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' Bank of Stockton Page 11 BofS (Cuter) Preliminary Soil and Groundwater Investigation Project No 102 2 April 8, 1997 4.0 DISCUSSION The laboratory analytical results from this investigation appear to confirm the conclusion of the earlier Geological Technics investigation that the area just north of the former "Oil Supply" building is near the locus of a gasoline UST release and a much smaller diesel release As shown in Figure 3, the contaminant plume in the vadose zone forms the approximate shape of an elliptical cone At the point where the cone intersects the saturated zone (+ 35ft), it has lateral dimensions of about 60 by 90 feet From the flanks the cone rises to its apex as little as 2 feet beneath the surface (beneath the former dispenser island) At the 35 foot level the area of the cone is about 4,300 square feet A rough estimate was made of the volume of gasoline impacted soil in the vadose zone using two methods The volume of a cone is given by the equation "v = nr2h/3" where r is the radius of the base of the cone, h is the height of the cone, and v is the volume The total area of the base of the cone can be approximated by a circle with a radius of 37 feet Using 35 feet for the height results in a volume of 50,177 cubic feet An alternate method involves estimated the area of the vadose zone plume at different depths and then calculating the volume of successive slices of the plume By adding the volume of each such slice, a total estimated volume of about 52,000 cubic feet was calculated These methods of estimating the volume of vadose zone soil impacted by petroleum hydrocarbons does not take into consideration the material that may be affected in the saturated zone The areal extent of the diesel impacted soil appears to be smaller at depth than at the 10 foot level Moreover, at the level of the top of the saturated zone, diesel fuel was detected in soil and / or groundwater samples in only two of the nine bonngs while gasoline was detected in all 9 bonngs The distnbution and concentration of gasoline range hydrocarbons in comparison to the much smaller concentration and distribution of diesel range hydrocarbons leads to the strong suspicion that the diesel range hydrocarbons have impacted the groundwater only in a small area and only because the diesel was entrained by the gasoline and earned to groundwater by the much more mobile gasoline In any case, it is clear the diesel impact to soil and groundwater is a tiny fraction of the impact due to the gasoline release It is noteworthy that bonngs B-6, and B-7 are in nearly the same position as borings B-5 and B-2 from the earlier Geological Technics investigation The TPH-G levels found at the 10 foot level in B-5 and B-2 were 5,530 and 5,530 ppm respectively The comparable samples from this investigation yielded values of 5,130 and 212 ppm respectively Moreover, the sample 136-15 contained non-detectable levels of petroleum hydrocarbons even though a distinct gasoline odor was present It is not known why the TPH-G level in sample 136-10 is 2937 Veneman Ave,#13240 Geo-Phase Environmental Inc Phone (209) 569-0293 Modesto, CA 95356 Fax (209) 569-0293