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<br /> RW 1 RW3 were installed onsite in May 1990 Well construction details are presented in
<br />' Table 1 The locations of wells are shown in Figures 2 and 3
<br /> Groundwater momtonng and sampling was initially conducted in January 1987 and has been
<br />' conducted on a quarterly basis since February 1989 Samples are submitted to a state-certified
<br /> laboratory and analyzed for BTEX and TPH-g Groundwater samples have also been analyzed
<br /> for MTBE since 1995 Historical groundwater analytical results and groundwater elevations are
<br />' presented in Table 2 Groundwater analytical results and groundwater elevations for the most
<br /> recent groundwater sampling event are graphically presented in Figure 4
<br />' In October 1990, EA Engineering, Science, and Technology (EA) submitted a Combined Work
<br /> Plan to Remediate Soil and Groundwater (EA 1990) Under this plan, EA proposed to remediate
<br /> groundwater through conventional technology available at the time a groundwater extraction,
<br />' treatment, and reinfection system Pursuant to the 1990 plan, EA installed the required wells for
<br /> this remedial approach From January to April 1991, wells IW1, PI, P2, V1, V2, and V4 were
<br /> installed onsite, and wells RW4, V3, MW11, and MW12 were installed offsite Figure 2 shows
<br />' the locations of former site facilities and the locations of all ExxonMobil wells on and off the
<br /> site The locations of offsite momtonng wells MW8, MW9, MW 11, and MW13 are shown in
<br /> Figure 3 Well construction details are presented in Table 1 Historical analytical results for soil
<br /> samples are summarized in tabular form in Appendix B
<br /> Aquifer tests and injection/percolation tests were performed at the site to gain a better
<br /> understanding of the hydraulic properties of soil underlying the site Aquifer tests were
<br /> performed to estimate aquifer parameters, radii of influence, and sustainable extraction rates The
<br /> injection/percolation tests were performed to assess if a hydraulic barrier could be created which
<br />' would prevent the migration of upgradient hydrocarbon-impacted groundwater from the Shell
<br /> service station to the former Exxon site during groundwater extraction onsite (EA 1991 a) Due to
<br /> problems in securing the permit for reinfection of treated groundwater, this remedial option was
<br />' not implemented
<br /> Liquid-phase hydrocarbons (LPH) had been found in wells MW3, MW6, RW3, V1, V2, and V4,
<br />' as documented in quarterly groundwater monitoring reports In December 1992, passive
<br /> skimmers were installed in wells RW3 and V1 to recover LPH The skimmers were checked for
<br /> LPH in May 1993, but no LPH was found in either of the skimmers or in any of the monitoring
<br />' wells During past quarterly groundwater sampling events, petroleum hydrocarbons have been
<br /> detected in groundwater samples collected from all wells except MW 11, which is located
<br />' downgradient and farthest from the site
<br /> A soil vapor extraction and air sparging (SVE/AS) system with two air sparging wells (SWI and
<br /> SW2) was installed in May 1994 The SVE/AS system operated from May 1994 to November
<br /> 1 1996 and recovered petroleum hydrocarbon vapors from wells MW2, MW3, MWS, and MW6
<br /> (screened from 10 to 30 bgs), wells V1 and V2 (screened from 20 to 40 feet bgs), and well V4
<br />' (screened from 25 to 40 feet bgs) A total of approximately 14,500 pounds of petroleum
<br /> hydrocarbons was removed from the vadose zone by the SVE/AS system (EA 1997) Based on
<br /> . previous site investigations (EA 1987a, 1987b, 1988a, 1988b, 1989a, 1989b, 1990, 1991b),
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