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- ------ <br /> r <br /> i='•. A S S O C I A T E S I N C <br /> 4.0 DISTRIBUTION OF PETROLEUM HYDROCARBONS <br /> 4=;1 4_1 Soil <br /> Based on the analytical results from soil samples collected on July 18, 1995, September 24, 1996, <br /> k and October 2, 1996, the vertical extent of soil contamination beneath the site appears limited to <br /> above 20 feet bgs. Three soil samples, B1-21.5, B2-25.5, and B3-26.5, were collected on July <br /> F_. 18, 1995, from depths below 20 feet bgs and none of the samples contained detectable <br /> ==j concentrations of petroleum hydrocarbon constituents. None of the soil samples collected on <br /> September 24 or October 2, 1996 were collected from below 10.5 feet bgs. The petroleum <br /> `=3 hydrocarbons appear to attenuate rapidly with depth. <br /> Field observations, PID measurements, and laboratory analytical data indicate the majority of <br /> j petroleum hydrocarbon-affected soils are present at depths from approximately 6 to 12 feet bgs. <br /> L-4 Cross section locations are shown on Figure 4. A cross section of the site in the downgradient <br /> direction is shown on Figure 5 and a cross section of the site in the crossgradient direction is <br /> shown on Figure 6. These figures illustrate the estimated vertical extent of soil contamination. <br /> Based on the analytical results from soil samples, the lateral extent of soil contamination beneath <br /> the site appears to extend from the former UST locations north less than 180 feet. Soil samples <br /> collected from borings MW-1, SBI, SB3, SBS, and SB6 contained no detectable concentrations <br /> of petroleum hydrocarbon constituents. The lateral extent of TPHg and benzene contamination in <br /> -_ the soil is shown on Figures 7 and 8, respectively. As indicated on Figure 7, the highest <br /> concentrations of TPHg are present in soil northwest of the former location of the gasoline USTs. <br /> Similarly, as indicated on Figure 8, the highest concentrations of benzene are present in soil <br /> €' northwest of the former location of the gasoline USTs. <br /> 4-2' Groundwater <br /> F. <br /> Based on the analytical results from groundwater samples collected from wells MW1, MW2, and <br /> MW3 on December 31, 1997, and from grab groundwater samples collected from borings SBI <br /> •`_ through SB7 on September 24, and October 2, 1996, the lateral extent of groundwater <br /> contamination beneath the site appears to extend from the former UST locations north and <br /> ~1 northwest less than 180 feet. Groundwater samples collected from borings SB5 and SB7 <br /> F= contained no detectable concentrations of petroleum hydrocarbon constituents and groundwater <br /> samples collected from borings SB 1 and SB3 contained only trace amounts of toluene. <br /> a The lateral extent of TPHg and benzene contamination in groundwater is shown on Figures 9 and <br /> 10, respectively. As indicated on Figure 9, the highest concentrations of TPHg are present in the <br /> groundwater north to northwest of the former location of the gasoline USTs. Similarly, as <br /> indicated on Figure 10, the highest concentrations of benzene are present in groundwater north to <br /> northwest of the former location of the gasoline USTs. The extent of contamination to the south <br /> is defined by well MWl and borings SBI and SB7, although directly south of boring SB6 the <br /> w:1951221reportslparcap.doc 5 <br /> j <br />