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• GT-7, creating multiple groundwater flow directions This is an unstable condition, and normally the water level would flatten out and become more uniform over such a small area,yielding a single flow direction The likely reason that the water level has not been able to equilibrate�is that the low permeability of the clay and silt bed from 40-60 feet has retarded groundwater movement up or down to a more uniform level In comparison to most other sites, the groundwater gradient beneath the Gillies Trucking facility has been highly variable This subject was addressed in the First Quarter 1999 Report, in which several different interpretations of the groundwater flow direction were illustrated Over the years, there have been substantial seasonal variations in both flow direction and gradient, making it difficult to identify the predominant flow direction or the primary controls on groundwater flow 4,3 Analytical Results Diesel was detected in three of the monitoring wells (Table 2) The concentration in GT-10 is considerably higher than in the recent past, but this is probably because the well was not purged because of the low water level in the well It is likely that diesel has collected at the bottom of the well as the water level declined Had the well been purged, the water level may not have recovered sufficiently to allow sampling The concentration was also higher in GT-6, perhaps because the static water level has now fallen and is within the screened interval of this well When the level was above 55 feet, samples were collected below the soil-water interface and may therefore have been slightly less contaminated In the same way, the low concentration (89 parts per billion—ppb) in GT-I may be indicative of concentrations below the soil-water interface in this well • Light-weight hydrocarbons in the gasoline range were again detected in GT-10, but concentrations are low relative to diesel Total Petroleum Hydrocarbons as gasoline were reported at a concentration of 920 ppb, while benzene and ethylbenzene were detected at less than 7 ppb Xylene and toluene may also be present, but the concentration is less than 1 ppb, and both compounds were present in GT-1 at about this concentration 5.0 SUMMARY OF EXTENT OF CONTAMINATION In previous reports, maps and dross sections have been presented to illustrate the lateral and vertical extent of soil contamination Maps have also been prepared showing the lateral extent of groundwater contamination In this section,this information is summarized and brought up to date 5.1 Vertical Extent Figure 5 is a graph of diesel concentration versus depth in three borings that are located in the immediate vicinity of the former UST facility All three borings penetrated the base of the Modesto aquifer channel between 50 and 54 feet, and the graphs illustrate that diesel concentrations increase rapidly downward through the channel from less than 2000 ppm to more than 15,000 ppm at the base of the channel In most borings, concentrations decline dramatically below the base of the channel and are near zero in the underlying Riverbank Formation, but a few samples between 60 and 70 feet in GT-8, GT-11, and GT-3 contained diesel fuel at concentrations of 700-5500 ppm, • 4