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relatively favorable in comparison to those that we have obtained at some other sates, and suggest that bacterial growth is probably not limited by nutrient availability 5.0 EXTENT OF SOIL CONTAMINATION 5.1 Lateral Extent I To date, 68 soil samples from borings drilled at this site have been analyzed for petroleum hydrocarbons In addition, 16 soil vapor samples have been collected, one of which was analyzed for hydrocarbons These data have been used to construct two contour maps that delineate the lateral extent of soil contamination at a depth of 20-25 feet (Figures 5 and 6) In constructing these maps, the samples at 20 feet and 25 feet were used In Figure 6, the concentrations of all BTEX compounds (benzene, toluene, ethylbenzene, and xylene) were summed to arrive at a total concentration, if samples were collected at both 20 and 25 feet, the totals were averaged and plotted on Figure 6 for contouring In Figure 5, the higher of the two values was used The extent of BTEX compounds is slightly greater than the extent of TPH Upgradient's July 1996 Assessment of Soil and Groundwater Contamination report suggested that the highest concentrations of both TPH-g and benzene are located between VM-1 and VM-2, beneath the former dispenser island The new data from VM-4 confirm that contamination extends only a short distance east of VM-2, but the data from HP-4 indicate that contamination is also more limited to the north of the dispenser island than previously thought The center of the plume is located fairly close to VM-1, and appears to indicate that the piping, rather than the dispenser, was the source of the leak As we noted in the above-mentioned July 1996 report, "analysis for gasoline oxygenates indicates that these compounds are probably not a significant problem at this site " None of the seven samples from HP-4 contained any oxygenates above the detection limit 5.2 Vertical Extent The vertical extent of contamination varies somewhat between borings In the first five soil borings drilled by Hunter& Associates in 1994, the maximum depth at which any hydrocarbons were found was 25-26 feet (SB-1) This was also the maximum (and only) depth at which hydrocarbons were found in VM-I Probably for this reason, most of the vapor probes within the mapped contaminant plume detected no hydrocarbon vapors Closer to the dispenser, strong odors and a vapor concentration of X500 ppmv were encountered at 12 feet in one vapor probe, indicating that contamination is present at shallower depths in this area In addition, hydrocarbons were detected at 15, 20, and 35 feet in VM-2 and as deep as 32 feet in HP-4 This depth corresponds to the base of the sandy channel deposit described in section 4 2 1, and implies that gasoline migrated downward through the vadose zone until reaching the low-permeability silt bed that underlies this channel (Figure 7) No contaminants have been detected in samples of either sand beds or silt beds below 35 feet However, gasoline and BTEX have been consistently 8