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Former Manna Pro Site Pae 8 Project No 207 3 ' Remedial Feasibility Study 5.0 VAPOR EXTRACTION WITH AIR SPARGING As discussed above, in recent years groundwater at the site has been recorded at historically ' high levels The surface of groundwater has recently been as high as 25 feet bgs, but as late as 1994, the groundwater surface was as low as 40 feet bgs This rise in groundwater elevation was coeval with six continuous years of greater-than-average rainfall in the northern San Joaquin Valley The highest concentrations of gasoline are known to lie from about 35 to 45 bgs or about 10 to 25 below the surface of groundwater Due to the fact that the bulk of the contaminant mass lies 10 to 20 feet below the groundwater surface, we ' postulated that remedial measures based solely on vapor extraction would not be sufficient to materially reduce the contaminant mass below groundwater in a reasonable period of time (see Section 3 0) Air sparging is a process in which air is infected into the saturated subsurface This, in-turn, introduces oxygen, thus helping to facilitate the growth of aerobic bacteria, and it also mobilizes contaminants bringing them to the groundwater surface thus ' enhancing the effectiveness of vapor extraction As these mobilized hydrocarbons moved through, and up, the water column, down-gradient vapor extraction wells could be used to extract these newly mobilized petroleum hydrocarbons ' Based on results of the pilot test, air sparging was effective in introducing oxygen into the subsurface and the PID field data suggested that it enhanced the effectiveness of vapor extraction by a factor of about two Moreover, the dramatic increase of dissolved oxygen in nearby observation wells demonstrates that the sparging was also effective in raising the oxygen levels in a semi-confined sand in which much of the contamination is contained ' The air sample analytical results from samples collected before and after the initiation of air sparging are inconclusive in supporting the supposition that air sparging increased the ' hydrocarbon concentration of influent air extracted from VEW-1 This is attributed to the relatively brief time the sparge system was in operation Nevertheless, field measurements ' showed that contaminant concentrations in evolved air increased markedly as a result of air sparging Prior to commencement of air sparging but after the vapor extraction system was activated, PID measurements of the extracted vapor from extraction well VEW-1 yielded ' readings which ranged between 52 and 33 ppm (from 05122100) During the operation of the air sparging system (from 05/30/00-06101/00), the PID readings from VEW-1 displayed a generally increasing trend from 52 to 109 ppm on 05130 to 109 to 120 ppm on 05/31 and 147 to 290 ppm on 06/01 As documented in the July 31 pilot test report, the increase in oxygen concentration was plotted against the respective distance each well from the sparge point The data points were plotted on a log-normal scatter chart A linear regression, best-fit curve was calculated in the Excel software and the radius of influence from sparging was taken as the intersect of the trend line with the 0 point of the oxygen increase Using this method, the radius of influence of the dissolved oxygen from the sparge well was estimated to be about 24 feet 2937 Veneman Ave , #8240 Geo-Phase Environmental Inc Phone(209)569-0293 Modesto, CA 95356 Fax(209)569-0295