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WwGEH3 DESIC1E0.5iL0161lfU/ib The wells were selected because of their position relative to the source area MW-9 is upgradient of the source area, MW-2A is in the source area, and MW-7 is downgradient of the source area The water quality parameters are summarized in Tables 4 and 5 and discussed below 2.2.7.1 Dissolved Oxygen Oxygen is required for aerobic biodegradation of hydrocarbons During aerobic biodegradation, dissolved oxygen concentrations decrease Anaerobic bacteria generally cannot function at dissolved oxygen concentrations greater than about 0.5 mg/L Measured dissolved oxygen concentrations were generally low and ranged from 0.96 to 1 12 mg/L The low oxygen concentrations are a strong indication that aerobic biodegradation of the BTEX compounds has occurred at the site. 2.2.7.2 Nitrate After dissolved oxygen has been depleted in the microbiological treatment zone, nitrate may be ' used as an electron acceptor for anaerobic biodegradation via denitrification Nitrates were either not detected or detected at low concentrations. The low concentrations suggest that denitrification will not be an important biodegradation process for fuel hydrocarbons 2.2.7.3 Ferrous Iron Iron concentrations ranged from 0 2 to 2 5 mg/L The highest iron concentration was detected in well MW-2A, which is screened across contaminated soil Iron is an electron acceptor for anaerobic biodegradation Iron is elevated in this well (MW-2A) with respect to background (as demonstrated in upgradient MW-9) suggesting that biodegradation of hydrocarbons by iron reduction is occurring ' 2.2.7.4 Sulfate Sulfate concentrations ranged from 2 3 to 91 mg/L. The highest sulfate concentration was detected in the downgradient well MW-7 and the lowest in source well MW-2A Sulfate is another potential electron acceptor that may be used for anaerobic biodegradation of petroleum hydrocarbons The association of elevated BTEX concentrations with depleted sulfate concentrations is a strong indication that anaerobic biodegradation of BTEX compounds through sulfangensis is occurring 2.2.7.5 Methane ' During methanogenesis, an anaerobic biodegradation process, carbon dioxide is used as an electron acceptor and methane is produced The presence of methane in the groundwater plume relative to background in the upgraded monitor well (MW-9) is indicative of strongly reducing conditions. Because methane is not present in fuel, the presence of methane in groundwater in contact with fuels is indicative of microbial degradation of fuel hydrocarbons L1PR0J ECMSANJOQ%HSA\ACT10NPL 12