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s. <br /> i <br /> . 15 June 1992 <br /> (GeoAudit CV 43C4-1. 1) <br /> Page 2 of 2 <br /> Upon reinitiation of the unit in March, the first monitoring <br /> periods indicate an increase of the organic vapor level compared to <br /> the results prior to repair. This phenomenon is called the "rebound <br /> effect" . Since we are not dealing with homogenous soil, the radius <br /> of influence within the same well varies with the different layers <br /> of the soil profile. As the air flow through the higher <br /> permeability layers reduces their volatile organic compounds <br /> (VOCs) , a vapor gradient is developed that moves VOCs by gas <br /> diffusion from the lower permeability to the higher permeability <br /> layers. Typically, this process results in an increase organic <br /> level after the unit is shut-down for a period of time and again .is <br /> attributed to diffusion procedures of VOCs into the well-circulated <br /> soil zones from lower less permeable ones. <br /> The organic vapor level has stabilized and is on the down trend. <br /> Upon further decrease of OV to approximately 50 ppm as isobutylene, <br /> or 500 ppm as hexane, additional vapor extraction from MW-1 is <br /> planned. The contamination at this well is assumed to be restricted <br /> • to the perforated casing and this well design appears to allow an <br /> . effective venting from this well. Hydrocarbon laden vapor from this <br /> well is expected to increase the concentration of OV. <br /> In summary, the compiled data indicates a good venting progress of <br /> the contaminated soil. <br /> Please contact our office if you have any questions. <br /> Sincerely, <br /> GEOLOGICAL AUDIT SERVICES, INC. Ong', <br /> Na Umi <br /> � Ea�+r.IWa 0► �r <br /> Bob Marty Joshua Ong, Ph.D. <br /> Staff Geologist Senior Project Hydrogeolo <br /> Registered Geologist No. 4725 <br /> Enclosures <br /> cc: Ms. Edna Roller. <br /> Mrs. Donna Bruno <br /> • <br />