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WORK PLAN FOR AQUIFER TEST Gillies Trucking Yard 3931 Newton Road Stockton,CA 1.0 INTRODUCTION Upgradient Environmental submitted a work plan for an aquifer pumping test to PHS/EHD in August 1999, and Mr. Jeffrey Wong of PHS/EHD requested further elaboration on the methods to be used and anticipated results. Upgradient submitted an addendum to the plan on October 20, suggesting that a step-drawdown test might be an appropriate first step to determine an optimum pumping rate for a longer-term constant- rate test. After fiarther consideration,however, Upgradient suggested conducting pre-test computer modeling as a way to determine the most effective pumping test procedure. Mr. Wong approved this approach in concept in February, 2000. During the following month, computer simulations using the software program VisualModflow® were performed, using data that had been obtained during earlier phases of site investigation. Based on these simulations, it now appears that a step-drawdown test is unnecessary and it is possible to proceed directly to a constant-rate test. The following sections describe the geological model and methods used in the simulations and present the predicted drawdown curve derived from the simulation. The subsequent section describes the proposed test procedures. 2.0 GEOLOGIC MODEL Data obtained during the assessment phase of this investigation indicates that the principal aquifer beneath the site is the Riverbank Formation. The majority of the petroleum hydrocarbons,however, are trapped in the overlying Modesto Formation,which normally lies within the vadose zone. In the site area,the Modesto Formation consists primarily of silt and fine-grained sand (Figure 1),but a medium-to coarse-grained sand body(Modesto Aquifer in Figure 1)is embedded within the lower portion of the formation. Diesel contamination is localized within this sand body,which fills an erosional channel. The hydraulic conductivity of the sand body is high(10"2 cm/sec in selected core samples),and during the spring months it is water bearing. The surrounding silt and fine-grained sand are overbank deposits that have a hydraulic conductivity of 10-6 and form a significant aquitard separating the Modesto Aquifer from the underlying Riverbank Aquifer. The Modesto Aquifer channel deposit trends east-west through the site and reaches a maximum thickness of 20 feet beneath the former UST facility(Figure 2).Diesel is present near the base of the channel (50-55 feet bsg)at concentrations that locally exceed 10,000 parts per million(ppm). Starting in 1998, groundwater began to rise into this channel,