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Ms.Lori Duncan Page 4 of 9 San Joaquin County Environmental Health Department j August 25,2004 HYDROCARBON DISTRIBUTION Hydrocarbons in the Unsaturated Zone Soil samples collected during drilling activities conducted between 1990 and 1993 indicated that hydrocarbon-impacted soils were confined primarily to the vicinity of the UST complex. Soil samples collected at 15.5 feet bgs during UST removal activities in February 1998 contained up to 8,200 mg/kg TPHg and up to 4.6 mg/kg benzene. Soil samples collected beneath the piping in February 1998 contained maximum concentrations of 12 mg/kg of TPHg and 0.03 mg/kg of benzene. Appendix C shows the lateral extent of petroleum-impacted soil prior to S VE/AS activities. Because the water table has risen to approximately 25 feet bgs,the historical soil impacts indicated in Appendix C are now below the water table. Influent analytical data for the SVE system, which operated from September 2000 through June 2004, indicated that the hydrocarbon concentrations in impacted soil were reduced by the SVE system. Tables summarizing historical soil analytical data,prior to the operation of the SVE system, are included in Appendices C through E. Hydrocarbons in Groundwater Groundwater at the site has been monitored regularly since site investigations began. Results of monitoring since 1995 are summarized in Tables 4 and 5. Table 6 summarizes construction details for all site wells. Groundwater analytical data from the August 2004 monitoring event are presented in Figures 6 through 14. Field sampling data sheets,chain-of-custody records, and laboratory analytical reports for the August 2004 sampling event are presented in Appendix H. Groundwater impacted by gasoline range organics (GRO) is generally confined to the southeastern portion of the site, extending from the area of the UST complex downgradient to MW-5 (Figure 6). The maximum concentration of GRO reported in August 2004 was in VW-3, at the southern end of the UST complex (7,000 micrograms per liter [pg/L]). Non-detectable concentrations of GRO at MW-5 suggest that significant downgradient migration of impacted groundwater has not occurred. lsoconcentration maps and cross-sections of GRO in groundwater are presented in Figures 6, 7, 12,and 13. Benzene-impacted groundwater is confined to the UST complex and extends downgradient (Figure 8). The maximum benzene concentration (260 p.g/L)reported in groundwater for August 2004 was in VW-1. An lsoconcentration map and cross-sections of benzene in groundwater are presented in Figures 8, 9, 14, and 15. Groundwater impacted by methyl tert-butyl ether(MtBE) is confined to the site in the area of the former UST complex; it extends east and northeast from the UST complex (Figure 10). The maximum reported concentration of MtBE reported during August 2004 was 280µg/L in VW-1. An MtBE isoconcentration map and cross-sections are presented in Figures 10, 11, 16, and 17. '-r Figures 6 through 17 show that petroleum-impacted groundwater is located primarily around the former UST complex and downgradient from the complex for approximately 50 feet. The r K:`Wprocessl15 BP ARCO\0054812004 SCM1Site Conceptual Model V2.doc