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Second Semiannua1201Groundwater Monitoring Report Former Texaco Service Station Chevron Site ID 309075 November 22, 2011 beneath the site are polar compounds, and are not dissolved hydrocarbons. The most likely source for most of the polar compounds at MW-1 is biodegradation of the residual petroleum. The polar compounds at MW-2, MW-3, MW-4, MW-5 and MW-6 may also be from biodegradation, but some of the peaks that are consistent between wells may be sampling/lab artifacts or background organic compounds. The polar compounds at MW-6D are a complex mixture that does not appear to be related to the polar compounds at MW-1, and likely result from an offsite source. In addition, the mixture of polar compounds at MW-6D is not likely related to the polar compounds at MW-1, because the concentration of polar concentrations at MW-6D (measured as TPH-DRO and TPH-MO without silica gel cleanup) is significantly higher than the concentration of polar compounds at MW-1, and most likely results from an offsite source The presence of the polar compounds in groundwater is evidence that in-situ biodegradation is active." Although the groundwater sample collected from MW-1 contained a concentration of 1,2-DCA above the California Primary maximum contaminant level (MCL), 1,2-DCA is stable and not migrating. 1,2-DCA has not been detected in any other well since groundwater monitoring began in 2008. The groundwater samples collected from the site monitoring well network did not contain concentrations of BTEX, MTBE, DIPE, ETBE, TAME, EDB, ethanol or methanol above their respective laboratory method detection limit (MDL). 5.1.3 Remediation System Updates A remedial system is not associated with the site. 5.1.4 Plume Stability ` Geochemistry Parameters �. Data gathered during the semiannual monitoring and sampling event indicates the plume is stable. In addition, geochemistry parameters (Attachment H) analyzed in groundwater samples collected on December 1, 2010, indicate generally mildly reducing conditions in groundwater across the site based on detected concentrations of nitrate in groundwater, with the exception of the MW-1 groundwater sample which potentially indicates the presence of more strongly reducing conditions in the vicinity of this well. Degradation of organic matter in groundwater, including petroleum hydrocarbons, can proceed via aerobic or anaerobic microbial processes. Bacteria present in soil and groundwater obtain energy for cell production and maintenance by facilitating thermodynamically advantageous reduction-oxidation (redox) reactions involving the transfer of electrons from electron donors to available electron acceptors. When sufficient dissolved oxygen is present in groundwater, aerobic biodegradation of hydrocarbons dominates. As oxygen becomes less available, anaerobic microorganisms consume electron acceptors in the following order of preference: nitrate, ferric iron oxyhydroxides, sulfate, and carbon dioxide. Anaerobic biodegradation is thus associated with decreased concentrations of nitrate and sulfate, increased concentrations of dissolved iron, production and consumption of carbon dioxide, and production of methane within the plume compared with background (Wiedemeier et al. 1999). g:\aproject\env\chevron\309075\6 deliverables\periodic monitoring\2011\1 sa11\temp pdf\text\309075 1 sa11 gmr fin 11222011.doc 6 V