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1 f GeolagicaI wtala t= Page 6 Additional Site Characterization Work Plan Project No.724.2 March 15,2005 3.0 SITE CONCEPTUAL MODEL In 2002 GTI developed a site conceptual model (SCM) in accordance with the SJC/EHD directives. The SCM data was included in our June 27, 2002 "Additional Site Characterization Work Plan" and consisted of cross sectional, contour diagrams and mass estimate calculations. Groundwater Figure 6 shows the lateral extent of the groundwater plume as interpreted in 2002; which also shows the proposed location of several multi-chamber well sets. This proposal was subsequently changed to include the current configuration. The model was updated in 2003 after the installation of the three additional well sets —MW- 7/107/207, MW-8/108/208 and MW-9/109/209. The groundwater plume was demonstrated to extend off-site to the northeast location of the MW-8/108/208 well set. The other two well sets define the lateral extent of the plume in their relative directions. Figure 3 shows the shallow well set TPH-G concentrations for the January 2005 monitoring event. A comparison of the two figures reveals a similarity in the core plume location on-site with uncertainty off-site to the northeast. The multi-depth screens of the well sets allowed better definition of the vertical distribution of the plume. GTI has modified Cross Section B —B' to illustrate the vertical distribution of the groundwater plume in the southwest -= northeast trend direction. Figure 8 shows an updated interpretation of the vertical extent of the groundwater plume in the on-site location beneath the former dispenser island (see Figure 2 for cross section locations). The SCM is incomplete because the plume is undefined down gradient(northeast) of MW-8/108/208. Soil The 2002 SCM model soil data was also updated after the installation of the three additional well sets. GTI's January 21, 2004 "Additional Site Characterization Report" included cross sectional, contour diagrams and mass estimate calculations for the soil contamination. This effort completed the definition of the soil plume that is concentrated beneath the former dispenser island as illustrated in Figure 7 and continues to source the groundwater plume. No additional characterization of the soil plume is recommended in the area under the former tank area. 3.1 Contaminant Distribution The groundwater data obtained from the proposed CPT borings will be used to extend the B — B' cross section in the down gradient direction and refine the site conceptual model.. Through interpretation and extrapolation of the data generated through the history of the project, the distribution of the -contamination, both laterally and vertically, will be