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dG- March 15, 1988 AGS 8696-3V Crystal Cream and Butter Company, Stockton, California n At the point of sample collection, the sampling assembly was lowered through the hollow center of the probe rod to collect the vapor sample without removing the rod from the sample depth. r� Contamination by atmospheric gas was thus eliminated, thereby giving a reliable and reproducible reading of the concentration of the hydrocarbon vapor in the soil specifically at the sample depth. Also, because each sample vial was the same size (i.e. , constant volume of gas) and was evacuated to the same degree (measured in inches of mercury) , the number of variables associated with vapor sampling was reduced. Reducing variation in the vapor sampling procedure provides a measure of consistency between gas samples from different depths and locations that cannct be achieved by other methods. Where additional samples were desired at greater depths, the probe was closed and pushed or driven to the deeper sampling point. Selection of Sample Locations and Degths Initial sample placement was based on the location of a previously buried gasoline storage tank situated beneath the concrete pad between the Crystal Cream and Butter Company ',.,...} warehouse and the American Protective Services Building. 00 Additional sample points were spaced out radially from the initial points. Soil probe locations are shown on the Generalized Plan, Plate P--2. Maximum sample depths were based primarily on the depth to ground saturation, since little or no vapor transport occurs below this depth. The saturated zone varies in thickness depending on soil type but is generally ani assumed to extend upwardly from the water-table a few feet (this depth is more accuratly determined after probing is initiated) . To remain above the inferred depth of the saturated zone, the deepest sampling horizon was established at a depth of 12 feet. Samples were also retrieved from a depth horizon of 7 feet to identify more shallow contaminant sources, such as product lines. At the completion of probing, probe holes were backfilled with bentonite pellets to within a few inches of surface grade and then topped off with asphalt or concrete. A Wild NA-24 auto level was used to survey probe hole locations relative to other facilities at the site. Description .of Chromatograph and Method of Analysis A 0.1.0-milliliter vapor sample was withdrawn from the sample vial using a 0.10-milliliter syringe and injected into a Photovac 10S70 portable gas chromatograph for analysis. The instrument is 3 `- Ajooiii?d Ge®SYSMIW