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each approximate 50 cubic yards of stockpiled soil These three samples were coin osited b the • PP Y P P P Y laboratory for analysis Results of these analyses indicated that only very low concentrations of TPHg and BTEX(no benzene) remained in the soil Based on the laboratory test results, the San Joaquin County Air Pollution Control District allowed uncontrolled soil aeration to mitigate the remaining petroleum hydrocarbons in the soil The soil was spread over the surface of an access road on the property and allowed to aerate On November 26, 1993, soil sample composites were collected from the aerated stockpile in the same manner as before A composite sample consisting of three individual samples was also collected from the substrate beneath the formerly contaminated soil to verify that contaminants had not migrated to soil below the aerated stockpiled soil Laboratory analyses of these samples indicated that TPHg and BTEX concentrations had been reduced to less than the nummum detection limits of the laboratory methods (Table 2) 5.0 FIELD INVESTIGATION The results of our field investigation indicated that subsurface soil and groundwater is significantly contaminated with TPHg and BTEX beneath the site and to a somewhat lesser degree offsite mainly toward the southwest The vertical and lateral extents of soil and groundwater contamination were defined during the plume delineation work, and the remediation feasibility studies at the site indicated that the hydraulic conductivity of the shallow groundwater-bearing zone is low, although vadose zone soil permeability appears relatively high 5.1 Soil Boring SEACOR drilled four initial soil borings at the site Two of the borings were located on the subject property and two of the borings were located offsite (Figure 3) Three of the borings (B2 through B4)were drilled to total depths of approximately 35 feet below grade (approximately 14 feet below the groundwater surface) for the purpose of installing 2-mch diameter groundwater monitor wells The remaining boring(B5) was drilled to approximately 25 feet below ground surface to install a 4- inch diameter vapor extraction well The proposed locations of these soil borings/monitor wells were selected to represent groundwater in the assumed upgradient, downgradient, cross-gradient directions (Figure 3) Drilling was performed by Geo-Environmental Drilling (Lic C57 9676923) of Yuba City, California using a CME55 drilling ng equipped with 8-inch hollow-stem augers on December 27 and 28, 1993 The augers were steam cleaned prior to drilling each boring to prevent downhole or cross-boring contamination Flowing sands were encountered in boring B2/MW2 at approximately 20 feet below ground surface where the shallow groundwater-bearing zone existed Directly beneath this 1- to 2- foot thick groundwater-bearing zone, a medium gray silty clay was encountered, which apparently has provided an effective aquiclude against the downward migration of contaminants SEACOR 014-01-1 par 5