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{ Dissolved oxygen concentrations in groundwater within the contaminant1 p ume range from 1.2 ppm (MW3) to 2.1 ppm (SB6) while concentrations outside the plume were 1.7 ppm as i exhibited in MW L Throughout the site dissolved oxygen concentrations were generally below, detection limits (<1.0 ppm) in the groundwater samples collected from the borings. Thus, �I these results are somewhat ambiguous for interpretation of bioactivity. Where dissolved oxygen was detected, the dissolved oxygen concentrations in the monitoring wells inside the plume (MW2 and MW3) were slightly lower than the well outside the plume. It appears that dissolved oxygen could be a good indicator for groundwater in the wells but additional data must be collected before a trend could be established. 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 are barely in this range. It appears that the groundwater under the site isenerall oxygen deficient since g Y Yg dissolved oxygen was generally not detected inside, and outside the plume, and the soil vapor data indicates that there is more than sufficient oxygen in the vadose zone for aerobic biodegradation (EPA 1994). However, the oxygen is not dissolving into the groundwater. Thus, aerobic biodegradation is probably not occurring or is occurring at a low rate at certain locations throughout the plume. Carbon dioxide concentrations which were generally higher within the contaminant plume than in the borings and wells outside the plume, which suggests microorganisms are present in the subsurface and biodegradation is occurring within the contaminant plume. Ferrous iron s (dissolved iron) was not detected in any wells or borings. Generally lower nitrate and sulfate . levels within the contaminant plume and relatively higher levels outside the plume 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 depletion of oxygen from aerobic activities. Concentrations of Eh are generally lower within the plume than outside the plume indicating bioremediation is occurring within the contaminant plume. Carbon dioxide, nitrate, sulfate and Eh appear to be good indicator parameters for intrinsic bioremediation at this site. With regard to dissolved oxygen, additional data must be collected before any trends can be established. The predominantly non-detectable concentrations of dissolved oxygen in conjunction with the lack of ferrous iron concentrations supports evidence that aerobic pp ob c bioremedration is not , occurring widespread in the contaminant plume. This evidence in conjunction with the pattern of nitrate and sulfate concentrations and the largely negative Eh levels indicate that anaerobic Lw:195I221Plumerpt 14 ` • SMTN r