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compounds under oxygen, nitrate, and manganese limited conditions via iron (III) reduction Iron (III) is reduced to iron (11) by microbial mediated oxidation of petroleum hydrocarbons Degradation of benzene via iron (111) reduction occurs as follows ' 60 H+ + 30 Fe(OH)3 + C6H6 =, 6 CO2 + 30 Fe 2+ + 78 H2O ' From the stoichiometry of anaerobic biodegradation of benzene, 41 1 mg of iron (III) are required to completely metabolize l mg of benzene and 1 mg of iron (1I1) is capable of facilitating the destruction of 0 024 mg of benzene Similar calculations can be made using the stoichiometry for toluene, ethylbenzene, and xylene degradation from iron (111) reduction Because of the difficulty in measuring the amount of iron (III) that is available for anaerobic biodegradation, the metabolic byproduct, iron (II), is commonly measured This means that for every 1 mg of iron (11) produced ' during biodegradation, 0 047 mg of benzene is destroyed Using the average mass ratio of iron (11) produced to total BTEX from the individual compound biodegradation stoichiometry, 1 mg of iron (I1) is produced for every 0 046 mg of total BTEX metabolized during iron (II1) reduction The EAC of groundwater for iron (III) reduction is determined as follows EACFe = 0 046 (Fem-FeB) Where EACF,_=Expressed Assimilative p Capacity, Iran (III) reduction 0 046 = mg/1 BTFX degraded per mg/I iron (1I) produced ratio Fetii = iron (II) concentration measured in plume (mg/L) FeB =background iron (11) concentration (mg/L) An increase in iron (II) and decrease in BTEX concentrations within an existing BTEX plume is a strong indication that indigenous anaerobic microbes are established and actively biodegrading ' petroleum hydrocarbons via iron (III) reduction 7 2,3) Sulfate Reduction Sulfate can be used as an electron acceptor by facultative anaerobic microbes to degrade BTEX compounds tinder oxygen, nitrate, manganese and iron (III) limited conditions vid sulfate reduction Anaerobic biodegradation of benzene by sulfate reduction occurs as follows 7 5 H++ 3 75 50. 2- + C6H6 ==,� 6 CO2 + 3 75 H2S + 3 H2O From ttie stoichiometryu of anaerobic degradation of benzene by sulfate reduction, 4 6 mg of sulfate are required to completely metabolize 1 mg of benzene and 1 mg of sulfate is capable of facilitating the destruction of 0 22 m; of benzene Similar calculations can be made using the stoichiometry for toluene, ethylbenzene, and xylene degradation by sulfate reduction Using the average mass ratio of sulfate to total BTEX from the individual compound biodegradation stoichiometry, 1 mg of sulfate is required to metabolize 0 21 mg of total BTEX The EAC of 17