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Mr. Marty Hartzell Page 7 of 10 • included in Attachment E. Based upon the data collected during the five days of operation at the site, a total of approximately 6 pounds of hydrocarbons (1.2 pounds per day) were removed using the IC Engine. If an initial mass removal rate of 7 pounds per day could be sustained, soil vapor extraction could theoretically cleanup the residual soil contamination within a period of about 3 months if the initial removal rate was maintained. However, experience at many other sites shows that influent vapor concentrations typically decrease 90% or more during the first month or two (700-1400 hours) of continuous operation. Since the previous investigation determined that most of the residual hydrocarbons in the soil exists within fine grained soils, the hydrocarbon removal rate would be limited by diffusion of contaminants from the contaminated zone into the moving vapor stream, since vapor flows around but not through the zone of contamination. Thus, we would expect that a much longer period of operation would be required to substantially remediate the residual soil contamination. Based upon our experience at other sites we feel that an operational period of at least 24 months would be required to achieve theoretical cleanup levels. The heterogeneous nature of the soil, for which vapor- phase diffusion is a limiting factor, suggests that residual contamination would remain in soil even after influent vapor concentrations decline to minimal levels. In other words, detectable levels of gasoline constituents will likely remain in the soil even if a full-scale vapor extraction system was operated at the site for an extended period of time. CONCLUSIONS Ground Zero previously evaluated soil remediation alternatives and determined that passive remediation, source excavation with off-site landfilling, and soil vapor extraction were the three most viable alternatives. Each of the three alternatives would meet the threshold criteria of providing overall protection of human health and the environment and compliance with Applicable and Relevant or Appropriate Requirements (ARARs), but the time frame required to meet those objectives would vary (Problem Assessment Report, May 14, 2001). Based upon balancing and modifying criteria presented in the Problem Assessment Report, Ground Zero concluded that soil vapor extraction would have greater acceptance by the State and should be implemented if feasible and/or warranted. Based upon the results of the recent investigation and soil vapor extraction test conducted at the site, however, it does not appear that the installation and operation of a full-scale soil vapor extraction system is warranted. This conclusion is supported by the following: • Soil samples collected in the subsurface indicate that gasoline contamination beneath the site is limited in extent, as evidence by the rapid lateral attenuation of contamination between borings SBI and VW1, which are located approximately 41/z feet apart. • Less than 300 kg, or approximately 650 pounds, of gasoline contamination remains in soil beneath the former gasoline tank. • The great majority of contamination is bound up in fine grained soil between the depths of • approximately 24 and 28 feet (clay) and approximately 38 and 42 feet (silt). O:DaG10R0[MD285gaual alEtcpwtAVEI7tPC.doc