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The depth to first groundwater beneath the Site as observed during the Fourth Quarter — 2008 was approximately 45 feet below the top of the well casings (TOC). This water is unconfined and represents the piezometric surface beneath the Site. Based on the Fourth Quarter—2008 measurements, the groundwater flow beneath the Site is directed generally towards the east-southeast at an average gradient of 0.001. A summary of the TOC elevations, groundwater depth measurements and Mean Sea Level elevations is presented on Table 5. 1.4 Purpose The purpose of the SCM is to summarize previous environmental engineering services conducted at the Site, describe current environmental conditions and summarize associated potential exposure pathways. 2.0 SITE CONCEPTUAL MODEL Generalized Site Conceptual Models"depicting the Site conditions as viewed looking west and north are presented on Figures 7 and 8, respectively. Soil boring logs for each boring and monitoring well advanced at the Site to date are presented in Appendix A. As shown on Figure 7, when viewing the Site looking west, residual hydrocarbons are limited to the capillary fringe and saturated soil near well VW-2 located east of the former UST excavation. The dissolved hydrocarbon groundwater plume appears to be sourced from the former UST locations, and extends to the east and southeast (downgradient) to wells MW-2, MW-14 and VW-2. Based on the order of magnitude decrease in concentrations in the samples from deep-screened well MW-2 compared to shallow-screened wells MW-14 and VW-2, extensive vertical migration of petroleum hydrocarbons in groundwater is not apparent. When viewing the Site looking north, the residual soil impacts appear to extend from the east end of the former gasoline UST location approximately 30 feet deep to groundwater. Impacted soil was not encountered in the soil samples collected from wells VW-1 and VW-2 above the groundwater surface; therefore, it appears that the impacted soil is limited to the area between the two wells, an area less than 30 feet across. The lateral extent of the dissolved hydrocarbon groundwater plume extends east of "the former UST locations to wells VW-2, MW-13 and MW-6. Impacts were not reported for the samples from well MW-1, apparently due to the presence of a submerged screen in this well, which again shows that extensive vertical migration of petroleum hydrocarbons in groundwater is not apparent. The lateral extent of the dissolved hydrocarbon groundwater plume may not be fully defined due to the submerged screens present in perimeter wells MW-5, MW-6,MW-10 and MW-11. Project No.Stockton Yard,Task Order.No.45 -3- Caltrans Contract 03AI368,EA No. 10-ON8300 Project No.S9300-06-45 January 28,2009