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EXECUTIVE SUMMARY <br /> Water-depth measurements indicate that static water levels in the three Riverbank aquifers differ <br /> slightly, but are sufficiently similar to demonstrate that the three zones are in hydrologic <br /> communication In each zone, the piezometne surface sloped southeastward in July 2005 <br /> Diesel fuel was detected in 16 of 21 water samples, and all three Riverbank aquifers are slightly <br /> impacted However, concentrations are low, ranging between 100 and 350 parts per billion in 15 of <br /> the 16 impacted wells The maximum concentration of 870 ppb was detected in the 1" Riverbank <br /> aquifer, and isoconcentration maps for the three zones indicate that the size and magnitude of the <br /> dissolved-phase plume decrease downward from the first to the third zone The plume is elongate in <br /> a north-south direction, but whether this is due to variability in groundwater flow direction over <br /> time or influence by the Gillies domestic well is uncertain <br /> t Results from the past three monitoring events have clarified the geometry of the Riverbank aquifers <br /> and the shape and extent of the diesel plume, and it does not appear necessary to perform additional <br /> assessment of the Riverbank Formation We recommend completing the assessment of the <br /> magnitude of contamination in the Modesto Formation, determining the mass of remaining <br /> hydrocarbons in this formation, and performing a fate and transport model of the potential for this <br /> mass to impact groundwater in the fixture Depending on the results of that analysis, it may be <br /> possible to prepare a Site Closure Report in the fourth quarter of 2005 <br />