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WORK PLAN FOR AQUIFER TEST Gillies Trucking Yard 3931 Newton Road Stockton, CA 1.0 INTRODUCTION Two underground storage tanks were removed from the Gillies Trucking yard on Newton Road in 1989, and diesel and gasoline contamination was detected in the soil. Assessment of the extent of soil and groundwater contamination began in 1990, and was completed in 1996, when Upgradient Environmental submitted a Problem Assessment Report delineating the extent of contamination and recommending corrective action. Initially,the local oversight agency(San Joaquin County, Public Health Services, Environmental Health Division) concurred, but later requested additional investigation of the vertical extent of contamination. The requested work was performed in March, 1999, and the results were reported in the First Quarter 1999 Report. Because diesel was detected at a high concentration,the report also recommended that the the next step should be to test the feasibility of groundwater remediation. Among other things,the report recommended an aquifer pumping test to evaluate the effectiveness of groundwater extraction. This work plan has been prepared to describe the procedures that will be used. 2.0 CONCEPT 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. The most contaminated part of the Modesto Formation is a narrow, relatively linear fluvial sand body that is embedded within finer-grained overbank deposits. Recently, groundwater has risen into this channel,flushing contaminants from the soil and becoming increasingly contaminated as the hydrocarbons dissolve in the groundwater. Extraction and treatment of this water thus offers a potential means to remove contaminants from the soil,which is too deep for excavation to be cost effective. Fortunately,the trend and orientation of the channel are now well-known,and monitor well GT-10 is optimally located for the purpose of extracting contaminated water from the channel. 2.0 PROCEDURES In order to measure the rate at which groundwater could be effectively withdrawn from the channel and estimate the time required and cost to reduce contaminant concentrations to acceptable levels, a 12-hour constant-rate groundwater pumping test of well GT-10 will be conducted. This well is screened from 30 to 60 feet, and therefore spans the contaminated