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' ENSR AEGON suggest that the Unocal plume is a relative) minor source of groundwater impacts at the Vintage Car Wash gg p Y 9 P g site. This conclusion is based on the following observations: 1. Groundwater concentrations in Zone A are below detection limits upgradient of the Vintage Car Wash site (i.e., at well MW-7), whereas Zone A groundwater concentrations at the Vintage Car Wash site (i.e., at well VX-1) have been comparable to source area concentrations at the Unocal site, indicating that a source area exists at the Vintage Car Wash site. 2. Groundwater concentrations in Zone B on the Vintage Car Wash site (i.e., wells MW-1, MW-2, and MW-3) are significantly higher than those detected at the most downgradient Unocal wells screened in Zone B (i.e., monitoring wells U-20 and U-21). 3. Groundwater concentrations of TPH, benzene and MTBE detected downgradient of the Vintage Car Wash site in Zone C are higher than those detected upgradient of the Vintage Car Wash site. Based on these observations, it is concluded that Unocal station #0187 does not contribute to groundwater contamination in Zone A at the Vintage Car Wash site, but may be a minor contributor to Zone B contamination at the Vintage Car Wash site. It is not clear whether the downgradient extent of impacts at the Vintage Car Wash site has been delineated. Dissolved-phase petroleum hydrocarbon concentrations in Vintage Car Wash site monitoring wells MW-12 and MW-13 are below MCLs. However, additional delineation by Vintage Car Wash may be necessary near the corner of Grant Street and Lindsay Avenue. ' The presence of a downward hydraulic gradient and the pattern of subsurface impacts are suggestive of a potential sinking plume at the site,caused by a downward component to the hydraulic gradient. While concentrations are declining downgradient of the site, the centerline of the plume may be becoming progressively deeper. The higher benzene concentrations detected in downgradient Zone C monitoring well U-21 relative to those detected in well U-12 are consistent with a sinking plume. TPHg concentrations at these two wells are not consistent with a sinking plume; however, due to the predominance of relatively hydrophobic aliphatic hydrocarbons in TPHg, its migration is more strongly affected by sorption than benzene, which may account for the difference in trends, as TPHg concentrations do not extend as far downgradient. 3.4 Groundwater concentration trends Groundwater concentration data are summarized in Table 3-3. Since the ozone sparging system became operational in December 2000, concentrations in several monitoring wells have declined. However, concentrations were declining prior to installation of the ozone sparging system in the majority of these wells. In addition, concentrations have been declining at monitoring well U-1, which is upgradient and beyond the area of influence of the ozone sparge system, suggesting that the ozone sparge system may not be wholly responsible for the decline in concentrations observed in wells near the system. A summary of concentration trends in selected wells is presented in Table 3-4. Trends were determined using a Mann-Kendall statistical analysis. Individual analyses spreadsheets and a description of the Mann-Kendall methodology are presented in Appendix A. Appendix A also includes graphical depictions of concentrations data referenced to the installation of the ozone sparging system. These graphs do not show an increasing rate of contaminant degradation following installation or expansion of the ozone sparging system. 3.5 Biodegradation parameters Dissolved oxygen data for wells in the immediate vicinity of the sparge wells(i.e., U-3, U-10, NP-1-22/65/111 and NP-2-221601116)do not show an increasing trend(see Table 3-5). These data suggest that the ozone sparging system may not be effective at treating impacts on site. However, this could be related to operational considerations(see Sections 3.4 and 5.3). r J:1Rem_Eng\Project Files1P60(6000- 6999)18940-UNOCAL Porttolio CEosure\6940- 3-3 January 2006 UNOCAL Portfolio Closure\She 0187-Stockton East MinoACSM ReporflCSM Report_Finat.doc