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Whereas in previous years MW-1 was located within or slightly east of the center of the plume, the map for December 2000 reveals that the core of the plume had begun to migrate southward. This is fundamentally different from the process of dispersion, where the core remains stationary but the plume gradually spreads and enlarges due to mixing of contaminated and uncontaminated groundwater. In 2000, the plume was narrowing in a north-south direction and migrating slightly southward, in a process referred to as advection. This change was probably due to the installation and initial operation of the remediation system at the Chevron site, causing the plume to move en masse toward the extraction wells. The groundwater elevation map for December 2000 (Figure 17) demonstrates that groundwater flow by the end of the year was radial toward the southeastern corner of the Chevron site. 4.7 Plume Contraction: 2001 In 2001, the TPH-g plume remained elongated east-west, and because concentrations were higher south of the Chevron tank pit than north of it, it appears that the core of the plume was still located south of MW-1 (Figure 28). It also appears that the plume began to separate in the center of the Chevron site, forming two "nuclei" centered near MW-1 and MW-4. Concentrations dgcreased below the detection limit in all of the Kwikee wells except for KF-3, implying that the plume began to contract toward the Chevron site as a result of active remediation there and the continued depression of the water table. 4.8 Continued Reduction: Early 2002 • Plume maps have been prepared for the first, second, and fourth quarters of 2002. The map for February 2002 is discussed in this section. Except in MW-8 and KF-5, concentrations were lower in all wells in February 2002 than they were in November 2001. On average, concentrations declined by 30-40%, resulting in further contraction in the size of the contaminant plume (Figure 29). The concentration dropped below the detection limit in MW-2 for the first time since this well was first sampled in 1987. The plume remained elongate, again with a long axis that trended slightly to the southeast. Groundwater flow was almost due westward across the Kwikee Foods site, so the fact that the TPH-g concentration rose above the .detection limit in KF-5 is somewhat unexpected. It could be due to the 3.5-foot rise in the water table that took place, although this seems unlikely. It could also be interpreted as an indication that gasoline that had previously migrated eastward beyond this well was being drawn back westward, causing the concentration to increase. However, slight changes in sampling procedure'are just as likely to cause such a modest increase. 4.9 Temporary Expansion: May 2002 In section 3.0 we discussed the effect on the water table caused by the shutdown of the Chevron remediation system in the second quarter of 2002. While the water level rebounded a few inches in the Chevron wells, no similar response was observed in the Kwikee wells, where the water level • 18