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Report-Pilot Remedial System Test Page 26 Former Manna Pro Site Project No 207 3 July 31, 2000 ' 8.0 CONCLUSIONS & RECOMMENDATIONS 8.1 Conclusions 1 The main contaminant mass currently lies 10 feet or more below the groundwater surface The interval subjected to effective vapor extraction most likely was the sand from ±20 feet below ground surface to the groundwater surface at 25 feet below grade The radius of influence, as calculated from the maximum vacuums measured from OB-4 & 5 was 55 feet Based upon this and the porosity of this formation from the geotechnical data, the radius of influence pore volume was 21,380 cubic feet 2 Vapor extraction as a stand-alone remedial method is unlikely to be effective so long as the groundwater surface remains substantially above the main contaminant mass Should the groundwater level recede, vapor extrication would become more effective Due to the fact that the groundwater was at an historic high, the effective area from which vapor was extracted was limited to about 5 feet of exposed sand Another limiting condition was the amount of vacuum which could be applied to VEW-1 The ICE was capable of producing greater vacuum and greater air flow, however, at greater levels of vacuum, groundwater mounding in the vapor extraction well would make the vapor extraction system cease operation In the future, as groundwater levels recede toward historically normal levels, it is likely that vapor extraction well head vacuum can be increased to more effectively strip sorbed hydrocarbons from the tsubsurface As the depth to water increases, it is assumed that the ROI will decrease for the same vacuum and flow rate applied to the vapor extraction well ' 3 Air sparging was effective in introducing oxygen into the subsurface and the OVM field data appear to suggest that it enhances the effectiveness of vapor extraction The overall increases in OVM readings from VEW-1 recorded during the course of the pilot test suggest that sparging may be mobilizing volatile organics in the subsurface The dramatic increase of dissolved oxygen in nearby observation wells demonstrates that the sparging was also effective, at least in raising the oxygen levels in the 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 has increased the hydrocarbon concentration of influent air extracted by VEW-1 This may, however, be attributed to the relatively brief time the sparge system was in operation ' 4 The radius of influence of the vapor extraction well was estimated to be 55 feet The radius of influence of the sparge well was estimated to be 24 feet (based upon the dissolved oxygen levels measured from 013-2, OB-3, and OB-4) 2937 Veneman Ave, #13240 Geo-Phase Environmental Inc Phone(209)569-0293 Modesto,CA 95356 Fax(209)569-0295