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diesel fuel remaining in the soil occur near the contact between the Modesto and Rivetbank Formations, so it is not surprising that the highest groundwater concentrations occur in the I" Riverbank aquifer and the plume is larger in this aquifer than in the underlying units It is a bit surprising that the highest concentration was detected in GT-16A, because this well is relatively far from the former UST facility,and only 200 ppb was detected in this well in March Neither the 2nd nor 3`d aquifers appear to be strongly impacted The maximum detected concentration was 299 ppb in GT-3, and no well in the 3`d Riverbank detected a concentration as high as 100 ppb In both zones the plume has a weakly defined center beneath the former UST cavity, but is stretched to the north toward the garage building This"stretching"may have been due to the influence of the Gillies domestic well (G-1), which was destroyed under permit earlier this summer Hence, if this well did cause some migration of diesel; we should observe a decline in concentrations in GT-12A and GT-12B the rest of the year Figure 9 plots the TPH-d concentration in GT-6 against time to illustrate concentration trends in the 1 st Riverbank aquifer When the depth to groundwater was more than 62 feet in 1995 through 1997, the TPH-d concentration was at its highest level During the high water years of 1999-2001, when the water table was above 55 feet, the concentration dropped to less than 100 ppb and on several occasions was below the detection limit During this time, contamination was largely confined to the Modesto aquifer When the water table began dropping again in late 2001, concentrations in the 1St Riverbank aquifer began to rise, and have been in the moderate range of 200-700 ppb since that time The highest concentrations have been detected when the water depth is between about 63 and . 65 feet below grade This corresponds to the top of the aquifer 4.0 SUMMARY AND CONCLUSIONS Three quarters of monitoring have demonstrated that there are slight differences in the elevation of the ptezometric surface in the three Riverbank'aquifers, but static water levels are within a few inches of each other in each of the nested wells, implying that all of the aquifers are in partial communication This is further demonstrated by the fact that the ptezometric surface in each aquifer sloped primarily southeastward during the second quarter This implies a relatively uniform groundwater flow direction, although the piezometric surface in the middle zone may be less planar than in the upper and lowei zones Twenty-one water samples from the 1st, 2nd and 3`d Riverbank aquifers were analyzed, and the laboratory detected diesel fuel in sixteen of these All three aquifers are impacted, but the degree of impact decreases downward and concentrations in the 3`d Riverbank are only slightly above the detection limit The concentration did not exceed 1,000 parts per billion in any of the wells, meaning that the impact is minor in all wells and all zones Longer-teen monitoring data for the I"Riverbank aquifer indicate that concentration values change with the depth to groundwater When the water table is high and groundwater is present within the Modesto Formation, concentrations in the Riverbank aquifers decline to or below the detection . limit When the water table is low, the diesel is carried downward into the Riverbank and 4