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31 <br /> concentrations measured during the February 4, 1999 monitoring event are summarized in <br /> ' Table 6 of Appendix E The distribution of dissolved oxygen, depletion of oxygen near the <br /> center of the hydrocarbon mass and lesser depletion away from the center, is consistent with <br /> ' the occurrence of biodegradation of the groundwater plume <br /> ' 6 5 3 2 Nitrates <br /> ' When nitrate is used as an electron acceptor m microbial degradation of a petroleum <br /> hydrocarbon plume, the concentration of nitrates in groundwater is expected to decrease <br /> ' Nitrate concentrations in samples collected during the February 4, 1999 monitoring event are <br /> summarized in Table 3 of Appendix E Figure 6-5 shows a isoconcentration plot of nitrates in <br /> i <br /> ' groundwater at the Site on February 4, 1999 Clearly, upgradient and cross-gradient <br /> concentrations of nitrate are greater than those found in and downgradient from the plume <br /> ' center The shape of the low nitrate area also coincides with the hydrocarbon plumes shown in <br /> Figures 6-1 and 6-2 Nitrate appears to acting as an electron acceptor for the biologically <br /> mediated degradation of the plume petroleum hydrocarbons Nitrate concentrations in well <br /> MW-6, upgradient from the plume center are above the MCL, while concentrations in wells <br /> ' downgradient of the plume center are reduced to concentrations below MCLs <br /> ' 6533 Iron (B) <br /> Once the available dissolved oxygen and nitrate 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 /> ' nucrobially oxidation in groundwater When iron (III) is used as an electron acceptor in <br /> nucrobial degradation of a petroleum hydrocarbon plume, the concentration of iron (II) in <br /> ' groundwater is expected to increase Iron concentrations m samples collected during the <br /> February 4, 1999 monitoring event are summarized in Tables 3 and 4 of Appendix E Figure <br /> ' 6-6 shows an isoconcentratlon plot of total iron concentration in groundwater at the Site on <br /> February 4, 1999 Since only iron (II) is in the soluble form (iron [III] is insoluble), and the <br />