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degradation pathways Indicators include microbial activity, <br /> • tY oxygen, <br /> nitrate, iron, manganese, and other electron acceptors The results are <br /> used to estimate the inherent capacity of an aquifer to degrade <br /> contaminants <br /> a <br /> IBR Capacity Estimation <br /> a <br /> The amount of biodegradation able to be supported by the ground water <br /> that moves through the zone of chemical impact is calculated based on the <br /> following rationale <br />' 1 Ground water upgradient of the source contains pgi' to ns electron acceptors <br />' 2 As the upgradient ground water moves through the source zone, free- <br /> phase petroleum product and contaminated soil release soluble <br /> hydrocarbons (e g, BTEX compounds) <br /> 3 Biological reactions occur until the available electron acceptors are <br /> consumed (Three exceptions to this conceptual model are the iron <br /> and manganese reactions, where the electron acceptors, iron (III), <br /> manganese (IV), dissolve from the aquifer matrix; and the methane <br /> reactions, where the electron acceptor CO2 is in a complex equilibrium <br /> with the alkalinity reactions For these reactions, the metabolic by- <br /> products, iron (II), manganese (11), and methane, can be used as <br /> proxies for the potential amount of biodegradation that occur from the <br /> iron-reducing and methanogenesis reactions) <br /> 4 The total amount of available electron acceptors for biological reactions <br /> can be estimated by 1) calculating the difference between upgradient <br /> concentrations and source zone concentrations for DO, nitrate, and <br /> sulfate, and 2) measuring the production of metabolic by-products <br /> (methane and divalent iron and manganese) in the source zone <br /> 5 Using stoichiometry, a utilization factor can be developed showing the <br /> ratio of the DO, nitrate, and sulfate consumed to the mass of dissolved <br /> hydrocarbons degraded in the biodegradation reactions Similarly, <br /> utilization factors can be developed to show the ratio of the mass of <br /> metabolic by-products that are generated to the mass of dissolved <br /> Ihydrocarbons degraded in the biodegradation reactions Wiedemeier, <br /> Wilson,et at, (1995) provides the following utilization factors based on <br /> l the degradation of dissolved hydrocarbon constituents <br /> I <br /> A-2 <br />