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ro ' CHAPTER I IN'ri-mDUCTION ARCO products Company (ARCO) authorized Brown and Caldwell Consultants (BCC) to conduct a site investigation at ARCO Station 2093, iocated at 3.125 Tracy Boulevard, Tracy, California. This report presents the methods and results of this investigation. The purpose of the investigation was to more fully determine the lateral extent of soil and groundwater contamination, both on site and off site. The information gained during this investigation ;s being used to assist in the development of an effective remediation plan. This investigation was required by the California Regional Water Quality Control Board, Central Valley Region (Regional Board) and the San Joaquin County Public Health Services Environmental Health Division (County) under the Regional Board Pilot Program. A work plan was prepared by BCC and submitted on behalf of ARCO to the County and Regional Board for approval. During the investigation three off-site monitoring wells and one on-site extraction well were installed. In addition to the wells, two soil boreholes were drilled near the location of the underground fuel storage tanks and a step drawdown test was performed on the extraction well. Figure 1-1 presents the locations of all of the monitoring wells and boreholes installed to date for this sit:. Currently all monitoring wells are sampled on a quarterly basis and quarterly sampling r reports are submitted to the Regional Board and the County. This report includes the quarterly sampling results for the quarter that ended in July 1994. Background In December 1986,one 500-gallon waste oil tank was removed and the underlying soil was sampled. Analytical results of the soil samples indicated the presence of hydrocarbons. In February 1986, the County directed ARCO to conduct a soil and groundwater characterization at the site. The purpose of the inve-'igadon was to determine the groundwater gradient, identify the presence or absence of product in the soil and/or groundwater beneath the site,evaluate the site geology,and obtain information necessary for evaluating potential remedial actions at the site. The investigation was conducted by BCC in March 1987 and included drilling four boreholes with groundwater monitoring wells(ATR-1 through ATR-4)installed in all of the boreholes (see Figure 1-1 for monitoring well locations). The results of this investigation are included in BCC's September 28, 1987 Problem Assessment Report.