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In anaerobic environments microorganisms may use other compounds such as nitrate, ferric <br /> iron and sulfate as electron acceptors. An increase in ferrous iron (i.e. reduced ferric iron), <br /> `' carbon dioxide and perhaps hydrogen sulfide and a corresponding decrease in nitrate and/or <br /> sulfate within a hydrocarbon plume indicate that anaerobic biodegradation is taking place. <br /> L. <br /> Additional indicators of anaerobic biodegradations include total alkalinity and redox potential <br /> (Buscheck and O'Reilly, 1995). The total alkalinity of a groundwater system is indicative of <br /> the water's capacity to neutralize acid. Alkalinity results from the dissolution of rock <br /> (particularly carbonate rocks), the transfer of carbon dioxide from the atmosphere, and the <br /> respiration of microorganisms. Therefore, an increase in alkalinity within a hydrocarbon <br /> plume is potentially an indicator of bioremediation occurring (Buscheck and O'Reilly, 1995). <br /> `- The redox potential of groundwater generally ranges from -400 millivolts (mV) to 800 mV. <br /> Under oxidizing conditions the redox potential of groundwater is positive while reducing <br /> �. conditions are negative. Therefore, with aerobic biodegradation depleting groundwater of <br /> oxygen within a hydrocarbon plume, the redox potential inside a hydrocarbon plume should be <br /> less than that measured outside the plume (Buscheck and O'Reilly, 1995). <br /> Indicators of potential intrinsic biodegradation occurring across a dissolved contaminant plume <br /> can be summarized by the following trends: <br /> A relative decrease in: A relative increase in: <br /> dissolved oxygen ferrous iron <br /> L. oxidation-reduction potential alkalinity <br /> nitrate carbon dioxide <br /> sulfate hydrogen sulfide <br /> 662 Conditions at Former Fayette Manufacturing Site <br /> Groundwater samples collected from each of the borings and each of the site's monitoring <br /> wells were analyzed for the indicator parameters noted above. Several of the geochemical <br /> parameters indicate intrinsic bioremediation is occurring within the dissolved petroleum <br /> hydrocarbon plume at the subject site. The results of these chemical analyses are presented in <br /> �., Table 2. Geochemical trends from the results are discussed below and are summarized in <br /> Table 5. <br /> 1 <br /> J <br /> w:1951221Plumerpt 13 <br /> • <br /> SMTN <br />