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ARCO Station 548 C O R 1612 West Hammer Lane,Stockton Remedial Action Plan May 31,2006 4.0 PETROLEUM HYDROCARBON DISTRIBUTION 4.1 Petroleum Hydrocarbons in Soil 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. Sal samples collected beneath the piping in February 1998 contained maximum concentrations of 12 mg/kg of TPHg and 0.03 mg/kg of benzene. Soil samples collected from MW-8 identified TPHg and benzene, toluene, ethylbenzene, and xylene (BTEX) from 45 to 98 feet bgs. Historical soil analytical results are shown in Appendix C. However, because the depth to water has risen to approximately 30 feet bgs, these historical soil impacts are now below the wa era e. The site conceptual model prepared by URS (URS, 2004) assumed that the remaining hydrocarbon mass in soil was negligible. This assumption was based on historical soil Y� analytical data, SVE/AS system performance, and the rise in groundwater elevations. The only analytical data indicating that impacted soil was present above 30 feet was-colle.cted_during the - t nk excavation in 1998_(Appendix C). This data indicates that minor soil impacts existed in the O` , d tank cavity, prior to remediation. SVE wells VW-5S and VW-5D were screened in and below b-3b the old tank cavity to remediate soil impacts in the area. These wells were calculated to have a radius of influence of 30 feet (URS, 2004). That radius of influence is large enough to allow VW-5S and VW-5D to remediate soil impacts in and around the tank cavity. In addition, depth to groundwater has decreased from approximately 50 feet bgs to approximately 30 feet bgs since site investigations began in 1991. This rise in groundwater elevations has effectively submerged the impacted soil at the site. The submersion of impacted soil has probably transferred contamination from soil to groundwater, effectively eliminating soil impacts above the former water table. Since SVE/AS operations have removed significant hydrocarbon mass from the site, and groundwater elevations have risen at least 20 feet, impacted soil is no longer considered to be significant to site conditions. 4.2 Petroleum Hydrocarbons in Groundwater Since February 1995, concentrations of petroleum hydrocarbons, specifically TPHg and Methyl Tertiary Butyl E), have historically been detected in on-site wells MW-1 and MW-5 and off-siteell MW-8. TPHg and MtBE were reported in well MW-5 in August 1996 at a maximum con ' in of 9,600 microgram per liter (lag/L) and 39 lag/L, respectively (Table 4). With the exception of occasional minor detections of TPHg, and MtBE, all other site wells have consistently reported non-detectable concentrations of petroleum hydrocarbons above laboratory reporting limits. Groundwater at the site has been monitored regularly since site investigations began in 1990. This data is included in Table 4. Table 1 summarizes construction details for all site wells. Groundwater analytical data from the March 2006 monitoring event are presented in Appendix D. Updated cross-sections including well AS-1 are included in Figures 21 through 26. Groundwater impacted by TPHg is generally confined to the area of the UST complex (Appendix D). The maximum concentration of TPHg reported in March 2006 was in VW-3; at the hBP-ARCO\04 BP Valley Portfolio15481Reports\RAP1548 RAP.doc 6