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Former Beacon Station No 12419 March 18,2002 Stockton California Problem Assessment Re ortlCorrective Action Plan . To estimate the mass of TPHg and benzene, it is assumed that the average reported TPHg and benzene concentrations reported in soil samples collected from the soil borings in the area considered (AS-1 and AS-2) are present throughout the impacted volume of soil Soli samples collected below 90 feet from the air sparge well AS-2 boring are not considered in the TPHg and benzene mass calculations because they are deeper than the estimated depth to groundwater level when initial soil impacts occurred as discussed in Section 71 Concentrations of TPHg ranging from 2 1 to 8500 ppm were reported in 10 of 26 soil samples and averaged 541 ppm (0 000541 TPHg) Concentrations of benzene ranging from 0 0056 to 48 ppm were reported in 15 of 26 soil samples and averaged approximately 3 42 ppm (0 00000342 benzene) The soil impacted by TPHg and benzene is interbedded sand, silt and clay and can be assumed to have an average approximate density of 110 pounds per cubic foot (pcf) The density of gasoline and benzene are 6 34 (1 gal/6 34p) and 7 31 (1 gal/7 31 p) pounds per gallon, respectively The following are calculations for estimated volumes of TPHg (Vgs) and benzene (Vbs) in impacted soil Vgs = (53,014 cubic feet of soil) x (I 10 pc f) x (0 000541 TPHg) x (I gal/6 34p) = 498 gallons TPHg, and Vbs = (53,014 cubic feet of soil) x (I 10 pcf) x (0 00000342 benzene) x (I gal/7 31 p) 2 73 gallons of benzene 8.2 Mass of TPHg and Benzene in Groundwater Groundwater analytical results for the latest quarterly monitoring event (November 2001) are summarized in Table 1 The distributions of TPHg and benzene concentrations in groundwater are shown on Figures 7 and 8 The upper portion of the saturated zone generally consists of silty sand and silt with minor clay fractions that collectively is assumed to have an average porosity of 30% (0 30) 419PARCAP3-02 doe 15 Project No 1419 23 HORIZON ENVIRONMENTAL INC.