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Former Beacon Station No 12419 March 18,2002 Stockton California Problem Assessment Report/Corrective Action Plan 7.1 Distribution of Petroleum Hydrocarbon-Impacted Soil Total petroleum hydrocarbons as gasoline (TPHg) and benzene soil analytical data are plotted on cross sections A-A' and B-B' (Figures 3 and 4) to illustrate their lateral and vertical distributions in soil The approximate lateral extent of total petroleum hydrocarbons as gasoline (TPHg) and benzene in soil is also plotted on the site map (Figure 6) Available analytical data indicate petroleum hydrocarbon constituents were reported primarily in soil samples from soil borings SB-5 and SB-6 (vapor wells VW-1 and 2, and air sparge well AS-1), monitoring well MW-9 and air sparge well AS-2 at depths ranging from 15 to 125 feet bsg as shown on the cross sections and summarized in the soil analytical data tables (Table 3 and Appendix A tables) The distribution of TPHg and benzene in soil, as shown on Figure 6 and the cross sections, suggests the source of petroleum hydrocarbons in soil is in the vicinity of air sparge wells AS-1 and AS-2 The lateral extent of TPHg and benzene in soil is defined by monitoring wells MW-6, MW-10, MWA, MW-7, SB-4 and air sparge well AS-3 The highest TPHg and benzene concentrations in soil (100 to 8500 ppb TPHg and 0 16 to 48 ppb benzene) were reported in soil samples from soil boring SB-6 (completed as vapor wells VW-1 and 2, and air sparge well AS-1) and from 90 feet bsg in the air sparge well AS-2 boring Relatively low concentrations of TPHg or benzene were reported in soil samples collected from soil boring SB-5 and the monitoring well MW-9 boring Concentrations of TPHg in soil samples collected from the air sparge well AS-2 boring at 100 to 125 feet bsg in sand are generally an order of magnitude Iess than that found at 90 feet bsg No TPHg or benzene was reported in the lowermost soil sample collected at 130 feet bsg in a sandy silt from the air sparge well AS-2 boring These data suggest that the groundwater level at the time of initial petroleum hydrocarbon impact stood at approximately 95 feet bsg 419PARCAP3-02 doc 12 Protect No 1419 23 HORIZON ENVIRONMENTAL INC.