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Former Discovery Chevrolet June 6, 1997 <br /> Tracy,Califomia Report of Additional Subsurface Investigation <br /> At sites where anaerobic biodegradation of petroleum hydrocarbons utilizing nitrates <br /> (denitrifying conditions) is occurring, groundwater samples collected from monitoring wells <br /> that do not contain dissolved petroleum hydrocarbons will have the highest nitrate (and <br /> nitrite) concentrations, and may be assumed to represent background nitrate levels. Samples <br /> from wells within the plume of dissolved petroleum hydrocarbons (where anaerobic <br /> degradation utilizing nitrates is occurring) will be relatively depleted in nitrate, due to the <br /> consumption of nitrates in the degradation process. In general, concentrations of nitrates <br /> were found to be higher as you move outside the dissolved hydrocarbon plume to the wells <br /> MW-3 and MW-5, as shown on Table 2. Nitrites were below the laboratory detection limits <br /> for all the wells. This would indicate nitrate is being utilized or has been utilized as an <br /> electron acceptor and anaerobic biodegradation of gasoline hydrocarbons is occurring or has <br /> occurred in the shallow groundwater within the plume. `�:'{ E�'s <br /> Ferric Iron as an Electron Acceptor N i`� ti ,+ ,b <br /> r K �' <br /> Once the available DO and nitrate are depleted, anaerobic microorganisms may use oxidized <br /> ferric iron [Fe(III)] as an electron acceptor. Large amounts of ferric iron are present in <br /> sediments comprising most aquifers. The most readily used ferric iron compounds occur in <br /> amorphous mineral forms (Lovely, 1991). The products of an oxidation-reduction reaction <br /> between petroleum hydrocarbon compounds and ferric iron are carbon dioxide, FeGII), and <br /> water. Siegel, et. al., (1992) have observed that total soluble iron concentrations tend to <br /> decrease down-gradient of the hydrocarbon source as ferrous iron is oxidized by slight <br /> amounts of DO to ferric chloride and other solid iron compounds (McAllister and Chiang, <br /> 1994). The water samples collected from wells at this site and analyzed for dissolved iron <br /> reported higher concentrations of iron within the dissolved hydrocarbon plume area(MW-2), <br /> while the remaining wells were near or below the laboratory detection limit(Table 2). <br /> Sulfate as an Electron Acceptor <br /> Organic compounds have been shown to undergo biodegradation by sulfate reducing <br /> : 1 microorganisms (Smolenski and Sutiia, 1987; Berry, et. al., 1987; Grbic-Galic and Vogel, <br /> 3001-11.rpt 9 HORIZON ENVIRONMENTAL INC. <br /> I <br />