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1 <br /> 1 Ms Margaret Lagorio <br /> Page 4 of 5 <br /> are summarized in Table 4 Laboratory reports and chain of custody documentation <br /> 1 � y are included in <br /> Attachment E <br /> Mass Removal <br /> 1 Mass removal was calculated utilizingthe average of the influent concentrations measured at the <br /> beginning and the end of each period of intermittent operation <br /> 1 Fa�.� <br /> During the three-month period of intermittent operation the system operated for a total of 1,186 hours It <br /> is estimated that the system removed approximately 520 pounds of hydrocarbons from the subsurface <br /> ' during this time The majority of the calculated removal occurred during the first period of intermittent <br /> operation because the beginning vapor concentration (1,000 ug/1) and ending concentration (170 ug/1) <br /> resulted in a mean concentration of 585 ug/1 This may actually overstate the average Subsequent <br /> 1 influent vapor concentrations were quite uniform, varying only between 200 and 280 ug/1 At shutdown, <br /> the influent concentration was 210 ug/l and the system was removing hydrocarbons at a rate of <br /> approximately 5 5 pounds per day The relatively uniform influent vapor concentrations observed during <br /> 1 2002 were higher than those present in late 1999 when the system was Iast operated This is attributed to <br /> the approximate 2-foot drop in water levels during the intervening period, which exposed additional soil <br /> column to vapor extraction <br /> ' The vapor extraction action system has now removed an estimated total of approximately 13,200 pounds of <br /> 1 gasoline hydrocarbons from the site The recent three-month period of intermittent operation accounts for <br /> approximately 4%of that total Operational periods and mass removal are summarized in Table 5 Figure <br /> 3 depicts influent vapor concentrations versus total hours of operation, the trend of which is asymptotic <br /> 1 The unit cost of the gasoline removed during the three-month period of intermittent operation is estimated <br /> to have been approximately $240/gallon Figure 4 depicts the cost per gallon during the entire period of <br /> vapor extraction system operation <br /> RECOMMENDATIONS <br /> ' Groundwater Monitoring and Sampling <br /> 1 Fieldwork for the third quarter is scheduled to be conducted in August 2002 EHD will be notified 48 <br /> hours prior to conducting the quarterly fieldwork Given the massive quantity of historical water level and <br /> dissolved contaminant data at this site, we see no necessity for monitoring and sampling on a quarterly <br /> 1 basis We recommend a reduction in monitoring and sampling frequency to an annual, or at least semi- <br /> annual, schedule <br /> Vapor Extraction System Operation <br /> 1 <br /> The current cost of operating the soil vapor extraction system is not justified by the relatively small <br /> 1 incremental effect on the total hydrocarbon mass removed from the subsurface However, water levels are <br /> on the decline and it is very possible that the San Joaquin Valley is entering a period of normal to below- <br /> normal ramfall which may result in continued decreases in the coming years If so, contaminated soil that <br /> is now saturated may become exposed We recommend removing the vapor extraction/thermal oxidizer <br /> ' roekNgrnr11gm2002 <br /> 1 <br />