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Similar reductions can be seen when comparing SB2 with S135 and SB7, or 92% and 85% for TPI-Ig respectively While Benzene, Toluene and Ethylbenzene have been reduced to trace amounts Xylenes have been reduced to 54% and 13% of their original values respectively Confirmation soil boring S137 was converted into a two inch PVC well MW13 This well and MW I, MW2, MW3 and MW 10 are currently being used for vapor extraction MW2 and MW13 are also used for air sparging (commencing on October 9, 1997) Sparging of these wells introduces and make oxygen available for biodegradation of the down gradient wells MW8 and MW12 Prior to shutting off the air sparging vapor extraction system on December 8, 1997, MW2, MW8, MW9 MW12 and MW13 were tested for the amount of DO in the groundwater Sparging had saturated MW2 and MW13 with oxygen, they registered 9 1 and 7 6 mg/L respectively Upgradient well MW9 contained 4 1 mg/L while downgradient wells MW8 and MW12 contained 1 3 and 1 8 mg/L Once the sparging and vapor extraction is turned off, the DO in the groundwater is depleted almost immediately Natural Attenuation is occurring at this site Field groundwater analysis obtained December 16, 1997 for Dissolved Oxygen, Nitrate, Sulfate and Ferrous Iron demonstrate that the available oxygen is being used by microorganisms and a reducing environment has been established at MW2 with the onset of methogenisis, see Figures Appendix F Bacteria native to the soil at hydrocarbon contamination sites normally degrade hydrocarbons The most effective hydrocarbon degraders (eaters) are the aerobic (oxygen using) bacteria The amount of available dissolved oxygen is usually the factor controlling the rate that these bacteria degrade the gasoline A much slower degradation process starts when the dissolved oxygen is depleted The plume begins to become anaerobic and the bacterium commences to reduce nitrate, ferric iron, and sulfate to further degrade the hydrocarbons Eventually, as these compounds and the oxygen are used, the bacteria begin methogenesis, in which the hydrocarbons are converted to methane The prior bioremediation sampling results also indicate that natural attenuation/bioremediation is active at this site 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 1 mg of CO2= 0 41 mg of CH2 (CH2(12+1+1 = 14) /(CO2 (12+16+16=34) 8 8603 Update#54/CLOSURE 3 12/98