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below the detection limit or in the 60-90 ppb range except on two occasions The eastern limit is defined by GT-21, and the concentration of 80 ppb in GT-20 suggests that this well is not fat trom the northeastern limit In the 2nd and 3rd Riverbank aquifers, the highest concentrations are located west of the former UST facility in borings GT-3 and GT-14 (Figure 15) These plumes appear to trend more northerly, but turn eastward near the garage building Wells GT-12A, GTA, and GT-17B provide northern and southern limits on the plume in the 2nd aquifer, but neither the eastern nor western Iimits aie well defined by the existing wells Wells GT-13B, GT-15B, and GT-18B provide good limits on the plume in the 3'd aquifer and imply that this plume is very narrow in the north-south direction, but neithei the northeastern end nor the southwestern end can be mapped with certainty fiom the available data 4 0 SUMMARY AND CONCLUSIONS Sia additional borings were drilled during the first quarter of 2005 to further investigate the vertical and lateral extent of fuel contamination in soil and groundwater of the Riverbank Formation Tluee of these were completed as nested wells for a total of nine new monitoring wells The borings ranged in depth from 80 to 125 feet Nine soil samples were analyzed for diesel-range hydrocarbons Diesel fuel was detected in one sample trom the base of the Modesto Aquifer at a concentration of 4100 mg/kg, in one sample hom the base of the 2nd Riverbank Aquifer at 2 7 mg/kg, and in one sample from the base of the 3rd Riverbank Aquifer at 16 mg/kg r There are now five monitoring wells in the 3`d Riverbank Aquifer, seven wells in the 2°d Rivetbank Aquifei, eight in the I" Riverbank Aquifer, and one in the Modesto Aquifer The piezometrrc surface in the 1 S` aquifer is relatively planar and slopes northward The prezometric surface becomes inei easrngly undulating in the deeper aqurfei s, and slopes generally eastward in both the 2' and 3rd agtufeis The slight differences in the elevation of the piezometrrc surface in each aquifer imply that these aquifers may be partially confined on a local scale, but at the scale of the overall site they aie a probably in coininunication The eastward slope to the piezometrrc surface in the 2nd and 3rd aquifers suggests that these two may be in somewhat better communication than they are with the I" aquifer, where the piezometrrc surface slopes northward On the other hand, the sand-on-sand contact between the I" and 2nd Riverbank aquiters in GT-6 and GT-18 (Figure 5) is a clear indication that these aquifers are in communication in some areas Twenty water saanples from the 1S' 2"d and 3rd Riverbank aquifers were analyzed This quarer was the exception to the rule that the highest diesel concentrations in the Riverbank Formation have been detected in the I" aquifer in GT-6 Previously, concentrations of 500-600 parts per billion had been detected, but in March the laboratory reported a concentration of 170 ppb A slightly higher concentration (200 ppb) was detected in GT-16 These two wells currently define the core of the plume in this aquifer, and concentrations decline outwaid to below the detection litnit The plume 9