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i 3.0 SITE CHARACTERIZATION ! The following sections incorporate data from both the site assessment phase and the remedial phase of the investigation, but many of the maps,graphs,and other figures are new and were not contained in previous reports Therefore, this section is somewhat more comprehensive than similar sections in these earlier reports 3.1 Extent of Soil Contamination Figures 9 and 10 are contour maps of soil vapor concentrations measured during the initial stage of the investigation in 1990 The maps use the data shown in Table 1 These maps were constructed for this report to illustrate the extent of soil contamination as recorded by the presence of hydrocarbon vapors Both maps define a north-south trending belt of contamination along the western edge of the property, generally coincident with the former location of the dispenser island At the time the data were collected, they implied that the soil is contaminated below 20 feet, but the depth range of the contamination could not be determined Based on laboratory analysis of the soil samples collected in the first phases of site assessment, GeoAudit concluded in its Monitoring Well Installation and Quarterly Report (November 1993) that contamination was limited to the depth range of 30-45 feet, "corresponding to the capillary fringe and upper few feet of the saturated zone" at the time (1992-1993) Subsequent drilling by Clearwater indicated that slightly contaminated soil was locally present as shallow as 20 feet near the dispenser island in spargmg well SW-1, and as deep as 75 feet in CPTB-7 (Table 2) The highest concentrations in SW-1 were encountered between 25 and 35 feet,within the capillary fringe and upper several feet of the saturated zone at that time The combination of the soil vapor survey and the results from SW-1 suggest that the leak was located in the product lines north of well SW-1 and east of boring B-3, so it is rather surprising that none of the samples collected at shallower depths in either boring exhibited high concentrations of hydrocarbons The subsurface investigation revealed that hydrocarbon contamination was restricted pnmanly to the saturated zone, but did not identify an obvious gasoline migration path from the leak point through the vadose zone to the saturated zone Gasoline isocontour snaps provide additional information about the probable location of the leak point (Figures 11 and 12) These maps, which illustrate the lateral extent of contamination within the most contaminated portion of the saturated zone(25-30 and 30-35 feet), suggest that the leak was located between MW-1 and SW-1 The shape of the contammant plume rising the soil data is similar to the shape using vapor data (c f Figures 9 and 10), but the plume is somewhat smaller and centered more to the east Tlus could be due to errors in locating some of the vapor probes or borings,but probably indicates that the soil vapors tended to spread out laterally and migrate preferentially toward the west as they rose through the vadose zone(see Figure 17 below) i 4