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The estimated mass of petroleum hydrocarbons remaining in Zones A and B is substantially the same as the mass reported in the Estimated Mass Remaining Letter Report(Ash Creek, 2012b). A copy of the report documenting the original estimate as well as updated estimated masses are provided in Appendix G. Extent in the C-Zone. Because it is composed of more continuous sand units (see Figures 3 through 5) than the AIB-zone, concentrations in the C-zone have migrated slightly further. However, concentrations do not extend more than about 500 feet from the Facility, as shown on Figure 14. The C-zone wells closest to the Facility were installed in 2009; C-zone analytical results collected since 2009 are contained in Table E-1 of Appendix E. Review of these results indicates that the petroleum constituents and MTBE concentrations in the C-zone are not increasing and the extent of the plume is stable. As discussed in Section 2.3, groundwater velocity in the C-zone is estimated to flow between 130 and 440 feet per year. The stability of the dissolved-phase hydrocarbon and MTBE concentrations in the presence of relatively high groundwater velocity indicates that microbial degradation is successfully limiting migration. It should be noted that concentrations shown in Table 3 and Appendix E represent samples that were not treated with silica-gel prior to analysis. When C-zone sample extracts are treated with silica-gel prior to analysis, resulting TPHd concentrations are generally below detection limits(see Table 5). The estimated mass of petroleum hydrocarbons remaining in the C-zone is substantially the same as the mass reported in the Estimated Mass Remaining Letter Report(Ash Creek, 2012b). A copy of the report documenting the original estimate as well as updated estimated masses are provided in Appendix G. Extent in the D-Zone. As for the C-zone, historical extent maps have not been prepared for the D-zone because most D-zone wells were installed in 2009 and 2010; however, analytical results from the most recent monitoring event (August 2012) are shown on Figures 15. Monitoring results for D-zone wells downgradient of the Facility are tabulated in Table 3 and Appendix E. The estimated mass of petroleum hydrocarbons remaining in the D-zone is substantially the same as the mass reported in the Estimated Mass Remaining Letter Report(Ash Creek, 2012b). A copy of the report documenting the original estimate as well as updated estimated masses are provided in Appendix G. 4.0 Conceptual Site Model A conceptual site model (CSM) was developed in the Investigation Summary Report (Ash Creek 2012a) submitted to the Water Board on January 6, 2012. This section presents the CSM, including the information used in its support and development. The CSM summarizes the potential receptors and exposure pathways of petroleum hydrocarbons associated with the Facility under current or reasonably anticipated future conditions. The CSM for the Facility considers physical properties,the nature and extent of dissolved-phase Revised Cleanup Plan Page 13 ak NuStar Stockton Terminal-Stockton California November 7,2012 1014-12