SITE INFORMATION AND CORRESPONDENCE_FILE 2

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First Semiannuai2ol f Groundwater Monitoring Report Former Chevron Service Station 94054 April 29,2011 Geochemical Parameters Degradation of petroleum hydrocarbon constituents in groundwater 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, manganese oxides, ferric iron oxyhydroxides, sulfate, and carbon dioxide. Anaerobic biodegradation is thus associated with decreased concentrations of nitrate and sulfate, increased concentrations of ferrous iron and dissolved manganese, production and consumption of carbon dioxide, and production of methane within the plume compared with background (Wiedemeier et al. 1999).' Concentrations of geochemical parameters in groundwater presented in Table 1. Results indicate mildly to strongly reducing conditions are present in site groundwater with the most strongly reducing conditions present in groundwater at MW-1A, MW-2, MW-4, MW-4B and MW-8 where concentrations of constituents are highest. These moderately to strongly reducing conditions are likely the result of ongoing microbial degradation of petroleum hydrocarbons present in site groundwater. This interpretation of site groundwater data is supported by multiple lines of evidence including: • Limited or no detections of nitrate indicating nitrate reduction processes have resulted in limited nitrate in site groundwater • Elevated concentrations of dissolved iron at MW-1A, MW-2, MW-4, MW-4B and MW-8 relative to upgradient well MW-14B indicating that reduction of iron oxides has resulted in release of dissolved iron to site groundwater • Elevated concentrations of dissolved manganese at MW-1A, MW-2, MW-4, MW-5, MW-6 MW-8 and MW-11 relative to most upgradient well MW-14B indicating that reduction of manganese oxides has resulted in release of dissolved manganese to site groundwater • Elevated concentrations of methane at MW-1 A, MW-1B, MW-2, MW-3, MW-4, MW-4B MW-5, MW-6 and MW-8 relative to most upgradient well MW-14B indicating the presence of strongly reducing conditions in groundwater in the vicinity of these well 5.1.3 Remediation System Updates An active remedial system is not associated with the site. 'Wiedemeier,T.H.,H.S. Rifai,C.J. Newell and J.T.Wilson, 1999. Natural Attenuation of Fuel Hydrocarbons and Chlorinated Solvents in the Subsurface.John Wiley and Sons, New York. \\oa,p\d Ol pmjecWfthevmn\94054\6 delivembles\periodic mnilonngMMe2011\isa11\temp p~U isall g, 94054_final7U11109W.do 9