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32 V groundwater samples for iron were filtered, total iron in the groundwater sample is assumed to • provide a close approximation of the iron (II) content Iron concentrations were greatest in the plume center and decreased with distance in the up-, cross-, and downgradient directions ' Again, the contour plot of iron(II) concentrations is coincident and similar in geometry to the hydrocarbon plume interpretations shown in Figures 6-1 and 6-2 The distribution of iron (II) ' at the Site is consistent with the occurrence of biodegradation of the groundwater plume 6 5 3 4 Sulfates ' When sulfate is used as an electron acceptor in microbial degradation of a petroleum hydrocarbon plume, the concentration of sulfates in groundwater is expected to decrease ' Sulfate concentrations m samples collected during the February 4, 1999 monitoring event are summarized in Table 3 of Appendix E Figure 6-7 shows a isoconcentration plot of sulfates in groundwater at the Site on February 4, 1999 Clearly, upgradient and cross-gradient concentrations of sulfate are greater than those found in and downgradient from the plume center The shape of the sulfate contours generally correlate to the shape of the hydrocarbon plumes shown in Figures 6-1 and 6-2, although more dispersed Sulfate appears to acting as an ' electron acceptor for the biologically mediated degradation of the plume petroleum hydrocarbons Sulfate concentrations in well MW-9, cross-gradient to the plume center are above the MCL, while concentrations in wells downgradient of the plume center are at concentrations below MCLs 1 ' 6 5 3 5 Methanogenes►s ' During methanogenesis, carbon dioxide (CO2) is used as an electron acceptor, is reduced by methanogenic bacteria and methane (CI-14) is produced When methanogenesis ' occurs, CHL concentrations increase above background The presence of methane in groundwater is indicative of strongly reducirig conditions Methane is not present in fuel, so its