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J . detected in the latter well on only two occasions when it was drilled in October 1996 and in late 1999, when the water level was above 52 feet The northern margin of the groundwater plume has usually been near GT-5 (concentrations have fluctuated between non-detect and 500 ppb since 1997) Prior to that, concentrations were above 1,000 ppb, when the static water level was within the uppermost Riverbank Formation at a depth of 60-65 feet As discussed in the previous section, diesel-range hydrocarbons are present at this depth in GT-3, GT-8, GT-10, and GT-11, which may explain the higher groundwater concentrations that were detected in GT-5 at that time Occasionally, diesel has been detected in the sample from the Gillies domestic well (G-1), apparently implying that the northern margin of the groundwater plume has sometimes been north of GT-5 This occurred Mostly in 1999, when the water level was at its historical high point and contaminated groundwater was able to migrate farther than normal within the Modesto channel At the present time, the plume does not extend as far north as GT-5 nor as far southwest as GT-2, and is apparently restricted to a linear trend between GT-6 and GT-3 (Figure 6) 5.3 Historical Trends in Contaminant Concentrations Changes in the water depth and diesel concentrations have been tracked for GT-6, and are illustrated in Figure 7 In this well, which is screened from 55-75 feet, concentrations were highest when the water depth was greater than 60 feet and was within the screened interval As the water depth rose above 50 feet in 1998, concentrations decreased to the non-detect level When the water depth began to fall again in late 1999, the concentrations increased to around 100 ppb but then dropped below the detection limit when the water depth rose again in early 2000 Since the middle of 2000 the water depth has tended to increase, and has been between 50 and 60 feet, near the top of • the screened interval Probably as a result, the diesel concentration rose above the detection limit and has been in the 50-150 ppb range until this quarter, when it dumped to 580 ppb These results agree well with the hydrogeologic interpretation of the site, which predicts that concentrations will peak when the static water level is at or Just below the base of the Modesto aquifer, where soil concentrations are at a maximum The graphs for GT-10 show a different trend (Figure 8) This well was first sampled to early 1999, and since then the depth to groundwater has been on the increase although the static water level has been within the screened interval (40-60 feet) throughout that period The gasoline concentration showed a general though irregular decline over that time, while the diesel concentration fluctuated rapidly and showed no relationship to the gasoline concentration or to the water depth Nor can the diesel concentrations be related to purging the concentration peak in March 1999 was recorded after the well was purged, but the nearly identical peak in December 2000 was recorded when the well was not purged Nonetheless, the large fluctuations in the concentrations are most likely due to slightly different sampling techniques, such as the depth to which the bailer was lowered In section 4 1, we noted that the only time that diesel concentrations have exceeded 1 ppm (1,000 ppb) in the deep wells was during late 1998-early 1999, when the water depth was at a minimum This is illustrated visually in Figure 9, which shows the historical record of concentrations in GT-1 This peak in concentrations occurred despite the fact that the static water level was at its maximum elevation above the top of the screened interval, when concentrations would normally be expected • to be at a minimum This fact corroborates our conclusion that groundwater concentrations rise 6