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i <br /> Subsurface investigation December 12, 1990 <br /> Former Unocal Station No. 2859, Stockton, California AGS 38012-5 <br /> HYDROCARBONS IN SOIL <br /> Laboratory results of soil samples collected from the maximum excavated depth of the <br /> former gasoline and waste oil tank cavities indicate hydrocarbonaceous soil was removed <br /> from under the former tanks during the work conducted in April and May 1988. Two <br /> Geologic Cross Sections (Plate P-14)show our interpretation of the stratigraphy across the <br /> site. The cross sections were constructed from the boring logs drilled during this and <br /> previous investigations. Shallow subsurface sediments consist mostly of silt and clay. The <br /> sediments of the water-bearing zone consist of fine-grained sand, silt, and clay that are <br /> underlain by clay and silt. <br /> The cross sections also include TPHg concentrations from soil samples collected in this and <br /> previous investigations. Hydrocarbons remain in the soil near borings KB3 and KB5 at <br /> approximately 20 feet below grade, but hydrocarbon concentrations are below detection <br /> limits for the methods used at 15 feet below grade in these two borings. Results of analyses <br /> of soil samples collected in our June 1988 work indicated our investigation delineated the <br /> extent of hydrocarbons in soil to the north of the tank cavity (boring B-1), to the west of <br /> boring K.B3 (boring B-3), and to the east of pump islands (boring B-2). Analytical results <br /> from our August 1989 work indicated our investigation delineated the extent of <br /> hydrocarbons in soil south of boring KB5 (boring B-6). <br /> HYDROCARBONS IN GROUND WATER AND FLOW DIRECTION <br /> Wells MW-1 through MW-5 have been monitored on a quarterly basis from August 1989 <br /> through August 1990. Laboratory results of monitoring events since August 1989 are <br /> _- -- <br /> ------ in-Appendix C-.-Results of the Nb <br /> - ovemer..-19.90.monitaring eYerst are pendirr <br /> - ----iricluded..in App _ _..--- <br /> 14 <br /> AopliScf G6mOSyst62MS <br />