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it . concentrations equal to or greater than the method detection limits in the groundwater samples 1 collected:;from MWl—MW4 and MW6. TPH-g was detected only in the groundwater samples 1 collectedfrom MW3, MW5, and MW7, in the range 53 µg/L to 110µg/L. Benzene was detected in the groundwater samples collected from MW5, MW7, and VEW2, at concentrations ranging from 1.2'µg/L to 2.6 µg/L:. MTBE was detected only in the groundwater samples collected from MW5 and VEW2, at concentrations of 11 ,ug/L and 100µg/L, respectively. 2.6 REMEDIATION SYSTEM EVALUATION In May 1993, AEGIS performed a pilot-scale soil vapor:extraction(SVE) test in vapor extraction wells VEWI and VEW2 to assess the feasibility of soil remediation at the site (AEGIS, 9 September 1993). In November 1993, AEGIS submitted a Remedial Action Plan which summarized site characterization and-feasibility assessment data, proposed SVE as a suitable remediation alternative, and described the implementation of the proposed remediation plan (AEGIS, 11 November 1993). In November 1995, an SVE system was installed at the site by FUGRO and operated from November 1995 to March 1996 (FUGRO, 18 January 1996). Performance data collected by FUGRO are presented in Table 3. Vapor extraction was performed at five wells (VEW1, VEW2, MW2, MW3, and MW5). ;'A new SVE system was installed by EA and was operated from February 1997 to March 1998. During the period February 1997 to March 1998, soil vapor was extracted primarily out of two wells (VEW I and VEW2), because the screened intervals for the other three wells used in the past for vapor extraction(MW2, MW3, and MW5)were nearly completely submerged: Influent TPH-g concentrations and the estimated total pounds of hydrocarbons removed by the system are shown in Table 3 and Figure 7. As can be seen in Table 3, the SVE system has removed approximately 1,551 pounds of TPH-g from the original start-up in November 1995 to the shut-down in March 1998. Laboratory analytical results for samples of the extracted soil vapor collected by EA are shown in Table 4. 1 The SVE.system was shut down from 5 February to 26 March 1998 to determine whether a Y �' rebound in extracted petroleum hydrocarbon concentrations would occur upon system restart. LThe SVE;,system was sampled on 30 March 1998, 3 days after restart. As can be seen iii Table 4, :,the sample of extracted soil vapor had non-detectable concentrations of TPH-g and BTEX. A `clear asymptotic trend in the cumulative mass of TPH-g removed can be seen in Figure 7. The decline in the concentration of TPH-g in the cumulative extracted soil vapor over time is shown in Figure i8, along with the average groundwater elevation over time and the screened intervals of the five vapor extraction wells. As shown in Figure 8, soil vapor has been extracted primarily from wells VEW1 and VEW2 located in the former UST area. Wells MW2, MW3 and MW5 have been partially or totally submerged for the duration of the soil vapor extraction remediation. The histogram of average groundwater levels and extracted soil vapor concentration, along with F 70137.WPM.MOS.WP.WPQ 5