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TERRA VAC ADDITIONAL SOIL AND GROUNDWATER INVESTIGATION WORK PLAN FORMER CHEVRON STATION 9-4054 2103 COUNTRY CLUB BOULEVARD STOCKTON, CALIFORNIA 1.0 INTRODUCTION Terra Vac Corporation (Terra Vac), at the request of Chevron Products Company (Chevron), has prepared this Work Plan for the additional site assessment of petroleum hydrocarbon impact to soil and groundwater at the former Chevron station 9-4054, located at 2103 Country Club Boulevard, Stockton California(Figures 1 and 2). The objective of the proposed work is to obtain additional soil and groundwater data utilizing push probe technology. The collected data will be used to delineate the soil and groundwater plumes. An assessment of the risks to human health and safety will be made based upon data collected during the additional site investigation. The results of the risk evaluation will be used to determine the most appropriate level of corrective action to be taken in remediating hydrocarbon impact to soil and groundwater. Once this determination has been made, the feasibility of at least two remedial alternative will be evaluated. The results of the feasibility evaluations along with the final remedial action will be proposed in a Corrective Action Plan (CAP) to be submitted to San Joaquin County Public Health Services, Environmental Health Division (PHS-EHD) and the California State Water Quality Control Board, Central Valley Region (RWQCB). Current Site Characteristics Available site data indicate that all subsurface fuel storage and dispensing facilities have been removed from the site. However, subsequent environmental investigations have identified petroleum hydrocarbon impact to soil and groundwater beneath the site. Current quarterly groundwater monitoring data indicate detectable dissolved benzene concentrations ranging from 10,000 parts per billion (ppb) in well MW-1 to 14 ppb in well MW-9. Wells MW-5 and MW-10 through MW-12 did not contain detectable concentrations of benzene. A recent plume stabilization study indicates that the dissolved benzene plume is stable along the leading edge of the Chevron plume. An isoconcentration map of the known benzene plume is presented as Figure 3. Data exist which imply that other hydrocarbon plumes are present proximal to the site. These sources appear to be located cross gradient, west (Boulevard Auto) and east (Kwikee Foods) of the site. Additional data resulting from investigations conducted by each of the potential offsite sources is needed to further define the extent of the respective dissolved benzene plumes. Figure 4 shows dissolved benzene concentrations reported for Project 30-0239 August 14, 1997