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REVISED WORK PLAN FOR AQUIFER TEST Gillies Trucking Yard 3931 Newton Road Stockton, CA 1.0 INTRODUCTION Date 10/14/99 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 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. A work plan for an aquifer pumping test was submitted to PHS/EHD in August 1999, and Mr. Jeffrey Wong ofPHS/EHD provided oral comments to the plan on October 5. Mr. Wong requested Upgradient to elaborate on the methods to be used during the test and provide further discussion of the anticipated use of the results. He requested that the revised plan be submitted by October 20. 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. This channel deposit trends east-west through the site and reaches a maximum thickness of 20 feet beneath the former UST facility. 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, and hydrocarbons have been flushed from the soil and become dissolved 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. In order to determine whether pumping from this well would be effective in