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flushing hydrocarbons from the soil and contaminating groundwater within the channel deposit. In order to satisfy regulatory agency questions about the maximum depth of soil and groundwater contamination at the site and install a well that might be useful in later groundwater remediation,Upgradient drilled well GT-10 in 1999. The well is located on the axis of the Modesto channel and penetrated highly contaminated soil in the lower part of the Modesto Formation at a depth of 45-60 feet. Groundwater samples have been collected from this well on several occasions since then,and TPH-d concentrations are consistently greater than 25,000 parts per billion. Hence,this well is optimally located for the purpose of extracting contaminated water from the channel. The purpose of conducting a pumping test in GT-10 is to deter nine how effective groundwater extraction from it would be in containing and/or remediating the groundwater plume. 3.0 COMPUTER SIMULATION Due to its limited thickness and lateral extent,the volume of the Modesto Aquifer is rather small. Furthermore,the normal static water level in the area is 50 feet or greater. Therefore, during much of the year,the aquifer contains little or no water.During the spring months, however, when the static water level rises to a typical depth of 45 feet, the basal 5 to 7 feet of the channel are saturated and hydrocarbon flushing takes place. Therefore, using the computer program VisualModflow®, we simulated a pumping test of GT-10 during the month of May, using an initial water depth of 40 feet. The simulation assumes that the Modesto Aquifer is unconfined (storage coefficient=0.01)but not perched, although the possibility of a perched water table cannot be entirely ruled out by groundwater monitoring data. A pumping rate of 10 gallons per minute (gpm)was used in the first simulation,but at this rate GT-10 quickly went dry before the effect of pumping could be felt at any of the surrounding observation wells. The simulation was re-run using a pumping rate of 3 gpm, producing the time-drawdown curves shown in Figure 3. The curves predict that the water level in GT-10 will be lowered by approximately 6.5 feet within the first hour of the test, and will then stabilize as withdrawal from the pumping well is balanced by recharge fron other portions of the aquifer. Drawdown at distances of 20 and 90 feet (represented by GT-1 and GT-2 in Figure 3)approach 3 and 2 feet in the simulation. When pumping is terminated after 4 hours,the water level recovers to the initial level within 30 minutes. Figure 4 shows the expected cone of depression after 4 hours. Due to the linear geometry of the aquifer,the drawdown is not actually cone-shaped, and no drawdown is predicted to occur beyond the margins of the channel because of the low permeability of the overbank sediment. A drawdown of up to 2 feet is predicted over most of the diesel- impacted area and between 6 and 12 inches of drawdown are predicted farther away. The simulation predicts that no response would be felt in the Riverbank Aquifer.