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1 ' significant. During the September 13, 2004 event, benzene was reported at 36 ug/L for a removal efficiency of 99.9% after approximately 3'/z months of operation. The successive decline of benzene in EW I from the 35,000 ug/L on June 3, 2004 in each of the monitoring events following was also significant. During the September 13, 2004 event, benzene ' was reported at 57 ug/L for a removal efficiency of 99.8% after approximately 3%months of operation. During the third quarter monitoring period, concentrations of benzene from MW3 and MW4 ' declined. The decline of benzene in MW3 does not appear to have accelerated after system start- up. It appears to have been ongoing at a similar or greater rate of decline. In addition, dissolved ' oxygen (DO) concentrations in MW3 appear to have been stable. MW4 is approximately 16 feet from the closest Spargepoint® (IW4). The decline of benzene in MW4 may have accelerated after system start-up from the 1500 ug/L on June 3, 2004 to later ' monitoring events (i.e., July 16, 2004, August 13, 2004 and September 13, 2004). During the September 13, 2004 event, benzene in MW4 was reported at a concentration of 3.0 ug/L. If this reduction was attributed solely to system activities this would equate to a removal efficiency of ' 99.8% after approximately 3V2 months of operation. (Note: In addition, DO concentrations may have increased very slightly in MW4. The slight increase in DO after system start-up may indicate influence by the C-SpargeTM process.) Benzene maps were plotted using data from March 17, 2004, June 3, 2004 and September 13, 2004. The March 17, 2004 and June 3, 2004 sampling data were obtained prior to system start- up. The September 13, 2004 sampling data was information obtained approximately 3 '/2 months after system operation start-up. Although there are not apparent reductions in the areal extent of benzene, significant reductions of benzene concentrations are apparent along the injection well ' corridor (Figure 21). 4.5.2 MTBE During this monitoring period, MTBE concentrations in most wells appeared to remain fairly consistent (i.e., MW2, MW3, MW5) or show fluctuating trends (MWI, MW4 and EW1) when compared with concentrations before system start-up on June 17, 2004. The MTBE concentrations appeared to decrease slightly, however, in MW8. ' Concentrations of MTBE in MWI, MW2, MW4 and EW 1 are significant and will be used to evaluate the remediation system effectiveness over time. MWI has the highest concentrations successively followed by E W 1, MW4 and MW2 (Table 2). Increases and decreases of MTBE in MWI may be related to periods of system operation and non-operation. However, the conditions need to be further evaluated over the duration of the remediation. One item that supports the analysis that the fluctuating concentrations for MTBE in MWI may be a result of system operation is that the DO also increased significantly after system start-up. The DO concentration in MWI increased from 0.36 mg/L on June 3, 2004 to 4.72 mg/L ' on September 13, 2004. S:126823%QM reports12004 3QR.doc 9 ATC Associates Inc,