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. 5.7 Vertical Extent of Groundwater Contamination At the present time, groundwater between the depths of 48 feet in GT-4 and at least 65 feet in the other borings is contaminated Water samples have not been collected from discrete depth intervals to determine the thickness of the groundwater plume in the Riverbank aquifer, but the low concentrations in most borings suggest that the plume probably does not extend far below 65 feet However, because groundwater levels have risen from 85 to 65 feet in recent years, it is possible that groundwater may be contaminated at low concentrations to a depth of 80 feet or more 5.8 Lateral Extent of Groundwater Contamination As shown in Table 5, diesel-range hydrocarbons are present in all of the wells sampled during this phase of investigation except for the drinking water wells These results imply that groundwater contamination is considerably more extensive than previously suspected In particular, contamination is not restricted to the area downgradient (south) of the UST facility, the highest concentrations of hydrocarbons are located upgradient(north)of the facility in GT4 and GT-8(Figure 16) The location of GT-9 near the southern limit of the plume appears to be justified by the low concentrations detected in the well sample and the absence of contamination in the hydropunch sample However, because of the unexpected trough in the piezometnc surface (Figure 9), contaminated groundwater may extend farther to the southwest of GT-9 than shown in Figure 16 . The Gillies drinking water well is north of the plume The high concentration in the hydropunch sample from GT-8 creates the impression that the northern margin of the plume is closer to G 1 than to GT-8, but if the concentration from the well sample(3,600 pgll) had been used in Figure 16, the limit would be closer to GT-8 Based on Figure 16, the plume is estimated to cover an area in excess of 26,000 square feet, or slightly more than 112 acre 6.0 CONCLUSIONS Several conclusions can be drawn from the investigation of this site These form the basis for the recommendations offered m section 7 0 1) The primary contaminant at this site is the diesel used to fuel the trucking fleet This fuel was contained in a 10,000-gallon UST until 1989 A secondary contaminant is gasoline, which was contained in a 2,000-gallon UST Toluene, ethylbenzene, and xylene have been detected infrequently and at low concentrations, but benzene has been detected extremely rarely and methyl-tertiary-butyl- ether has not been detected in any samples 2) The vertical and lateral extent of soil contamination have been well delineated and related to the subsurface geologic conditions and processes controlling contaminant distribution The shape of the contaminant plume changes with depth and is controlled primarily by the geometry of a mayor stream channel deposit and secondarily by cemented sandstone ("hardpan") zones The bulk of the i 15