SITE INFORMATION AND CORRESPONDENCE_1985-1997

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KEI-P88-077 July 26, 1988 Page 2 INTRODUCTION This proposal presents the work plan for the next phase of effort at Unocal Service Station #5098 located at 5606 Pacific Avenue, Stockton, California. The first phase of the investigation conducted at the referenced site was performed by J. H. Kleinfelder and Associates. In mid 1985 Unocal removed three underground tanks (1 - 10, 000 gallon unleaded, 1 - 10, 000 gallon unleaded supreme and 1 - waste oil) . A preliminary site investigation, by J. H. Kleinfelder indicated hydrocarbons and solvent contamination. Additional investigation was conducted by J. H. Kleinfelder and Associates in 1987 to define the lateral and vertical extent of contaminants found during the preliminary investigation. Over the course of two years a total of six monitoring wells and nineteen borings were installed. The location of these wells and borings are shown on the location map included with this report. Soil contamination was encountered down to the level of groundwater which was approximately 43 feet below grade. Contamination varied significantly by depth. The most predominant levels were encountered in borings B-2 , B-8, B- 10, and B-13 (see attached Table 1) . The details of the investigations are described in June, 1986; March, 1987 ; July, 1987 ; and August, 1987 reports. These reports have been submitted by Unocal to the concerned regulatory agencies. During, the course of reviewing the Kleinfelder investigations, the following conclusions were reached: * Approximate lateral extent of the zone of contamination was defined in the soil. Four major hot spots were located and the soil contamination varied in depth. * The vertical extent of the contamination was apparently limited to about 40 feet below ground surface. * Depth to groundwater is approximately 43 feet below grade. Direction of the flow is south to southwest. In the uppermost water bearing unit transmissibility ranges from 160. 6 gpd/ft to 415 gpd/ft. * Monitoring wells MW-1 and MW-2 are the most significantly contaminated wells. Benzene levels in both wells exceed the action level set by the RWQCB. * The impact to off-site agricultural wells is unknown. * Soil excavation and on-site aeration were recommended.