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The diesel distribution is consistent with typical migratory patterns through vadose zone soils <br /> Migration is areally limited horizontally and vertical migration occurs until a confining layer <br /> or groundwater is encountered <br /> In January 1991, four soil vapor extraction wells were installed at the site in a distribution <br /> similar to the soil borings Soil samples collected during installation of the wells revealed <br /> diesel concentrations in samples from the three of the wells. The greatest concentration of <br /> diesel of 4900 ppm was encountered at 25 feet below grade in well MW-6, located in the <br /> former tank location The two underlying samples from this well, collected at 35 and 40 feet <br /> below grade, revealed no detectable diesel concentrations Analytical reports for these <br /> samples can be found in Versar's "Interim Remediation/Pilot Study Report", 1992 <br /> The soil vapor extraction system was in operation for a total of 66 days and extracted 0 5 <br />' gallons of diesel fuel from the subsurface <br /> In October 1993, two soil boring were drilled by IT through the former tank location to a <br /> depth of 40 feet below grade. Soil samples were collected at 10 foot intervals and upon <br /> analysis only one sample, collected at 15 feet below grade revealed detectable diesel <br /> concentrations (2900 ppm) <br /> 5.0 Conclusions <br /> Review of soil sample analytical data collected to date reveals a significant decrease in diesel <br /> concentrations over time. Although the soil vapor extraction (SVE) system was not <br /> successful in extracting significant concentrations of diesel from the subsurface, it appears <br /> that through increasing the oxygen flow to the subsurface, the system was successful in <br /> stimulating biodegradation of the fugitive diesel As expected, the greatest diesel <br /> concentrations had historically been encountered beneath the former tank location Current <br /> investigative results indicate that diesel fuel remains in the soil at 15 feet below grade beneath <br /> the former tank location. Underlying samples revealed no detectable diesel concentrations. <br /> Upon review of the substrata underlying the former tank location, it appears that the <br /> remaining diesel concentrations are encountered in a silt/clay layer which exists between 7 5 <br /> and 18.5 feet below grade This may be due to the fact that SVE system would have <br /> introduced more oxygen to the more permeable substrata (sands/silty sands) then to this silt <br /> MZ/12-08-93/ENG/93-1065 nbr <br /> 7 <br />