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It is felt, that through vapor extraction and limited over-excavation of the UST area the source (soil contamination) has been removed and the soils portion of the remedial action efforts has been accomplished. GROUNDWATER Grab samples of the water obtained on June 4 and 5, 1997 from soil borings SB5, SB6 and SB7 were compared to the historic water samples obtained on December 7, 1988 from SB2 and SB3,•see Table - and Charts in CONFIRMATION OF THE EFFECTS OF THE VAPOR EXTRACTION SYSTEM, dated July 31, 1997. Comparing S133 to SB6 a 93% reduction in the amount of TPHg has been accomplished. Benzene, Toluene and Ethylbenzene have been reduced to below laboratory low=er detection limits and Xylenes have been reduced by 99%. Similar reductions can be seen when comparing SB2 with SB5 and SB7, or 92% and 85% for TPHg 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 SB7 was converted into a two inch PVC well MW13. This well and MW1, 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). It is anticipated that sparging of these wells will introduce 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 MW 12 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 MW 12 contained 1.3 and 1.8 mg/L. Once the sparging and vapor extraction is turned off, the DO in the groundwater is used up almost immediately, see Figure 10. Natural Attenuation is occurring at this site. Figures 11, 12 and 13 represent Nitrate, Sulfate and Ferrous Iron groundwater analysis obtained on December 16, 1997. These figures along with Figure 10 (Dissolved Oxygen in groundwater) demonstrate that the available oxygen is being utilized by microorganisms and a reducing environment has been established at MW2 with the onset of methogenisis. 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. 8 R603 Update##52/CLOSURE RV 7/98