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fl ( O%TC . A S S O C I A T E S I N C . 13,2004). During the December 13, 2004 event, benzene in MW4 was reported at a concentration of<0.5 ug/L. If this reduction was attributed solely to system activities this would equate to a removal efficiency of 99.9% after approximately 6 months of operation. (Note In addition, DO concentrations have increased notably in MW4. The increase in DO after system start-up would indicate influence by the C-SpargeTM process.) Benzene maps were plotted using data from March 17, 2004, June 3, 2004, September 13, 2004 and December 13, 2004. The March 17, 2004 and June 3, 2004 sampling data were obtained prior to system start-up. The September 13, 2004 and the December 13, 2004 sampling data was information obtained approximately 3 and 6 months, respectively 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 decreased in all wells except MW 10b. This reduction of MTBE is significant and will be used to evaluate the remediation system effectiveness over time. MW1 had the highest concentrations successively followed by EWI, MW4 and MW2 (Table 2). MTBE concentrations in MW1 have historically fluctuated, however the overall MTBE trend may be a true declining trend. If the trend is actually declining and it is associated with the ongoing remediation, some mixing may have occurred around the time of the August 13, 2004 monitoring event. Since DO also increased significantly after system start-up the results may be due to remediation system operations. (Note: Where the C-Sparge system is effective, results of the groundwater sampling may indicate an immediate rise in concentration of MTBE due to mixing followed by a progressive drop in concentration. The agitation of the groundwater and capillary pores by the fine bubbles strips adsorbed fractions. The mixed concentrations are often a better measure of total mass for treatment than solely the aqueous fraction.) The concentration of MTBE in MW1 has decreased from 190,000 ug/L prior to system startup to 49,000 ug/L on December 13, 2004. The DO concentration in MW1 increased from 0.36 mg/L on 3 June to 6.79 mg/L on December 13, 2004. In MW2, MTBE concentrations decreased from June 3, 2004 (140 ug/L) to December 13, 2004 (96 ug/L). DO has also increased after system start-up in MW2 from 1.39 mg/L on June 3, 2004 to 2.29 mg/L on December 13, 2004. MTBE was not detected at method detection limits in MW3 during this monitoring period. In MW4, MTBE concentrations decreased from June 3, 2004 (9200 ug/L) to December 13, 2004 .. . . . .(2600 ug/L). Although MTBE concentrations are decreasing with time, fluctuating conditions are evident. If the trend is actually declining and it is associated with the ongoing remediation, some mixing may also have occurred around the time of the August 13, 2004 monitoring event. DO has increased after system start-up in MW4 from 0.38 mg/L on June 3, 2004 to 2.20 mg/L on December 13, 2004. Since DO also increased significantly after system start-up the results may be due to remediation system operations. SA268231QM reports12004 4QR.doc 10