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include well construction details, groundwater elevations, and concentrations of TPH-g, benzene, <br /> and MTBE over time <br /> The SVE system was shut down from 5 February to 26 March 1998 for replacement of the <br /> activated carbon and to determine whether a rebound in extracted petroleum hydrocarbon <br /> concentrations would occur upon system restart (EA 1998) The SVE system was sampled on 30 <br /> ' March 1998, 3 days after restart As can be seen in Table 3, analysis of this soil vapor sample did <br /> not contain detectable concentrations of TPH-g or benzene An asymptotic trend in the <br /> cumulative mass of TPH-g removed can be seen in Figure 7 as well as the decline in <br /> concentration of TPH-g in the extracted soil vapor over time <br /> To assess the impact of rising groundwater over time on the effectiveness of the SVE, the <br /> average depth to groundwater across the site over time, screened intervals of the five vapor <br /> extraction wells, and the soil vapor influent concentrations are presented as Figure 8 As can be <br /> seen in Figure 8, as well as the hydrographs in Appendix B, the SVE extracted primarily from <br /> wells VEW1 and VEW2, which are located in the former UST area Wells MW2, MW3, and <br /> MW5 were partially or totally submerged for the duration of remediation The significant <br /> decline in TPH-g influent concentrations occurred prior to March 1997 and before rising <br /> ' groundwater encountered the screened interval of deeper vapor extraction well VEW2, which <br /> has a total depth of 35 feet bgs The average depth to water at the site during this time, as <br /> ' presented in Figure 8, was between 35 and 40 feet bgs This indicates that the decline in SVE <br /> concentrations is due to an effective removal of petroleum hydrocarbons from the vadose zone <br /> by the SVE system, rather than a change in groundwater elevations As demonstrated in Table 3 <br /> and Figure 7, the SVE system has removed approximately 1,551 pounds of TPH-g from the <br /> original start-up in November 1995 to when the system was shut down in March 1998 <br /> h�pro][ct5�70137t Nastei4Wp'Llosuic l 10010137ciiext due, 6 <br />