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remained depressed. This created a steep eastward hydraulic gradient along the western margin of the Kwikee site. As Figure 30 illustrates, this unstable condition was temporally associated with a minor expansion of the gasoline plume, as concentrations increased in both KF-3 and KF-5, as well as in MW-6 and MW-7. The concentration also increased by 5,000 ppb in MW-1, because contaminated water was pooling there and not being withdrawn by the remediation system. Hence, the effect of the shutdown is visible not only in the.water elevation data but also in concentration data for at least five wells (the effect may also be recorded by slight increases in the xylene concentration in some of the Kwikee wells). Regarding this increase, EHD commented in its September 2002 letter that "To interpret the increased contaminant concentrations detected in sample KF-3 as contradicting EHD's January 2002 suggestion that higher contaminant concentrations may be in soil east of the well stretches the importance of the rather modest concentration increase". As the review of the Chevron data shows, this increase was not limited to KF-3,and should not be discounted as an insignificant"blip"in the data for a single well. 4.10 Renewed Contraction: November 2002 After their temporary increase in the second quarter of 2002, TPH-g concentrations have declined again in all-wells on both sites (except MW-2, which experieliced a slight increase, and perhaps MW-1, which was not accessible for sampling in November). Declines of 30% and 32% were registered in KF-3 and KF-5, and declines ranging from 14% to 42% were observed in the wells at the Chevron site. This has resulted in a slight contraction of the plume, with the 500 ppb contour retreating west of both KF-3 and KF-5. The largest decline (42%) occurred in MW-4. This well has • not responded as clearly to the remediation system as most of the other wells, and TPH-g concentrations have fluctuated between about 13,000 and 30,000 ppb since the system became operational. This lack of a clear downward trend in concentrations in this well, in contrast to wells farther east, may be due in large part to the absence of the middle hydrogeologic unit (aquifer sand) in this well (Figure 3, cross section A-A'). The uniformly fine-grained sediment in the vicinity of MW-4 no doubt inhibited westward migration of gasoline that leached from the UST facility in the eastern portion of the Chevron site, and in the same way may inhibit itseastwardmigration during operation of the existing remediation system. As explained in section 2.1, eastward thickening and widening of the middle hydrogeologic unit created a preferential pathway for contaminant migration, particularly under the east-to-northeast groundwater flow regime that was dominant prior to 1996. 4.11 Methyl Tertiary Butyl Ether Concentrations MTBE concentrations at both the Chevron and Kwikee sites have been determined using both EPA method 8020 and method 8260. Examination of the MTBE data for the past several years indicates that concentrations determined using method 8020 are generally 2 to 10 times greater than those determined using method 8260, especially in wells where the concentration of all analytes is fairly high. This is clearly illustrated in Figure 32 for the most contaminated well at each site. For example, the concentration in KF-3 in August 1997 was reported at 290 ppb by method 8020, whereas the concentration less than 3 months later was only 58 ppb according to method 8260. 19