just below the capillary fringe and flowing conditions during drilling may,have <br /> released contaminants from desorbtion by the flowing. The April 2001 deeper samples <br /> were collected well below the capillary fringe, and lower concentrations would be <br /> expected in deeper portions of the aquifer. When compared to the quarterly monitoring <br /> data, these reconnaissance sample plumes overlay each other in the area of the historic <br /> plume location. Data from PB-20 and PB-21 show the dissolved plume present to the <br /> west of the trailers at very low concentrations, which are attributed to dispersion and <br /> micrological degradation at the periphery of the plume over time. <br /> Subsurface Conditions Site Conceptual Model <br /> A total of twenty-one exploratory borings were drilled and nine monitoring wells at the <br /> locations shown on Figure 1, the deepest of which was advanced to a termination depth of <br /> 26 feet. The site is underlain by silty clay to depths of about 8- to 10-feet. This clay is <br /> underlain by interbedded clay, clayey sand, silty sand, and sand to depths of about 20- to <br /> 24-feet, and compose the aquifer strata. The sand is locally very clean and tends to flow <br /> and caused partial or total borehole collapse in all borings. The aquifer strata are underlain <br /> by interbedded silty clay, clayey sand and clayey silt, which compose the underlying field <br /> confining strata at which the borings were terminated in the range of about 24-to 26-feet. <br /> Groundwater occurred at depths of about 12- to 14-feet and rose to about 9-feet <br /> indicating the aquifer strata are unconfined to very weakly confined seasonal water,level <br /> fluctuations appear to vary from about 0.5 to 0.8 through out the year. On the basis of <br /> selected field groundwater measurements in uncased boreholes, groundwater appears to <br /> flow to the north; however there was insufficient time for levels to equilibrate for adequate <br /> data to plot a contour flow direction_ The historic groundwater flow direction as mapped <br /> in the existing monitoring wells is consistently northerly. The data revealed by this.work <br /> i agrees with the previous site assessment work performed by Wright and EVA—k. (see <br /> References). <br /> Regional lithoiogic lateral and vertical variation are present in the shallow aquifer strata <br /> and underlying.confining strata, which are typical for river and floodplain deposits in this <br /> portion of the San Joaquin Valley (see Geologic Cross Section, Figure 2). While regional <br /> lateral and vertical variation are present in the aquifer and confining strata, these strata as <br /> a whole form the aquifer stratigraphy between about 10- to 24-feet. The underlying <br /> confining strata display some lateral lithologic variations and were not observed to yield <br /> water, however, it is possible that these could strata leak. In our opinion, on the basis of <br /> these.observations, both aquifer strata and the underlying confining strata are interpreted <br /> to have some lateral extent underlying the immediate site vicinity. <br /> i <br /> Pace 9 <br />