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• implying that the intervening aquitard was absent or too thin locally to prevent some downward f migration However, concentrations are at or near zero below 70 feet in all borings, and soil below this depth is incapable of causing significant groundwater contamination In large part, that is why diesel concentrations in groundwater samples from the deeper wells (GT-1, GT-2, and GT-3) have been less than 1 ppm in 34 out of 39 samples When concentrations have exceeded 1 ppm, they have always been less than 4 ppm All five samples were collected in 1999, when the depth to groundwater was at its highest level (45-48 feet) and the static water level was above the base of the Modesto aquifer Concentrations began to decline as the water level dropped, and have remained below 1 ppm since then Hence, we conclude that the diesel that is occasionally detected in groundwater samples from these deeper wells is not due to the presence of a significant contaminant source below 70 feet Rather, the hydrocarbons desorbed into the groundwater when the level was above 50 feet and migrated downward as the water level dropped to its present depth The significant zone of residual contamination is from 40 to 60 or perhaps 65 feet (Figure 5) 5.2 Lateral Extent The lateral extent of soil contamination was thoroughly examined in 1996, when five borings (GT- T through GT-9) were drilled around the perimeter of the known area of contamination Seventeen samples from these bonngs were selected for laboratory analysis The sample depths ranged from 40 to 65 feet, which by that time had already been identified as the zone of significant contamination (see above) Soil contamination in these samples was minimal, and only one boring yielded more than one contaminated sample (GT-8) It is noteworthy that GT-8 is the nearest boring to the axis of the Modesto channel, which explains the higher concentrations that were detected • Hence, even though it is farther than either GT-5 or GT-7 from the UST facility, concentrations were higher because the geology was more favorable for petroleum migration along the channel axis As documented in the Problem Assessment Report and updated in the First Quarter 1999 Report after GT-10 and GT-11 were drilled, hydrocarbons extend northeastward beyond GT-8 and eastward beyond GT-1 within the Modesto channel Within the aquitards that overlie and underlie flus channel, hydrocarbon concentrations were low in both of these borings and organic vapor readings measured by photo-ionization detector (PID) in the field were low By analogy with other borings where PID readings were also low and hydrocarbons were not detected in the laboratory, we concluded that contamination below 60 feet is very limited laterally beyond GT-1 and GT-8 Attempting to locate the zero limit of soil contamination precisely has little benefit, particularly since the depth and soil type preclude effective remediation of any contamination that may be present near the contact between the Modesto and Riverbank formations The lateral extent of groundwater contamination has been illustrated by diesel isoconcentration maps in previous reports The first map to illustrate this was prepared in October 1996, and was included with the Problem Assessment Report The plume was depicted as elliptical in shape, centered around boring GT-4 (where a water sample obtained by hydropunch had a diesel concentration of 1,730,000 ppb), and extending slightly beyond all wells and borings where diesel had been detected Later maps demonstrated that the southern margin of the contaminant plume has is usually been a short distance south GT-6, GT-1, and GT-3 and north of GT-9 Diesel has been 5