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and 7 1 parts per million in the 3rd Riverbank aquifer, were detected Samples I the mudstone aquitards that separate these aquifers were below detection limits, except for one sample between the Modesto and 1St Riverbank aquifers A concentration of 4400 parts per million was detected in GT-15 As reiterated in many previous reports, a large body of data demonstrates that this aquitard absorbed a large quantity of diesel fuel and Was the primary constraint limiting the downward leaching of diesel fuel into underlying aquifers Monitoring wells were constructed in the 1St, 2nd, and 3`d Riverbank aquifers in various wells Three wells were installed in the 3rd aquifer, and a groundwater elevation map was derived for this aquifer using the three-point method A gradient of 0 001 ft/ft was calculated, with groundwater flow to the south Three wells were also installed in the 2nd aquifer, and the groundwater elevation data from these wells suggest that groundwater in this aquifer flows westward, at least in the vicinity of the UST cavity One well was installed in the 0 Riverbank aquifer, and water was present at the bottom of well GT-13A when it was sampled However, data from most wells indicate that this aquifer is currently above the water table and is within the unsaturated zone Hence, the source of the water in GT-13A is uncertain and it may be more representative of the 2"d Riverbank aquifer than the 1St aquifer A groundwater elevation map for the 1St Riverbank aquifer was constructed using data from three of the previously installed wells and GT-13A, and the contours imply that groundwater is flowing northward in this aquifer Thirteen water samples from the 1", 2nd and 3`d Riverbank aquifers were analyzed The Gillies domestic well continues to be free of hydrocarbons A new well 20 feet to the south had a diesel concentration of 59 parts per billion in the 2"d Riverbank aquifer, implying that the northern mat gin of the contaminant plume is between these two wells A sample from a well south of the former UST cavity was also below the detection limit, helping to delineate the plume in that area In between these two areas, concentrations ranged from 300 to 600 parts per billion, and the plume appears to be centered north of the former UST cavity This is fairly consistent with the apparent direction of groundwater flow in this aquifer All three water samples from the 3`d Riverbank aquifer were slightly contaminated with diesel Concentrations of 56 and 72 parts per billion were detected south and west of the UST cavity From Figure 12, this suggests that the southern margin of the plume is delineated and is cross-gradient to present groundwater flow A concentration of 390 parts per billion was detected nearer the UST cavity, and we infer that this plume is located within the immediate vicinity of the UST cavity The maximum concentration in this aquifer isrelativelylow(probably less than 500 parts per billion) 5.0 RECOMMENDATIONS In previous sections of this report, we have made specific recommendations for minor changes in the next phase of drilling Based on our interpretation of the data collected during this phase, we recommend drilling wells GT-16 through GT-21 in one additional phase, rather than separating them into two phases, as originally proposed in the July 2004 work plan Because groundwater 13