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i Introduction This workplan is for conducting final subsurface characterization at the Disco property located at 110 North Filbert Street in Stockton,California (Figure 1). At a meeting on January 24, 1997 and subsequent phone conversations involving representatives from San Joaquin County Public Health Services(PHS),Del Monte Foods, and Parmaceast,PHS requested a workplan for the following activities: • Regrading of surface soil in the area of the former Disco gasoline islands to reduce/eliminate surface water ponding • Additional soil sampling and groundwater characterization beneath the former Disco gasoline pump islands • Groundwater Investigation downgradient of the former Disco tanks • Installation of one upgradient monitoring well that intersects the current water table The subsurface characterization activities listed above are primarily necessitated by significant changes in groundwater levels and flow directions that have occurred during the past 2 to 3 years. Groundwater levels have risen over 10 feet and are above the top of well screens for the existing monitoring wells. Groundwater flow directions have changed from northerly to southerly. The information gathered from new monitoring wells installed as part of this workplan will be used to supplement existing monitoring well data. As part of this investigation,2 to 4 shallow (i.e.,water table) and 2 to 3 deep (i.e.,vertical extent) monitoring wells will be installed. The new shallow wells will be utilized to determine water quality at the water table interface. It is assumed that the existing network of monitoring wells is adequate for understanding shallow groundwater flow and gradient information because of the general consistency in geology in the upper shallow groundwater zone. The deep monitoring wells will be installed to determine the vertical extent of groundwater contamination. It is anticipated that the existing network of deep monitoring wells will be supplemented by the installation of 2 new deep wells for purposes of improving understanding of groundwater flow in the deeper zone. However,we understand that a third deep well may be necessary if the existing deep wells are not screened at the same interval as the two new deep wells. After the investigations described in this workplan are completed, Del Monte and Parmaceast plan to update the Problem Assessment Report and Final Remediation Plan (CH2M HILL, April 1995)by issuing a Final Corrective Action Plan which will include a human health and ecological risk assessment. i