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The water level continued to rise after 1995, and between 1998 and 2001 the water level vaned between 54 feet and 45 feet. At this depth, it was within the Modesto Formation channel deposit in some wells or just below it in others. The channel deposit is known from previous soil samples to be impacted by diesel fuel. Maximum diesel concentrations were detected in water samples from all wells during these years, regardless of whether the static water level was within the screened interval or not. For example, in GT-2 a concentration of 3,000 ppb was detected in October 1999, when the depth to groundwater was 54 feet (21 feet above the screened interval of the well). The maximum concentration in GT-1 was 3,400 ppb in March 1999, when the depth to groundwater was 45 feet (30 feet above the screen). Hence, it was not until after the water level rose into the Modesto Formation that diesel concentrations began to exceed 1,000 ppb. On this basis, we have concluded in past reports that these elevated concentrations are due to dissolution of diesel fuel when water is present in the shallower (Modesto) formation and that concentrations decline when the water level drops below the Modesto Formation. The data from GT-3, for example, strongly suggest this. Between March 1999 and October 1999,the water depth dropped more than 9 feet from the Modesto channel into the aquitard below this channel, and the diesel concentration declined from 1,400 ppb to 96 ppb. In our view, such data imply that groundwater contamination within the basal part of the Modesto Formation below a depth of 55 feet or within the upper part of the Riverbank Formation below 65 feet is extremely minor. If contamination were significant within groundwater in the Riverbank Formation, one would expect concentrations to increase when the water level is within the screened interval of this formation. Our view is further supported by the rapid decrease in diesel concentrations in soil,samples below the base of the Modesto Formation, and the fact that diesel concentrations are less than 25 ppm in 10 of 12 soil samples from the Riverbank Formation. This view is also consistent with the fact that diesel has a low solubility in water, and is therefore seldom present more than a few inches below the static water level when overall concentrations are low. Hence, one can argue that low concentrations (50-500 ppb in the recent past) of diesel have impacted groundwater in the Riverbank Formation and the depth of this impact is dependent on the depth to the soil-water interface, but one cannot reasonably argue that detectable diesel concentrations extend more than a few feet below the soil-water contact at any specific moment in time. Further, the only significant impact occurs when the soil-water contact is within the Modesto Formation,at a depth of less than 55 feet. Regarding further vertical assessment, EHD has suggested using a cone penetrometer to identify stratigraphic intervals that may- have high permeability and to collect grab groundwater samples. Two such hydrologic units are known from previous drilling and laboratory testing, as discussed below in Item 4. Item 4.Hydraulic Properties of the Sand Bodies EHD was also unable to locate information regarding the hydraulic properties of the sand bodies. Data regarding the hydraulic conductivity of the sand bodies were presented in the Problem Assessment Report.