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�.+ A S S O C I A T E S I N C L •.. groundwater generally ranges from -400 millivolts (mV) to 800 mV. Under oxidizing conditions the redox potential of groundwater is positive while reducing conditions are negative. Therefore, with aerobic biodegradation depleting groundwater of oxygen within a hydrocarbon plume, the redox potential inside a hydrocarbon plume should be less than that measured outside the plume (Buscheck and O'Reilly, 1995). Indicators of potential intrinsic biodegradation occurring across a dissolved contaminant plume can be summarized by the following trends: A relative decrease in: A relative increase in: dissolved oxygen ferrous iron oxidation-reduction potential alkalinity nitrate carbon dioxide sulfate hydrogen sulfide L 4.1.1 Conditions at Former Fayette Manufacturing Site Between September 1996 and June 1997, groundwater samples collected from each of the site's monitoring wells were analyzed for specific geochemical parameters to determine if intrinsic bioremediation is occurring at the site. In addition, grab groundwater samples were collected from soil borings during this time period and were also analyzed for specific geochemical parameters. Dissolved oxygen concentrations in groundwater within the contaminant plume ranged from <1.0 to 1.2 mg/kg (MW3) to 2.1 ppm (SB6)while concentrations outside the plume ranged from <1.0 to 1.8 mg/kg (MW I). Throughout the site dissolved oxygen concentrations were generally below detection limits (<1.0 ppm) in the groundwater samples collected from the borings. Thus, these results are somewhat ambiguous for interpretation of bioactivity. Where dissolved oxygen was detected, the concentrations in the monitoring wells inside the plume (MW2 and MW3) were slightly lower than those outside the plume. It appears that dissolved oxygen could be a good indicator for groundwater in the wells. According to Buscheck and O'Reilly, 1995, sufficient dissolved oxygen concentrations for aerobic biodegradation of BTEX are >1 to 2 mg/L. The concentrations of dissolved oxygen in the plume beneath this site generally fall within this range. Thus, aerobic biodegradation is most-likely occurring within the dissolved contaminant plume. r Carbon dioxide concentrations, which were generally higher within the contaminant plume than in the borings and wells outside the plume, suggests microorganisms are present in the subsurface and biodegradation is occurring within the contaminant plume. Ferrous iron (dissolved iron) was not detected the borings. However, it was detected in the monitoring well samples at generally higher concentrations within the plume. Generally lower nitrate and sulfate levels within the �., contaminant plume and relatively higher levels outside the plume during the initial sampling event also supports evidence of anaerobic activity. Nitrate and sulfate are evidently acting as electron acceptors throughout the plume where the subsurface environment is oxygen deficient due to wA95122Vepods\Request for NFA doc 10 V