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URS Mr Mike Infurna Page 4 San Joaquin County Environmental Health Department January 5, 2004 HYDROCARBON DISTRIBUTION Hydrocarbons in the Unsaturated Zone Hydrocarbons in the unsaturated zone appear to be confined primarily to the former UST area Soil samples collected at 16 feet bgs during UST removal activities in August 1993 contained up to 4,000 mg/kg TPHg and up to 2 7 mg/kg benzene Soil samples collected beneath the piping in August 1993 contained maximum concentrations of 1,600 mg/kg for TPHg and 0 089 mg/kg for benzene Soil samples collected during drilling activities conducted between 1991 and 1993 indicated that hydrocarbons in soil were primarily confined to the vicinity of the UST complex Figures 4 and 5 show the lateral extent of impacted soil prior to SVE at depths of 15 to 40 feet bgs, and 40 to 60 feet bgs,respectively These figures show that the lateral extent of soil impacts increases with depth,particularly in the area beneath the pump islands However,because the water table has risen to approximately 40 feet bgs, the soil impacts shown in Figure 5 are now below the water table The mass of TPHG in soil prior to remedial action at the site was calculated to be 10,782 pounds of TPHg (Attachment D) The operation of the SVE system removed approximately 9,109 pounds of TPHg (Attachment E) Therefore,a net of 1,673 pounds of TPHg remains at the site The remaming contaminant mass may be present either in the soil or groundwater Tables summarizing historical soil analytical data, prior to operation of the SVE system, are included as Appendix F Hydrocarbons in Groundwater TPHg concentrations reported in groundwater during the December 2002 monitoring event are shown on Figure 6 TPHg impacted groundwater is generally confined to the vicinity of the dispenser islands and the UST complex Relatively low concentrations of TPHg at MW-6 suggest that significant downgradient migration of impacted groundwater has not occurred Benzene concentrations in groundwater are primarily located in the former UST and dispenser island areas An isoconcentration contour map of benzene concentrations in groundwater,as reported during the December 2002,groundwater monitoring event(Table 2),is presented as Figure 7 Figure 8 illustrates the vertical and lateral extent of the dissolved benzene concentrations at the site The maximum benzene concentrations reported in groundwater have been in vapor extraction wells VW-8 and VW-10, which are located approximately 20 feet south-southeast of the dispenser islands The first groundwater sample collected from VW-10 in December 2002 had a reported benzene concentration of 2,800 µg/l_ A block diagram of the dissolved benzene plume is presented as Figure 9 This figure shows that the highest dissolved benzene concentrations are immediately south of the western dispenser island and the lateral extent of the benzene plume has not moved off site An Mtl3E isoconcentration ma for the December 2002 groundwater analytical details resented as Figure 10 p �' Yt P 1� Figure I1 depicts vertical and lateral extent of dissolved MtBE in groundwater at the site Dissolved concentrations of MtBE have migrated off-site at the southeastern site boundary Concentrations of MtBE up to 8 86µg/1.have been reported in off-site well MW-6,which is located downgradient from the pump islands and UST complex However, MtBE has not been detected in MW-6 since September 1999, suggesting that K 1WprocessU5 SP ARCM2130\Site Conceptual Model Report\2130 SCM Rev 2 doc