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19 June 1992 <br /> (GeoAudit CV 30-34E3-1. 33) <br /> Page 2 of 4 <br /> 2 . a) Typically, in-situ cleanup methods for hydrocarbons can <br /> not achieve non-detectable levels of contaminants, <br /> including the vacuum extraction method. The objective is <br /> to remove the volatile hydrocarbons (such as the aromatic <br /> compounds) , which are more toxic than the less volatile <br /> counterparts, to a point of a diminishing return. The <br /> cut-off level of 500 ppm as hexane or 50 ppm as <br /> isobutylene is used as an indication that the vast <br /> majority of the vapor has been removed. It is not <br /> intended as a cleanup level, but merely as an indication <br /> that the confirmatory soil samples are necessary. It has <br /> been our experience that that level in the recovered <br /> vapor would yield soil samples of less than 100 mg/kg of <br /> TPH and non-detectable levels of benzene and toluene. <br /> b) If a vacuum pressure is applied in the extraction <br /> well (s) , a vapor circulation takes place. The fact that <br /> we are not dealing with homogenous soil would, indeed, <br /> yield varying radii of influence within the same well for <br /> the different layers. As the air flow through the higher <br /> permeability layers reduces their volatile organic <br /> compounds (VOCs) , a vapor gradient is developed that <br /> moves VOCs by gas diffusion from the lower permeability <br /> to the higher permeability layers. A similar analogy is <br /> presented by the "Leaky Aquifer" condition in <br /> hydrogeology. If the pump of a vacuum extraction unit is <br /> shut down for a period, a higher VOC level in the <br /> recovered vapor is obtained upon re-starting the unit. <br /> This phenomenon, named "rebound effect" among vacuum <br /> extraction practitioners, is attributed to diffusion <br /> procedures of VOCs into the well-circulated soil zones <br /> from lower less permeable ones. <br /> In total, up to 4 extraction wells are planned to be <br /> installed at the site. A continuous coring for the pilot <br /> boring of the first well can be performed to accurately <br /> document and describe the continuous soil profile. This <br /> data can be used to modify the design of the remaining <br /> wells at a later date, if necessary. The first and main <br /> vapor extraction well will have a perforation from 4 to <br /> 70 feet. The design of the remaining wells will be <br /> adjusted according to the soil type. If there are some <br /> layers that have not been mitigated to the satisfaction <br /> of the SJEHD during the confirmatory drilling/sampling, <br /> additional extraction wells targeting those zones can be <br /> installed in the future. <br />