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3.1 Soil SPH released from AST 3302 was present in the upper 3 to 4 feet of the silt unit that extends to at least 10 feet bgs near the AST. In 2003, the majority of soil containing SPH in the area of the AST was removed. Although a significant portion of the SPH was removed during the 2003 removal action, residual hydrocarbons remaining in soil adjacent to subsurface utilities were inaccessible due to the potential for damaging the utilities during excavation. In addition, residual hydrocarbons likely present beneath AST 3302 were not accessible for removal. While SPH was observed in well ST-MW-1 in 2003 following source removal and was occasionally observed in well ST/MW-14, it has not been observed in either of these wells since 2006. This suggests that SPH is no longer present in the subsurface soil at the Facility. The estimated mass remaining in soil was presented in the Estimated Mass Remaining Letter Report(Ash Creek 2012b) and submitted to the Water Board on May 15, 2012. No new information on petroleum mass in soil has been obtained since the submittal of the May 15, 2012 report. 3.2 Groundwater As described in Section 2.6, investigations have defined the lateral and vertical extent of petroleum hydrocarbons and related constituents in groundwater. The following paragraphs discuss the nature and extent of these constituents in the A/B-, C-and D-zone groundwater. Extent in the A/B Zone. Groundwater monitoring conducted since the early 1990s has shown that the dissolved-phase concentrations in the A/B-zone do not extend more than approximately 275 feet downgradient of the Facility. Downgradient migration of the concentrations has been limited by: (1) the discontinuous nature of the A/B-zone sands; (2) a low-permeable silt ridge located immediately east of the Facility(Figure 4);and (3)ongoing biodegradation of dissolved-phase concentrations. Figures 10 through 13 illustrate the extents of dissolved TPHg, TPHd, benzene, and MTBE, respectively, in A/B-zone groundwater in 1995, 2005, and 2012. Individual concentration maps were prepared for each of these years for each constituent to develop these maps; for reference, copies of the individual concentration maps are contained in Appendix D, and historical analytical data from groundwater monitoring in the southern leasehold is tabulated in Appendix E. As shown on Figures 10 through 13, the extents of the dissolved-phase hydrocarbons and MTBE have been stable or decreasing since 1995. Concentration trend plots for TPHg, TPHd, benzene, and MTBE in wells PS/P-11, PS/P-12, PS/P-13, and PS/MW-15 are contained in Appendix F. Wells PS/P-11, PS/P-12, PS/P-13, and PS/MW-15 are located directly downgradient of the former source area at the Facility. As shown on the trend plots, TPHg, TPHd, BTEX, and MTBE concentrations are generally stable or decreasing in these wells. MTBE concentrations increased in well MW-15 following the 2002 gasoline release from AST 3302; however, concentrations have been decreasing since 2008. Revised Cleanup Plan Page 12 NuStar Stockton Terminal-Stockton,California November 7,2012 1014-12