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3, illustrating the lateral extent of soil contamination at a depth of 15 to 20 feet, where the highest concentrations were detected. As the map shows, very little contamination escaped the tank cavity. Because all of the samples were collected from the saturated zone, it appeared likely that the benzene was present as an adsorbed phase on soil particles due to migration of contaminated groundwater, rather than due to downward leaching through the vadose zone from the underground _ tanks. Further, the absence of gasoline oxygenates did not support the idea of a recent release from these tanks, but favored an older release pre-dating the addition of oxygenates to gasoline. This was the first substantial evidence suggesting that the contamination was due to a leak from an off-site source, such as the former Chevron station, which had been demolished several years earlier. Groundwater monitoring continued in 1998 and 1999 but no new investigation was initiated. Groundwater flow varied from east to north during this period except on two occasions when it appeared to be southwestward. However, this turned out to be due to damage of KF-3 during the 1998 tank removal work, causing the casing in this well to drop slightly and yield erroneous water elevation measurements. After this well was re-surveyed in July 2000, this erroneous gradient was corrected. 3.3 Phase 3—Extent of Contaminated Groundwater The third phase of the assessment took place after the soil investigation had been completed. Upgradient Environmental again revised the work plan, proposing to install three on-site monitor wells and one off-site well on the residential property immediately east of Kwikee Foods. These wells were installed in the third quarter of 2000 with hollow-stem drilling rig, Geoprobe®, and portable drilling equipment. The results were reported in the Third Quarter 2000 Report (November 1, 2000). Wells KF-4 and KF-5 (Figure 2) were screened from 25 to 5 feet after being drilled to a depth of 25 feet and encountering water at about 10 feet (Appendix A). Wells KF-6 and KF-7 were drilled with limited-access equipment to a depth of either 15 or 20 feet, and are screened to a depth of 5 feet with 1-inch diameter PVC casing. As shown in Figure 3, all of the new wells were located beyond the limits of soil contamination. Little or no hydrocarbon odor was detected during drilling, and only two samples from KF-4 and four from KF-5 were preserved for laboratory analysis. TPH-g was reported at 2.98 ppm at a depth of 11 feet in one sample (Table 2). Gasoline was initially detected in the water in both KF4 and KF-5, but concentrations have since decreased below detection limits in KF-4 (Table 3). In addition, hydrocarbons have been absent from both KF-1 and KF-2 for the last three quarters. • 4