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Report—Remediai Action Feasibility Page 24 Cutter Lumber—Western Lift Site ' GPE Project No 1011 August 1,2003 ' 6 The most intensely contaminated interval (35 to 45 feet) roughly corresponds to a clay- and silt-rich layer These fines-laden soils readily adsorb petroleum ' hydrocarbons and greatly reducing their mobility, as exemplified by the failure of the contaminant mass to rise with the rising surface of groundwater table after years of being under the groundwater surface The adsorbed state of the bulk of ' the contaminant mass in conjunction with the depth of the mass below the groundwater surface would appear to render vapor extraction as a stand-alone process or in conjunction with groundwater pumping, less than feasible We ' conclude that the groundwater surface would need to fall about 8 to 10 feet (to about 40 feet bgs) in order for these methods begin to become feasible If such a change in the water table were to occur, the sands, which are present roughly between 35 and 45 feet, could be readily treated by vapor extraction In addition, the vapor extraction would tend to desiccate the clay and silt which would ' increase permeability and further enhance the effectiveness of the vapor extraction method Unfortunately there is no way of knowing when such a change in natural ambient groundwater conditions might occur ' 7 Vapor extraction with groundwater pumping is similar to vapor extraction alone in that both methods would be likely to affect only the contaminated soils in the ' vadose zone and in the upper few feet of the saturated zone Insofar as most of the contaminant mass lies at 5 to 15 feet below the groundwater surface, neither of these methods is likely to remove a substantial portion of the gasoline contaminant mass 8 Given the current aquifer conditions, it seems apparent that unless water levels ' drop considerably over the next few years, the only reasonable active remedial option of those that were herein assessed is vapor extraction with air sparging Natural attenuation would also work, but we estimate the time needed to 1 adequately mitigate the release by natural attenuation be at least 25 years 8 A rough estimate of costs in constant dollars over the lifetime of project for the ' two methods that are technically viable (i a natural attenuation and vapor extraction with sparging) and it appears clear that the proposed corrective strategy ' is also the most cost-effective ' 2937 Venernan Ave , #13240 Geo-Phase Environmental Inc Phone(209) 569-0293 Modesto,CA 95356 Fax(209)569-0295