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San Joaquin County Environmental Health Division Project 2267-E December 29, 1999 Page 3 benzene to a depth of 45 feet below the ground surface Studies in this area indic Lite that the hydrocarbon concentrations in the soil and groundwater are limited to the area around the location of the leaking underground storage tanks which were removed from the southeast corner of the site in 1967, as shown in Appendix A HYDROGEOLOGIC SETTING The subject property is located on a relatively level parcel, in an urbanized area, at an . elevation of approximately 15 feet above mean sea level Surface water drains to the west, toward San Francisco Bay, within a culvert south of the subject property Older alluvial sediments of the Calaveras River and vicinity, consisting of interbedded, poorly consolidated sands and clays, underlie the subject property Boring log data from Monitoring Well U-7, located in the northeast corner of the site, indicates that there is 42 feet of clay overlying 10 feet of sandy clay at this site The boring logs also indicate that there is no apparent hydrogeologic pathway for migration of the shallow groundwater from this site to the northeast, toward the contaminated Unocal site Currently, the depth to groundwater in this area vanes from 25 to 27 feet below the ground surface It flows to the northeast at a gradient of 0 01 to 0 003, see Appendix A The piezometric surface was at an elevation of 50 to 60 feet below the ground surface in 1964 By 1970 the groundwater had risen approximately 30 feet to its current level The contaminated shallow groundwater is non-potable and subject to intrusion by saline bay water The nearest water well is more than 1,500 feet down gradient from this site TENTATIVE WORK PLAN . The pnmary purpose of this phase of our review is to determine if there is any petroleum hydrocarbon contarrunation in the shallow groundwater at the subject property, and, if so, i� YS&tom&A 3 ix HYDRO-GEO CONSULTANTS,INC