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i Final Baseline Human Health Risk Assessment San Joaquin Human Services Agency Building IDecember 2001 • 0 Contaminant loss from leaching downward does not occur 1 • Source degradation and transformation is not considered (e g , biodegradation, ' hydrolysis, etc ) • Concentration at the soil particle surface/soil pore air space interface is zero ' • Convective vapor flow near the building foundation is uniform ' • Contaminant vapors enter the building through cracks and openings in the walls and foundation at or below grade • Convective vapor flow rates decrease with increasing contaminant source- building distance ' • All contaminant vapors directly below the building will enter the building, unless the floor and walls are perfect vapor barriers This implies that a constant pressure field is generated between the interior spaces and the soil surface and that the ' vapors are intercepted within the pressure field and transported into the building This assumption is inherently conservative in that it neglects periods of near zero pressure differential (e g , during mild weather when windows are left open) ' • • The building contains no other contaminant sources or sinks and represents a well mixed air volume It therefore neglects contaminant sinks and the room-to-room ' variation in vapor concentration due to unbalanced mechanical and/or natural ventilation ' 3.0 INDOOR AIR MODELING A risk-based analysis was conducted for the indoor air pathway using guidance that was Idownloaded from the national United States Environmental Protection Agency (U S EPA) Superf ind Risk Assessment WEB site The address for this website is as follows http //www epa gov/oerrpage/superfund/programs/risk/ IThis guidance was developed to address concerns raised about the potential for adverse impacts to indoor air quality from contamination in subsurface groundwater and soil In response to this Iconcern, EPA developed a series of risk-based spreadsheets (and a User's Guide) that allow for site-specific application of the Johnson and Ettinger Model (1991) The user's guide is titled User's Guide For The Johnson and Ettinger (1991) Model For Subsurface Vapor Intrusion Into Buildings (Revised) It was prepared by Environmental Quality Management, Inc for submittal I ISan Joaquin County GW Monitoring Report2(Appendix D)v 2 2 12/27/2001