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Appendix A Site Closure Request July 2,2010 Speedy Foods-Stockton Page 10 of 13 16.0 Discussion of Risk Assessment Model 16.1 Soil Screening Levels Vadose zone results collected from the upper 65 feet of confirmation borings CB 1 and CB2 indicate TPHg, benzene, and MTBE concentrations are below the 'Tref Environmental Screening Levels (ESLs) for residential land use (Table C-1) published by the San Francisco Bay RWQCB in the document Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater (revised May 2008). The chemicals of concern are not found in soil concentrations that would pose a risk to human—Fiea-ltli or the environment. • Final residential screening levels for TPHg in deep soil where groundwater is a potential source of drinking water is 83 milligrams per kilogram (mg/Kg). Samples collected in the upper 65 feet of CB 1 and CB2 (vadose zone) indicated that TPHg was below laboratory detection limits of 1 mg/Kg. • Final residential screening levels for benzene in deep soil where groundwater is a potential source of drinking water is 0.044 mg/Kg. Samples collected in the upper 65 feet of CBI and CB2(vadose zone) indicated that benzene was below laboratory detection limits of 0.005 mg/Kg. • Final residential screening levels for MTBE in deep soil where groundwater is a potential source of drinking water is 0.023 mg/Kg. Samples collected in the upper 65 feet of CB1 and CB2 (vadose zone)indicated that MTBE was below laboratory detection limits of 0.005 mg/Kg. 16.2 Groundwater Vapor Emissions to Indoor Air–TPHg After calibrating the ESL models for benzene and MTBE,Grotuid Zero ran models for TPHg using the physical-chemical and toxicity values in Table J of the San Francisco Bay RWQCB's Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater (revised May 2008). There were significant variations in the literature regarding normal boiling point, critical temperature, and enthalpy of vaporization. The upper and lower ends of the variables were run and the most conservative model indicated that a TPHg concentration 32,000 gg/L would generate a Tier I hazard quotient of 0.2. This conservative model does not include the adjustment upward of by a factor of 10 as recommended in Section 2.4.2.2 of Appendix 1 of the San Francisco Bay RWQCB's Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater (February 2005). The model assumes groundwater is 10 feet below ground surface. The groundwater table at the Site fluctuates between 56 and 76 feet bgs. TPHg concentrations are below the calculated Tier I ESL for TPHg at the Site. TPHg concentrations ranged between 13,000 µg/L and-1 5,000 µg/L during the last two semi-annual groundwater monitoring events. Johnson and Ettinger models showing the variations in the assessment of TPHg vapor emissions from groundwater as well as calibration models for benzene and MTBE are included in Attachment D.