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The soil and groundwater investigation performed by Weston in February 1991 F detected petroleum hydrocarbons in the soil and groundwater. The subsequent quarterly monitoring events have detected petroleum hydrocarbons in the Lgroundwater. The results of the Weston assessment and monitoring are summarized in Tables 2-2 and 2-3. The Weston assessment report and quarterly monitoring results are included as Appendix B. C. Site Geology _ Based on soil boring to s from previous investigations, thegeology of the site consists of two sedimentary units. The upper unit extends from the surface to approximately 10 feet below ground surface (bgs) and consists of clay Y Y and silt clay. The lower unit extends from 10 feet bgs to approximately 25 feet,bgs and consists of fine grained sand and silty sand. D. Site Hydrogeology The site is basically flat and at approximately 5 feet above mean sea level (msl). Surface water drains to the east. Groundwater has been documented at approximately 10 to 12 feet bgs. Previous studies reported groundwater gradients ranging from - 0.0037 feet per foot (ft/ft) to.0.0065 ft/ft and directions ranging from due west to due south. L ( 3.0 PLAN FOR DETERMINING SOIL CONTAMINATION L Two soil borings will be drilled with a truck-mounted continuous-flight hollow j stem auger (CFHSA) drilling rig. These borings will be drilled primarily to install two additionalg roundwater monitoring wells, but also. to determine if the soils have { been impacted by the contaminants. The proposed-soil boring locations are shown on L. Figure 4-1 as MW-8 and MW-9. LA. Sampling Methods and Procedures During the drilling of the borings, soil samples will be collected at 5-foot r intervals, at changes in lithology, and at areas of obvious contamination. The soil L samples will be collected with a modified split-spoon sampler fitted with 6-inch brass 93822 East Bay Municipal Utility District 40429200.wp 8 Work Plan-Stockton Service Center I