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Evaluation of Natural Attenuation_ 7500 West Eleventh Street, Tracy, C4. Page 35 <br /> concentrations of ferrous iron (Fe-'2 depleted oxygen, depleted nitrates and elevated <br /> concentrations of manganese (Mn ). Under strongly reducing conditions, sulfate <br /> concentrations may be high in areas within a plume relative to sulfate concentrations <br /> outside the area of the plume. Methane may also be produced under strongly reducing <br /> conditions; such methanogenesis may use carbon dioxide as an alternative electron <br /> } acceptor. Because methane is not present in fuel hydrocarbons, it can be used as an <br /> .� indicator of anaerobic biodegradation. However, both aerobic and anaerobic <br /> biodegradative processes can yield high quantities of carbon dioxide so that, while <br /> elevated concentrations of carbon dioxide in groundwater can indicate active bio- <br /> remediation, it,by itself, cannot discriminate between aerobic and anaerobic processes. <br /> -; In the following sections, the geochemical indicators of the presence of active bio- <br /> } remediation are generally discussed in the context of their relationship to the <br /> concentration of BTEX in the primary groundwater plume. The relationships reviewed <br /> have generally similar parallels when the indicators are evaluated in the context of other <br /> analytes of concern, such as TPHg, TPHd, and MTBE. To render the discussion <br /> accessible and avoid subjecting the reader to an unmanageable volume of material, SJC <br /> <i has not presented individual discussions and graphical presentations of the relationship <br /> between geochemical indicators and the concentrations those latter analytes. However, <br /> the relevant relationships can be readily visualized by simple observation of the data <br /> compiled in Table 5 and the readers, if they so desire, are invited to undertake that <br /> exercise by themselves. <br /> -` 7.2.2.1 Ferrous Iron in Groundwater <br /> As is noted above, the presence of ferrous oxide in groundwater within a plume of <br /> petroleum hydrocarbons is indicative of active anaerobic bioremediation. This is because <br /> -� when ferric iron (Feef3) is available in the subsurface it can be used as a terminal electron <br /> acceptor by micro-organisms that yield water-soluble ferrous iron (Fe+), which becomes <br /> oxidized to ferrous oxide (FeO). <br /> -t <br /> As was described in Section 4.0, SJC measured the concentration of ferrous oxide in the <br /> monitoring wells at the site as part of a field test. In the laboratory, the total iron in <br /> samples recovered from the wells was measured by EPA Method 6010B. Both of those <br /> parameters are recorded in Table 5. Using Equation 7 below, these parameters permit <br /> . : calculation of the amount of ferrous iron generated by the anaerobic bioremediation of <br /> the components of fuel hydrocarbons in the subsurface. <br /> k.=..! Weight of Fe+3 per Liter =Total Fe per Liter [Atomic Weight of Fe/(Atomic Weight of <br /> Fe+Atomic Weight of O)] x Weight of FeO per Liter (Equation 7) <br /> The atomic weight of iron is 55.847 and the atomic weight of oxygen is 15.9994 so that <br /> the concentration of ferric iron in the sample is computed as follows. <br /> ?, f Fe+3 = Total Fe—0.777FeO (Equation 8) <br /> r <br /> ! SJC <br /> f__I <br />