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site, the water table slopes either southeastward or southwestward, at a relatively moderate gradient of 0.0046 ft/ft. South of the site, the gradient is generally northward, at a steep gradient of 0.015 ft/ft. Between the Chevron and Kwikee sites the gradient is also 0.015 ft/ft, in a westward direction. Combined, these various gradients create a north-northeastward trending linear groundwater depression on the Chevron site that is similar in trend to that seen in 1997, 1998, and 2001 but is unlike the more circular cone of depression that existed earlier in 2002. Elevation contours on the Kwikee site indicate a somewhat unusual condition there, with a lower water table on the eastern and western margins than in the middle of the site. This pattern suggests that a slight "ridge" is present beneath the site, with flow both westward and eastward from the center of the site. This may be a temporary situation, because it is so unusual and is probably unstable. 4.0 MOVEMENT OF THE GROUNDWATER PLUME The movement of the plume of gasoline-impacted groundwater, as recorded by changes in TPH-g and MTBE concentrations over time, is depicted in a series of maps for the same quarters as those discussed in section 3.0. The maps were constructed after the groundwater elevation maps were completed, and therefore had no influence on-the groundwater elevation mapping. This allows the extent of groundwater contamination to be compared directly to the direction of groundwater flow at that particular moment in time. These maps thus provide a series of snapshots of the plume over the past eight years. 4.1 First Kwikee Monitoring Year: 1995 1n August of 1995, the gasoline plume was larger and more extensive than at any time since then, and extended entirely across both sites and quite far to the north (Figure 22). Only one dissolved- phase plume of gasoline can be discerned, and it originated between MW-4 and MW-1 at the Chevron site. The arrows denote the local groundwater flow directions taken from the groundwater elevation map on the same date, and the coincidence of the plume's axis and the predominant flow direction is unmistakable. Southeastward elongation of the plume is evident, and downgradient spreading of the contours in that same direction is a hallmark of longitudinal dispersion of the contaminants away from their source. Lateral dispersion toward MW-7 is also apparent, and the presence of gasoline in MW-9 illustrates the strong influence that the long-term regional northeastward hydraulic gradient exerted on the plume, causing widespread dispersion toward the northeast and forming a very broad and diffuse northern margin. Even the sharply defined southern limit of the plume near MW-8 is easy to explain in terms of the steep northeastward hydraulic gradient that existed at the time, "squeezing" the TPH-g contours-as the influx of relatively clean water from the southwest prevented the plume from dispersing southward. As discussed in section 2.4.3, the.period of relatively stable TPH-g concentrations in MW-1 came to an end in early 1995, and concentrations were on the rise, perhaps due to remobilization of previously adsorbed hydrocarbons. The Kwikee site was not sampled in August, but samples from . 15