SITE INFORMATION AND CORRESPONDENCE FILE 2

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subsurface. The conversation concluded that if the lack of seal at the of the wellheads were the reason for the increase in well VEW-3, then it was the responsibility.of Exxon, not Palisades,to investigate and mitigate the impact. Palisades also hired an outside consultant to perform a reconnaissance of the site vicinity and conduct a file review to identify other possibilities that could have contributed to the increase in petroleum hydrocarbon concentrations in well VW-3. The consultant identified three possible scenarios that could have lead to the observed rise in concentrations. These scenarios and associated logic are discussed below. A. The consultant indicated that there are other potential underground facilities that could contribute petroleum hydrocarbons to the subsurface. A 7-11 service station facility is located immediately adjacent to the Palisades' facility. A Safeway store(new USTs)is located approximately 250 feet south west of the Palisades'facility.A Shell service station is located approximately 600 feet east of the Palisades' facility. The Shell station is located cross gradient from the site and therefore was not evaluated further. The 7-11 facility and Safeway store are located upgradient. Monitoring wells or other evidence of subsurface assessment were not observed at the Safeway store. The 7-11 Store does have a history of petroleum hydrocarbon impact, and associated monitoring wells were observed. The consultant did not have the opportunity to review the files for the 7-11 facility. Given that the predominant groundwater flow is toward the northeast,it is feasible that a release from the 7-11 facility or the Safeway store,could have migrated beneath the Palisades property to be detected in well VEW-3. However,concentration of petroleum hydrocarbons in wells MW-1,MW-7 and VEW-1 (located onsite and situated between well VEW-3 and two potential off-site sources),did not increase similar to VEW-3. This lack of an increase in concentrations in wells MW-1,MW-7,or VEW-1 argues against an off-site migration from these sources,yet does not entirely rule out the possibility(i.e.preferential pathways for groundwater flow). B. A review of the historical groundwater elevations and concentration data indicates that the recently observed increase in MTBE concentrations in well VEW-3 may be due to fluctuations in groundwater elevations. The initial high MTBE,TPHG and TAME and TBA concentrations were observed in March of 2002 when groundwater elevation was recorded at 7.25 feet bgs in well VEW-3. The well was dry in the previous two quarters(TD of the well is 10 feet). It is possible that the rising groundwater could have desorbed residual petroleum hydrocarbons in soil related to the older Exxon release. ` i