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' Former Manna Pro Site Page 7 Protect No 207 3 ' Remedial Feasibility Study • 4.0 SOIL VAPOR EXTRACTION Remedial pilot tests have been conducted at the site on two separate occasions One of these (by GTI) included groundwater pumping, while the recent second test included air sparging ' Both pilot tests included a component in which soil vapor extraction was applied without other methods operating simultaneously At the time of the first test, in 1994, the groundwater surface at the site was about 40 feet below grade ' During this test, soil vapor was extracted at the rate of about 15 scfm, and the extracted soil vapor was reported to have a contaminant load concentration of 280 mg/l At the extraction ' rate of 15 scfm, which was used by GTI, these figures convert to a gasoline extraction rate of about 376 lbs/day Applying the same gasoline vapor concentration to an air extraction rate ' of 45 scfm, the largest sustainable extraction rate for this pilot test, the initial gasoline removal rate would be about 1,128 lbs/day I During the pilot testing recently completed by Geo-Phase Environmental, the groundwater elevation was about 25 feet below grade, and 10 feet above the highest level of the most intensely impacted soil The water column lying above the main contaminant mass greatly reduced efficiency of hydrocarbon removal using the vapor extraction system During the test, only the vacuum applied to the system was modest (30-35 inches of water), as only approximately 5 feet of screen was exposed above the groundwater table During the vapor- extraction-only part of the pilot test, only about 1 to 2 pounds of gasoline per day were removed Although the conditions of the vapor extraction test were less than ideal, they were better than in the GTI test in terms of vacuum and air flow rates applied, and yet with the lower water table, GTI was able to remove approximately 1,000 times as gasoline vapor It seems clear that the principal difference in the gasoline concentration of evolved soil vapor in of the two extraction pilot tests was related to the height of the water column over the main contaminant mass In comparing the results, it also seems clear that vapor extraction as a stand-alone system has the potential to substantially reduce the gasoline load in the subsurface, but only when the groundwater surface is at relatively low levels At the recent high levels of groundwater, vapor extraction is clearly neither efficient nor cost effective remedial method It may be anticipated that, as groundwater levels fall, the effectiveness of vapor extraction would increase proportionally Unfortunately, there is no way to predict future fluctuations in the level of the groundwater surface The GTI report estimated the radius of influence of 18 feet at the extraction rate of 15 cfm In the more recent pilot test conducted by Geo-Phase Environmental, the maximum radius of influence of the vapor extraction well VEW-1 is estimated to be 71 feet If the cut-off pressure is set to I inch of water pressure (rather than 0 inches), then the radius of influence is estimated to be about 55 feet 2937 Veneman Ave,#13240 Geo-Phase Environmental Inc Phone(209)569-0293 Modesto,CA 95356 Fax(209) 569-0295