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' Report—Fist Phase Remedial System Page 20 Former Manna Pro Site ' Project No 2011 • June 1$,2002 ' 7.0 CONCLUSIONS & RECOMMENDATIONS ' 7.1 Conclusions I The main contaminant mass currently lies 10 feet or more below the groundwater ' surface The interval extracted from, most likely consisted of the sand from ±20 feet below ground surface to the groundwater surface at about 30 feet below grade The radius of influence, as calculated from the average vacuums measured from OB-4 & 5 ' was 42 feet Based upon this and the porosity of this formation from the geotechnical data, the radius of influence pore volume was 24,940 cubic feet ' 2 The vapor extraction system was effective in removing a calculated 850 pounds of gasoline petroleum hydrocarbons The extraction rate was estimated to be 32 pounds of gasoline per day at the time of system startup The rate declined over time ' following an approximately exponential rate to about 3 5 pounds per day at the end of 95 days During the last few weeks, a distinct flattening of the decline rate was noted We suspect that instrument error in the vapor extraction rate may have overstated the ' to extracted air volume by as much as 30 percent A 30 percent error would equate to a hydrocarbon mass of about 600 pounds rather than the calculated mass of 850 pounds 3 Airs ar in was effective in introducing p g g g oxygen into the subsurface and based on an earlier pilot test, it undoubtedly enhanced the effectiveness of vapor extraction The rate of injection was, for the most part, limited to about 5 scfm due to the operating characteristics of the compressor The decision was made to limit the injection rate ' rather that incur the risk of injecting entrained lubricating oil into the sparge wells Greater injection rates can be expected to increase the effectiveness of the vapor extraction system, but certain improvements will be required to eliminate the problem of oil blow-by 4 The overall effectiveness of the VES / Sparge System, for the most part, verifies the ' engineering concepts and design features set forth in the CAP There are, however several modifications that are justified The extracted soil vapor is generally too lean for use of a thermal oxidizer as a final gasoline destruction method Given the ' relatively lean gasoline content of the extracted vapor, a thermal oxidizer would require an extremely high percentage of propane to generate a combustible mixture This would render the use of a thermal oxidize a cost-ineffective method of ' hydrocarbon destruction A rough estimate was made that suggested if propane has been used to combust extracted gasoline vapors, the fuel cost alone would have increased the cost of the project several thousand dollars per month, and the O&M costs would also be greater with a thermal oxidizer system i ' 2937 Veneimn Ave , #13240 Geo-Phase Environmental Inc Phone(209)569-0293 Modesto, CA 95356 Fax(209) 569-0295