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' Former Manna Pro Site Page 11 Project No 207 3 ' Remedial Feasibility Study ' 7.0 CONCLUSIONS & RECOMMENDATIONS 7.1 Conclusions 1 The total contaminant load as gasoline range hydrocarbons was estimated to be about 22,000 pounds with the dissolved phase being about 460 pounds These figures seem to be in reasonable agreement with the GTI estimate of 1994 2 Given the recent high stand of groundwater, in conjunction with the thick clay / silt unit, it does not appear likely that either vapor extraction, or vapor extraction with groundwater pumping offers a viable remedial alternative The most intensely ' contaminated interval (35 to 45 feet) roughly corresponds to a clay- and silt-rich layer These fines-laden soils adsorb petroleum hydrocarbons, greatly reducing their mobility, as exemplified by the failure of the contaminant mass to rise with the rising ' surface of groundwater table 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 t groundwater pumping, less than feasible We conclude that the groundwater surface would need to fall about 8 to 10 feet (to about 35 feet bgs) in order for these methods to become feasible If such a change in the water table were to occur, the sands, ' which are present roughly between 20 and 30 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 ' 3 Both vapor extraction and vapor extraction with groundwater pumping 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 8 to 20 feet below the groundwater surface, neither of these methods is likely to remove a substantial portion of the gasoline contaminant mass By including air sparging in the ' remedial system, the rising air in the saturated zone will help to mobilize and vaporize some of the absorbed gasoline contaminant mass, bringing it to the groundwater surface where vapor extraction method should readily remove the material from the ' subsurface for destruction 4 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 is that of vapor extraction with air sparging Natural attenuation would also work, but we estimate the time needed to adequately mitigate the release by natural attenuation to lie between 25 to 100 years i r2937 Veneman Ave , #13240 Geo-Phase Environmental Inc Phone(209)569-0293 Modesto,CA 95356 Fax(209) 569-0295 I