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groundwater is at approximately 13 feet bgs, which means that these petroleum hydrocarbons entrapped in soil at the site are submerged. This issue, coupled with soil types encountered at the site, will be discussed in greater detail in other sections of this report as it relates to past and future efforts at the site. 2.3 PREVIOUS ENVIRONMENTAL ACTIVITIES AT THE SITE Il Between January 1987 and April 1988, groundwater monitoring wells MW 1—MW7 were installed onsite, and wells MW8—MW10 were installed offsite. Groundwater recovery wells RW1—RW3 were installed onsite in May 1990. The locations of wells are shown in Figures 2 and 5. Groundwater monitoring and sampling was initially conducted in January 1987, and has been conducted on a quarterly basis since February 1989. Samples are submitted to a state certified laboratory and analyzed for benzene, toluene, ethylbenzenc, and total xylenes (BTEX), and for TPH-g. Groundwater samples have also been analyzed for MTBE since 1995. Historical groundwater analytical results as well as groundwater elevations are presented in Table 1. The analytical results of the most recent groundwater sampling event (June 2001), as well as groundwater elevations, are graphically presented as Figure 6. 1 In October 1990, EA submitted a Combined Work Plan to Remediate Soil and Groundwater (EA 1990). Under this plan, EA proposed to remediate groundwater through conventional technology available at the time: a groundwater extraction, treatment, and reinjection system. Pursuant to the 1990 plan, EA installed the required wells for this remedial approach. From January to April N.) 1991, wells TW 1, P1, P2, V1, V2, and V4 were installed onsite, and wells RW4, V3, MW11, and MW12 were installed offsite. Figure 2 shows the locations of former site facilities and the locations of all ExxonMobil wells on and off the site. The locations of offsite monitoring wells MW8, MW9, MW11, and MW13 are shown in Figure 5. Well construction details are presented in Table 2. Historical analytical results for soil samples are summarized in tabular form in Appendix C. - i Aquifer tests as well as injection/percolation tests were performed at the site to gain a better . understanding of the hydraulic properties of soil underlying the site. Aquifer tests were performed to estimate aquifer parameters, radii of influence, and sustainable extraction rates. The injection/percolation tests were performed to assess if a hydraulic barrier could be created which would prevent the migration of upgradient hydrocarbon-impacted groundwater from the Shell service station to the former Exxon site duringgroundwater extraction. onsite EA 1991 a . Due to g ( ) problems in securing the permit for reinjection of treated groundwater, this remedial option was not implemented. Liquid-phase hydrocarbons (LPH) had been found in wells MW3, MW6, RW3, V1, V2, and V4, as documented in quarterly groundwater monitoring reports. In December 1992, passive skimmers were installed in wells RW3 and VI to recover LPH. The skimmers were checked for LPH on 18 May 1993, but no LPH was found in either of the skimmers or in any of the monitoring wells. During past quarterly groundwater sampling events, petroleum hydrocarbons have been detected in groundwater samples collected from all wells except MWI1, which is located downgradient and farthest from the site. G%Piojects173942%VASTERIW?P R408011i"t2.doc 3 1