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• The results of prior bioremediation sampling indicate that natural attenuation/bioremediation i s active at this site All of the wells show the impact of active biodegradation. It is not therefore possible to determine a background level of the compounds. The closest approximation for background levels at this site is the highest concentrations of electron acceptors and the lowest levels of by-products. Compound Function Concentration Well Dissolved Oxygen (02) Electron Acceptor 0.3 mg/l MW 7 Nitrate Electron Acceptor 2.2 mg/l MW 6 Sulfate Electron Acceptor 102 mg/l MW 4 Ferrous Iron By-product 3.11 mg/l MW 2 Methane By-product VAPOR MW 8 SAMPLE Carbon Dioxide By-product VAPOR VAPOR SAMPLE EXTRACTION AND MW 8 The presence of Ferrous iron in the wells indicates that biodegradation has progressed to the point that the system is oxygen deficient and the bacteria have started to reduce the iron to provide oxygen for the degradation. Biodegradation in MW2, which is in the heart of the plume, has consumed all of the available electron acceptors. The carbon dioxide (CO2) levels in the vapor extraction effluent stream indicate that a portion of the hydrocarbons have been degraded. CO2 and water are the final byproducts of the biodegradation of hydrocarbons. The carbon in CO2 results from the oxidation of the hydrocarbon radical CH2 and as such I mg of CO2= 0.41 mg of CH2 (CH2(12+1+1 = 14)/(CO2 (12+16+16=34). The presence of methane (CH4) indicates that a number of the wells had progressed into methogenesis. The levels of electron acceptors present and the presence of the reaction products, carbon dioxide, methane and ferrous iron indicate that the bacteria in the soil and the compounds in the groundwater have the capability to consume a significant amount of hydrocarbons. The gasoline range hydrocarbon plume associated with this site is declining in concentration and should continue to do so, since the removal of the source in the soils beneath this site. Total Petroleum Hydrocarbons are represented on Figures 6A, 6B and 6C. Benzene in soil is represented on Figures 7A, 7B and 7C. • 9 R603 Update 452/CLOSURE RV 7198