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`..e `,.l 02 May 2013 AGE-NC Project No. 04-1168 Page 6 of 10 Figures 4 and 5, the greatest adsorbed hydrocarbon concentrations were encountered within the former tank pit area near borings B1 and MW-1. Utilizing B1 as a mid-point, the adsorbed hydrocarbon plume extends toward the north approximately 13 feet, toward the east approximately 8 feet, toward the south approximately 3 feet and toward the west approximately 7 feet (Figure 5). Vertically,the adsorbed hydrocarbon plume appears to extend to a depth of approximately 45 feet bsg. Based on the investigation results, the adsorbed TPH-g plume is defined vertically by samples collected from MW-1 at 60 feet and MW-3 at 70 feet bsg (Figure 4). Laterally, the adsorbed hydrocarbon plume is defined to the east by B3 to the northwest by MW-3 and to to west by MW-2. The plume is marginally undefined south of B1 but based on spacial non-detections surrounding the impacted area, it would appear that the adsorbed plume terminates in a location at a short lateral distance from B1 (Figure 5). /4.2. MASS OF REMAINING ADSORBED PETROLEUM HYDROCARBONS Utilizing Figures 4 and 5 to model the adsorbed TPH-g impact, the mass of TPH-g in soil was determined by separating the soil plume into two contoured zones(i.e. 100 mg/kg and 1 mg/kg). Based on mass-volume estimates,AGE calculated an approximate 26.9 pounds (lbs), or 4.3 gallons of TPH-g impact soil at the site. The assumptions for the calculations of TPH-g adsorbed in soil are presented in Appendix E. 3. EVALUATION OF ENVIRONMENTAL SCREENING LEVELS AGE compared confirmation soil analytical results (assumed mass remaining)to the San Francisco Bay Regional Water Quality Control Board's (SFBRWQCB) Environmental Screening Levels (ESLs) for gross contamination and direct exposure. Utilizing the SFBRWQCB ESLs to evaluate deeper impacted soils (i.e. greater than 10 feet bsg), both commercial land and residential land usage ESLs were exceeded for TPH-g (131-25) and EDB (132-35): AGE believes the above referenced soil contamination left in place will not pose a significant threat to human health or to the environment based on the following: • Soil samples collected during the 2004 and 2005 investigation have likely further attenuated through natural process since samples were collected and will continue to attenuate over time. Advanced GeoEnvironmental,Inc.