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M ` Former Beacon Station No. 12474 March 18,2002 Stockton California Problem Assessment Report/Corrective Action Plan is Id evaluating the masses of TPHg and benzene present in soil, the surface area of the approximate lateral extent of TPHg and benzene is assumed to be a circle measuring 55 feet in diameter or 2376 square feet (ft), as shown on Figure 6. The vertical distance of the assumed volume of impacted soil is the approximate depth to groundwater measured during the past four years of quarterly monitoring or approximately 60 feet (Appendix B). Therefore, the assumed volume of impacted soil is the surface area times the depth of 2376 ft2 x 60 feet= 142,560 cubic feet (ft). To estimate the mass of TPHg and benzene, it is assumed that the average reported TPHg and benzene concentrations reported in all samples collected from the soil borings in the area considered are present throughout the impacted volume of soil. Concentrations of TPHg were reported in 24 of 46 soil samples and averaged 665 ppm (0.000665 TPHg). Concentrations of benzene were reported in 20 of 46 soil samples and averaged approximately 5.0 ppm (0.000005 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 (1ga116.34p) and 7.31 (lgal/7.31p) pounds per gallon, respectively. The following are calculations for estimated volumes of TPHg (Vgs) and benzene (Vbs) in impacted soil: Vgs = (142,560 cubic feet of soil) x (110 pco x (0.000665 TPHg) x (lgal/6.34p) = 1,645 gallons TPHg 10J12-> Iva Vbs = (142,560 cubic feet of soil) x (I 10 peo x (0.000005 benzene) x (1 gal/7.31 p) = 11 gallons of benzene 1.5 6.2 Mass of TPHg and Benzene in Groundwater - Groundwater analytical results for the second quarter 2001 monitoring event (June 200 1) are summarized in Table 1. Concentrations of TPHg and benzene were reported in groundwater 474PARCAP3-02.doc 9 Project No. 1474.23 HORIZON ENVIRONMENTAL INC.