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1 <br /> ARCADIS GERAGHTY&MILLER <br /> the nitrate data is not conclusive, the data is consistent with the interpretation that nitrate <br />' appears to acting as an electron acceptor for the biologically mediated degradation of the <br /> plume petroleum hydrocarbons <br /> 6533 Iron 1I <br /> Once the available dissolved oxygen and rutrate in the groundwater have been depleted, <br /> iron(III) can be used as an electron acceptor The reduction of insoluble iron(III), also known <br /> as ferric iron, to the soluble form iron (II), also known as ferrous iron, occurs through <br />' microbially oxidation in groundwater When iron (III) is used as an electron acceptor in <br /> microbial degradation of a petroleum hydrocarbon plume, the concentration of iron (II) in <br />' groundwater is expected to increase Iron concentrations in samples collected during the <br /> February 5, 1999 monitoring event are summanzed m Tables 3 and 4 of Appendix D Figure <br /> 6-6 shows an isoconcentration plot of total iron concentration in groundwater at the Site on �. <br /> February 5, 1999 Since only iron (Il) is in the soluble form (iron [III] is insoluble), and the <br /> 4 ��! <br /> groundwater samples for iron were filtered, total iron in the groundwater sample is assumed to N <br /> provide a close approximation of the iron (II) content Iron concentrations were greatest in <br />' the plume center and decreased with distance in the up-, cross-, and downgradient directions <br />' The contour plot of iron (H) concentrations is coincident and similar in geometry to the <br /> hydrocarbon plume interpretations shown in Figures 6-1 and 6-2 The distribution of iron (II) <br />' at the Site strongly suggests the occurrence of biodegradation of the hydrocarbon plume <br />' 6 5 3 4 Sulfates <br /> When sulfate is used as an electron acceptor in microbial degradation of a petroleum <br /> hydrocarbon plume, the concentration of sulfates in groundwater is expected to decrease <br /> Sulfate concentrations in samples collected during the February 5, 1999 monitoring event are <br /> summanzed in Table 3 of Appendix D Figure 6-7 shows an isoconcentration plot of sulfates <br />' 45 <br />