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Former Discovery Chevrolet June 6, 1997 <br /> TTacy,California Report of Additional Subsurface Investigation <br /> 1987; and Schink, 1985). The hydrocarbon-sulfate oxidation-reduction reaction produces <br /> hydrogen sulfide compounds and bicarbonate (HCO3). The net result is a decrease in sulfate <br /> concentration. <br /> At sites where anaerobic biodegradation of petroleum hydrocarbons utilizing sulfate is <br /> occurring, groundwater samples collected from monitoring wells that do not contain <br /> dissolved petroleum hydrocarbons will have the highest sulfate concentrations. Samples <br /> from wells within the plume of dissolved petroleum hydrocarbons where anaerobic <br /> degradation utilizing sulfate is occurring will be relatively depleted in sulfate. The water <br /> samples collected from wells at the site and analyzed for sulfate indicate that sulfate <br /> concentrations in the groundwater was higher in MW-5, which did not contain dissolved <br /> hydrocarbons and is outside of the plume. The sulfate concentrations in the remaining wells <br /> (MW-1 through MW-4), which are both in and on the edge of the plume, were lower than <br /> MW-5, indicating utilization of sulfate as an electron acceptor. Therefore, it appears that <br /> anerobic biodegradation utilizing sulfate as an electron acceptor may be occurring or has <br /> occurred. <br /> Conclusions Regarding Intrinsic Bioremediation <br /> The relationships discussed above demonstrate that intrinsic bioremediation is a viable and <br /> active method of remediation at the site. Published rates of BTEX degradation under <br /> aerobic and anaerobic conditions range from 0.3 to 1.3 percent per day, depending on initial <br /> petroleum hydrocarbon concentrations (McAllister and Chiang, 1994; Chiang et. al., 1989; <br /> Kemblowski et. al., 1987; Salanitro, 1992). Additionally, geochemical data appears to <br /> indicate that gasoline hydrocarbon compounds desorbed from soil are actively remediated by <br /> microorganisms in groundwater, it also could be concluded that intrinsic bioremediation will <br /> continue to decrease the levels of gasoline hydrocarbons dissolved in the groundwater <br /> beneath this site. The general overall trend has been a reduction of dissolved petyroleum <br /> hydrocarbon concentrations. <br /> 3001-11.rpt 10 HORIZON ENVIRONMENTAL INC. <br />