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AReport—Remedial Action Feasibility Page 21 Cutter Lumber—Western Lift Site ' GPE Project No 1111 August 1,2003 ' not withstanding, the pilot vapor extraction test resulted in an initial extraction rate of less than 3 pounds per day, and this decreased to non-detectable levels after less than three days ' The surface of groundwater has recently been as high as 25 feet bgs, but as late as 1994, the groundwater surface in the area was as low as 40 feet bgs Should a series of ' abnormally dry years result in a decrease in the groundwater surface to the vicinity of 40 feet, it is possible that vapor extraction could be an effective corrective measure Such an occurrence would expose more of the contaminant mass to the vadose zone, and would ' also permit contact with sandier intervals lying below 30 feet 9.3 Soil Vapor Extraction with Air Sparging r As discussed above, the highest concentrations of gasoline are known to lie from about 35 to 45 bgs or about 5 to 15 below the surface of groundwater In view of this condition, ' we initially 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 Sections 6 0 & 9 2) 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 dramatically effective in mobilizing hydrocarbons and bringing them to the surface where they were available to be removed by the vapor extraction system It also appears that the relatively large diameter of the ' soil borings in which the service wells were constructed was effective in promoting this effect Air sparging was doubtlessly also effective in introducing oxygen into the impacted zone, and can be expected to assist with bacterial degradation of the plume On ' commencement of sparge operations, the gasoline extraction rate essentially leapt from less than 1 pound per day to about 18 pounds per day, and this rate increased over the ' ensuing two days The last part of the test entailed switching the vapor extraction locus to MW-7, (through the former tank pit) and this resulted in an instantaneous extraction rate of over 150 pounds per day of gasoline ' As documented above, the pressure exerted on nearby wells due to sparging was measured and analyzed The data suggest that the effective radius of influence due to ' sparging is about 40 feet The rather dramatic figure as compared to vapor extraction radius of influence is attributed to a much greater transmissivity of the soil at the depth were sparging occurs The increased transmissivity is attributed to the presence of a ' sandy layer from about 3 5 to 45 feet below grade ' 2937 Venenian Ave, HB240 Geo-Phase Environmental Inc Phone(209) 569-0293 Modesto,CA 95356 Fax(209)569-0295