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During the second quarter 2005 monitoring period, benzene concentration levels were not
<br /> detected at method detection limits of 0 5 ug/L in MWS, MW6a, MW6b, MW6c, MW7a,
<br /> MW7b, MW7c, MW8, MW9a, MW9b, MW9c, MW10a, MW10b and MW10c, which is
<br /> consistent with recent monitoring events
<br /> Composite benzene concentration maps were plotted using data from March 17, 2004, June 3,
<br /> 2004, September 13, 2004, December 13, 2004, March 16-17, 2005, and the June 21-22, 2005
<br /> events The March 17, 2004 and June 3, 2004 sampling data were obtained prior to system start-
<br /> up The June 21-22, 2005 sampling data was information obtained approximately 12 months
<br /> after system operation start-up Reductions in the areal extent of benzene are apparent over time
<br /> Significant reductions of benzene concentrations are apparent along the injection well corridor
<br /> (Figure 21)
<br /> 4.5.2 MTBE
<br /> During this quarter MW6c had the highest MTBE concentrations successively followed by
<br /> MW6b, MW4, EW1, MW1, MW2, MW7c, MW10b, MW6a, MW5, MW7b, MW7a, and
<br /> MW10a The MTBE concentrations in MW5, MW6a, MW7a, MW7b, and MW10a are below
<br /> typical clean-up levels (Table 2)
<br /> MTBE concentrations in MW 1 have historically fluctuated but appear to be declining in the
<br /> second quarter of 2005 The fluctuating MTBE concentrations in MW 1 may be a result of
<br /> periods of system operation and non-operation The concentration of MTBE has decreased from
<br /> 190,000 ug/L prior to system startup to 280 ug/L during the June 21-22, 2005 groundwater
<br /> monitoring event DO also increased significantly after system start-up in MW1 from 0 36 mg/L
<br /> on June 3, 2004 to 10 54 mg/L during the June 21-22, 2005 event
<br /> In MW2, MTBE concentrations have fluctuated but remain similar, only slightly decreasing from
<br /> June 3, 2004 (140 ug/L) to June 21-22, 2005 (100 ug/L) DO has fluctuated after system start-up
<br /> to MW2 from 139 mg/L on June 3, 2004 to 167 mg/L during the June 21-22, 2005 monitoring
<br /> event
<br /> MTBE has not been detected in MW3 at or above method detection limits since March 13, 2003
<br /> DO has increased overall in MW4 but slowly since system start-up in June 2004 and MTBE
<br /> concentrations may be decreasing slowly with increasing time, however the trend is not without
<br /> fluctuations
<br /> MTBE concentrations remain not detected at method detection limits for MW3, MW8, MW9a,
<br /> MW9b, MW9c, and MW l0c The MTBE concentrations in MW5, MW6a, MW7a, MW7b, and
<br /> MW10a are below typical clean-up levels (Table 2)
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<br /> S 1268231QM reports12005 2ndQR doc 10
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