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1 <br /> 4.0 SUBSURFACE CONDITIONS <br /> Soils encountered beneath the Site consist primarily of elastic silt (MH) to a depth of <br /> approximately 10 to 13 feet below grade, and lean to fat clay (CL, CH/CL) from approximately <br /> 13 to 20 feet below grade, the maximum depth explored Apparent discontinuous sand lenses <br /> 1 were noted at varying depths The orientation of a stratigraphic cross-section is shown on <br /> Figure 3 Site stratigraphy is shown on Figure 4. <br /> Groundwater beneath the Site was first encountered at approximately 15 to 16 feet below grade <br /> After well completion, the piezometric surface rose to approximately 12 to 13 feet below grade <br /> The subsequent rise in fluid levels may be due to the slow yielding nature of the low <br /> permeability soils at the Site, or, alternatively, may suggest that groundwater exists under <br /> somewhat confined conditions Based on March 2, 1992 monitoring data (Table 4), static <br /> groundwater is approximately 11 5 feet below grade and flows to the south under a gradient of <br /> 0 011 ft/ft or 58 feet per mile, which is consistent with low permeability soils A groundwater <br /> I <br /> gradient map is presented on Figure 5. <br /> 5.0 CONCLUSIONS <br /> ' 5.1 Soil Contamination <br /> Laboratory analyses and field PID readings from the three soil borings suggest that soil beneath <br /> ' the Site has been impacted by gasoline hydrocarbons Gasoline hydrocarbons were detected in <br /> each boring (Table 2) Detected hydrocarbons, most notably benzene and TPHG, are <br /> ' concentrated near the groundwater interface at depths between 10 to 15 feet below grade Soil <br /> contamination at this depth is likely due to migration of hydrocarbons along the silt/clay <br /> interface or due to fluctuations of hydrocarbon impacted groundwater The absence of soil <br /> contamination in Bonngs MW2 and MW3 at a depth of 5 feet below grade and the shallow <br /> piezometnc surface (11 5 feet below grade) suggest that shallow soil contamination across the <br /> Site is due to dissolved contaminants in the capillary fringe Consequently, the soil <br /> icontamination should be addressed as a groundwater contaminant <br /> a5mikv-d ��Jj H_ �.jj <br /> ' R-61292 TL 6 <br />