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Similar results are obtained when samples are analyzed by both methods. As an example, the first time the MTBE concentration was analyzed both ways in MW-1, the laboratory reported 1,130 ppb by method 8020 but only,120 ppb by method 8260. Because of the inaccuracy of the 8020 data at both sites, the 8020 results were not used in constructing contour maps of the MTBE plurrie, although they are shown in parentheses in the reaps that follow. In section 4.5, clockwise rotation and southward or southeastward advection of the TPH-g plume were discussed. Figure 33 illustrates this further, using the MTBE data. Because only the Kwikee samples were analyzed by EPA method 8260 in December 2000, the map is strongly constrained only by the Kwikee data, but qualitatively comparing. the 8020 results for both sites over the previous several quarters suggests that MTBE concentrations have generally been higher in Chevron wells than in Kwikee.wells. Therefore, the core of the MTBE plume is mapped in Figure 32 west of the Kwikee site, but the apparent absence of MTBE in MW-2 and MW-3 suggests that the MTBE core was not located west of MW-1. Further qualitative evaluation of the rapid decrease in MTBE concentration between MW-1 and MW-2 suggests that the contours are tightly compressed between these wells, whereas quantitative evaluation of the data from KF-3 and KF-5 indicates that contours on the east end of the plume are spread farther apart. This is another example of longitudinal dispersion downgradient from the contaminant source, which further supports the view that the source was located west of the Kwikee site. However, due to the greater mobility of MTBE in groundwater, it is able to move en masse by advection more easily than other gasoline compounds, which would explain why the core of the MTBE plume appears to have moved east and south of MW-1 while the core of the TPH-g plume did not. • Figure 34 shows another snapshot of the MTBE plume, using data from February 2002. This map is constrained by EPA 8260 data from both sites, and it is evident that MTBE concentrations were nearly the same in MW-1, MW-6, and KF-3 at that time. This adds further support to the interpretation that the center of the plume was located between these three wells. Further, note that the MTBE concentration in MW-1, as determined by EPA method 8020, was approximately 200 ppb, and compare this value to that determined using the same method during the previous four quarters (<1,000 ppb, 1,130 ppb, 1,800 ppb, and 1,200 ppb). These data can be interpreted as indicating that the core of the MTBE plume was closer to MW-1 during those quarters than it was in the fourth quarter of 2001, or that MTBE concentrations declined during the fourth quarter. The decline from 53 ppb in December 2000 to 7.6 ppb in February 2002 in KF-5 supports the possibility of an overall concentration decrease, but the concentration in KF-3 was nearly unchanged, supporting the alternate possibility that the plume merely rotated clockwise farther and was oriented more northwest-southeast. Figure 34 incorporates both possibilities. The maximum concentration is shown decreasing from>125 ppb to just over 100 ppb and the concentration in MW-6 is interpreted to have decreased from about 125 ppb to 64 ppb. The plume has also been rotated about 5° clockwise (the same amount of rotation as was seen in the TPH-g plume in 2000). The MTBE plume had the same orientation in May 2002, but concentrations increased in all of the impacted wells and the plume evidently migrated eastward (Figure 35). This migration coincided perfectly with the strong eastward gradient (Figure 20)' that developed after the Chevron 20