Page 12 of the PAERAB's Spring 2011 Community Update Newsletter

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Text Box: wasn’t until investigative work of the Massachusetts Department of Environmental Protection (MassDEP) in the 1990s that regulators began to understand the significance of vapor intrusion. State and federal regulators are now in the process of examining older remedial decisions involving groundwater contamination to assess whether the vapor intrusion pathway warrants reopening closed cases.” On Page 12... “Chemicals volatilize from impacted soil and/or groundwater beneath a building and diffuse toward regions of lower chemical concentration (e.g., the atmosphere, conduits, basements). Soil gas can flow into a building due to a number of factors, including barometric pressure changes, wind load, thermal currents, or depressurization from building exhaust fans. The rate of movement of the vapors into the building is a difficult value to quantify and depends on soil type, chemical properties, building design and condition, and the pressure differential. Upon entry into a structure, soil gas mixes with the existing air through the natural or mechanical ventilation of the building.” On Page 13... ‘Both diffusion and advection are mechanisms of transport of subsurface soil gas into the indoor air environment. Diffusion is the mechanism by which soil gas moves from

Text Box: high concentration to low concentration due to a concentration gradient. Advection is the transport mechanism by which soil gas moves due to differences in pressure. These pressure differences can be generated by atmospheric pressure changes, temperature changes creating natural convection in the soil, or forced pressure changes due to building ventilation systems. Advective transport is likely to be the most significant in the region very close to a basement or a foundation, and soil gas velocities decrease rapidly with increasing distance from the structure (USEPA 2004b). Once soil gases enter the “building zone of influence,” they are generally swept into the building through foundation cracks by advection due to the indoor-outdoor building pressure differential. The reach of the “building zone of influence” on soil gas flow is usually less than a few feet, vertically and horizontally.’ “To define the vapor intrusion pathway as a complete exposure pathway, a source, migration route, and receptor must be identified…” On Page 14... ‘Building depressurization may cause soil gas from soil and/or groundwater contamination to be drawn into buildings through holes and cracks in the foundation. Heating systems, basements, and strong winds promote vapor intrusion into buildings by reducing the internal air pressure and creating a vacuum effect that enhances advective flow from underlying soils and/or groundwater into buildings (“the stack effect”).’

Text Box: On Page 15... ‘Once the pathway is identified, a “phased” approach is recommended to evaluate whether exposures present significant risk to the receptors, as defined by the regulatory agencies. Where there are obvious signs of vapor intrusion, such as odors or explosive hazards, these potential acute risks should be addressed immediately. For probable chronic exposure, the site may be evaluated on either a generic or site-specific basis in a timely manner.’ “…a general framework for evaluating vapor intrusion from subsurface media contamination … builds on the premise of conservative but flexible guidance, within the confines of the regulatory agency. Ultimately, the intent is to be able to differentiate sites where the pathway presents or does not present an unacceptable risk, while erring on the side of caution when there is uncertainty. Some key principles that guide this framework are summarized below…: implementation of a community outreach program that provides timely information to concerned citizens and property owners use of a phased approach that allows for the collection and use of both generic and sitespecific information/data development of an accurate conceptual site model (CSM) that is representative of site

ITRC Vapor intrusion intro — continued

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