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Final Baseline Human Health Risk Assessment <br /> San Joaquin Human services Agency Budding <br />' December 2001 <br />' • HUMAN HEALTH RISK ASSESSMENT <br /> SAN JOAQUIN COUNTY HUMAN SERVICES AGENCY BUILDING <br /> METHODOLOGY AND RESULTS FOR THE MODELING OF INDOOR AIR <br />' CONCENTRATIONS <br />' 1.0 BACKGROUND AND OBJECTIVE <br />' Benzene was selected as the only chemical of potential concern (COPC) in groundwater for the <br /> purposes of assessing potential risk from indoor air exposures The exposure scenario identified <br />' for evaluation of groundwater beneath the San Joaquin County Human Services Agency Building <br /> (i e , "the site") are identified below <br />' • Current workers inside the building and <br />' • Future workers inside the buldmg <br />' The primary objective of this air evaluation is to address the potential for exposure to benzene <br /> • through the indoor air pathway described above and to estimate any associated risks that may be <br /> Ipresent with this pathway <br /> The model used to evaluate the indoor air pathway is the Johnson and Ettinger (1991) model for <br /> subsurface vapor intrusion into buildings (Johnson and Ettinger, 1991) Discussion of the <br /> methodology used in this model is presented below All data tables associated with the human <br /> health risk assessment are located at the end of this report <br /> 2.0 JOHNSON AND ETTINGER MODEL <br />' The Johnson and Ettinger (J&E) model is a risk-based model which incorporates both convective <br /> and diffusive mechanisms for estimating the transport of contaminant vapors emanating from <br /> either subsurface soils or (in the case here) groundwater to indoor air spaces located directly <br /> above or in close proximity to the source of contamination The J&E model incorporates the <br /> following assumptions <br /> • • Soil is homogenous such that the effective diffusion coefficient is constant <br /> ISan Joaquin county GW Monitoring Report2(Appendix D)v 2 1 12/27/2001 <br />