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v v CONESTOGA-ROVERS &ASSOCIATES February 17,2010 Reference No. 241661 The purpose of this comparison is to demonstrate that it is unlikely that MTBE would migrate a distance of 400 feet and not show more degradation than one order of magnitude. And demonstrating this would suggest that the MTBE detected in Shell's well MW-10 (located 400 feet from the subject site)is not likely to be related to the release at the Shell site. The data from the 7-Eleven site and the 3011 West Benjamin Holt Drive site show that MTBE concentrations typically attenuate to low levels at distances of less than 250 feet,and in some cases (the 7-Eleven site) in less than 100 feet. Analysis of the soil types beneath the Shell site and the 7-Eleven site indicate they are similar,so it is reasonable to assume that the degradation of MTBE from the Shell release would occur as it did with the MTBE plume from the 7-Eleven site. Soil types beneath the 3011. West Benjamin Holt Drive site were similar but contained more coarse-grained soil. The 7-Eleven site had a significantly larger gasoline release than the Shell site with initial MTBE concentrations as high as 610,000µg/L. Similar to the Shell site,MTBE was the primary constituent of concern for the 7-Eleven site. The 7-Eleven plume migrated between 75 and 90 feet downgradient of the 7-Eleven site and within this distance declined in concentration by 3 orders of magnitude. It is reasonable to assume that the Shell plume (which was a lower concentration release)would attenuate in a similar manner to the 7-Eleven plume,or perhaps even in a shorter distance. If this were the case,then the Shell plume would attenuate beneath the east-bound lane of Interstate 205, and not be detectable on the northern side of Interstate 205. Since it is not possible to install monitoring wells within the Interstate 205 right of way,we cannot properly verify the distance the Shell plume has migrated north of the site. Shell's offsite well MW-10 was installed north of the site on the northern side of Interstate 205 (400 feet downgradient of the Shell site). Based on the attenuation of the nearby 7-Eleven MTBE plume, it is not reasonable to conclude that the MTBE detected in MW-10 originates from the Shell site. It is likely that the MTBE detected in well MW-10 is from a release that occurred offsite in the immediate vicinity of MW-10. Based on the information and analysis presented above,we request that the EHD and CVRWQCB reconsider the Shell environmental case for closure. Worldwide Engineering.Environmental,Construction,and IT Services