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. where it has been breached by drilling. Groundwater is confined to the underlying sand bed because of the low hydraulic conductivity of the silty clay layer. The clay bed below the sand layer is also an aquitard, and samples of this bed were also damp but not saturated. There has been a significant rise in the piezometric surface since the monitoring wells were drilled in 1994. The depth to groundwater at that time was 19 feet, within a foot of the aqui tard-aquifer contact. The monitoring wells were therefore screened from 15 to 30 feet. With the rise in the piezometric surface, the static water level in the wells is now 9 feet above the screened interval. 5.4 Groundwater Gradient The data in Table 1 were used to contour the piezometric surface and interpret the groundwater flow direction, which are illustrated in Figure 6. The piezometric surface now slopes S30°W at a gradient of 0.0025 ft/ft. This represents a complete reversal in direction from the previous east- northeast gradient and suggests that groundwater should be flowing toward-smith Canal at present. The reason for the gradient reversal is probably related to the rise in the piezometric surface into the upper silty clay layer,but exactly how they are related is uncertain. 5.5 Laboratory Results As in the past, a gasoline odor was noticeable in all wells when they were purged. Gasoline and BTEX compounds were detected in all three wells, but at lower concentrations than in previous . monitoring events (Table 3). However, concentrations were still sufficiently elevated to prevent Sequoia from attaining the low detection limits that are desired for the gasoline oxygenates. Bence, although it is clear that at least four of the oxygenates arc not present at concentrations in excess of 100 µg/1 (parts per billion), it is possible that one or more may be present above the 35 ppb level that has been used by many regulatory agencies to require further monitoring and/or remediation of MTBE. This possibility is suggested by the fact that the concentrations of TBA and MTBE reported by the previous laboratory (McCampbell Analytical) in November, 1997, are below the detection limits attained by Sequoia. However, due to the fact that analyses for the oxygenates have been in use for less than two years and are still not fully proven, the precision of McCampbell's detection limit is still questionable and it is possible that the results reported previously may be inaccurate. Figure 7 is a contour map of TPH-gasoline and benzene concentrations inn groundwater samples collected during the first quarter of 1998. The samples from MW-6 and MW-7 were collected by Blaine-Tech in February. The samples labeled "west water" and "east water" were collected from the floor of the open UST excavation by Upgradient in March; and the samples from KF-1, 2, and 3 were collected by Upgradient in early May. Hence, the contour map does not represent a single point(day) in time, but it is the best that can be achieved at the present time. The highest TPH-g concentrations were detected in the excavation samples, which gives the impression that the USTs were the source of the groundwater contamination. However, the highest benzene concentration was in KF-3. and the concentrations in the offsite monitoring wells (MW-6 7