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• f . AUGEAS Corporation California Natural Products Problem Assessment Report June, 1991 5.0 ANALYTICAL RESULTS AND DISCUSSION 5.1 Soil Samples Soil samples collected from each of the exploratory soil borings and monitoring wells were evaluated for total petroleum hydrocarbon compounds.The laboratory results are summarized and presented in Table 2. Complete analytical reports are located in Appendix 4. All samples submitted for analysis were collected from a depth of five to seven feet. The analytical results show that gasoline has been retained in the near-surface soil. Lateral spreading of l the fuel away from the above-ground tank was facilitated by the sandy nature of the soil within the upper :; • 10 feet of the soil column.The gasoline appears to have spread primarily to the east and north away from the tank.A silty clay layer at a depth of 10 feet below the surface served as a temporary barrier, slowing the downward migration of fuel to the water table. Nearly uniform saturation of a lower sand layer at the soil/water interface throughout the zone of influence, however, suggests that the gasoline has diffused through the clay layer and has reached the water table. Only exploratory boring SB-7 penetrated a completely clear column of soil; all other borings encountered at least detectable levels of gasoline beginning at a depth of approximately five feet below the surface.The zone of contamination extends to the soil/water interface at a depth of approximately 15 feet. PID readings declined in borings SB-4 and SB-9, suggesting that these points represent the boundary of the soil plume. 5.2 Water Samples On August 30, 1990, and June 10, 1991, Augeas Corporation collected groundwater samples from all three monitoring wells. A field blank, labeled MW-4 was also collected as standard quality assurance procedure.Each sample was evaluated for total petroleum hydrocarbon constituents in the gasoline range and purgeable aromatic compounds (EPA Methodology 602). The results of these analyses are sum- marized in Table 3. Complete analytical reports are presented in Appendix 4. Petroleum hydrocarbon compounds were found in two of the groundwater monitoring wells (MW-1 and MW-3).Although no free product was found in either MW-1 or MW-3,significant levels of benzene were detected at levels well above State Action Levels and EPA Primary Drinking Water Standards. MW-3 appears to be near the eastern margin of the main zone of contamination based on the concentration of gasoline found in the soil column. Although MW-I appears to be near the margin of the plume as shown • in Figure 3, the concentrations of BTEX constituents found in soil sample MW-1,5 are very similar to 9 i