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CONCLUSIONS <br /> Concentrations of hydrocarbons in the influent vapor stream have increased <br /> over the last two months, with a significant increase from June to July <br /> It has been our experience that influent vapor concentrations during active <br /> remediation periods will decrease over time until a point of asymptotic <br /> conditions are met Concentrations will usually only rise in a "rebound effect" <br /> when the system is off for a period of time In this case, however, the ASNE <br /> system has operated 24 hours per day, 7 days per week since early January <br /> 2002 It has only been off for a couple of hours every 8 to 10 days f o r <br /> maintenance reasons <br /> ASE can only speculate at this time, due to the limited number and frequency <br /> of influent air samples, as to the reason for the recent upturn in vapor <br /> concentrations The reasons could be as follows <br /> • The concentrations have risen since AS-1 and VE-1 were turned off in the <br /> middle of June 2002 It is possible that the additional air infected into a n d <br /> extracted from the 6 other remediation wells has caused a previously un- <br /> recovered pocket of contamination to become mobile, and thus recovered <br /> in the influent vapor stream ASE would expect the increase to be slight, as <br /> it was between May and June However, the significant increase between <br /> June and July would not be the expected result of turning off only one s e t <br /> of remediation wells, <br /> • This could be the result of a recent release from the UST system ASE is <br /> not aware of such a release The groundwater elevation within the on-site <br /> shallow groundwater wells has dropped between 1 to 3 feet since spring <br /> 2002 Typically, when the water table drops, concentrations of petroleum <br /> hydrocarbons also decrease However in this instance, the concentrations <br /> of hydrocarbons within the groundwater monitoring wells on site has <br /> increased slightly <br /> RECOMMENDATIONS <br /> All future influent vapor-stream and groundwater monitoring well samples will <br /> be analyzed for the current analytical suite plus ethanol, the current additive <br /> used by this Unocal station <br /> • <br /> Frank's One Stop Rernechation System Monthly Report — July 2002 <br /> -4- <br />