Page 14 of the PAERAB's Spring 2011 Community Update Newsletter

Text Box: off-gassing from dry cleaning, exhaust from attached garages). Even a simple activity like a hot shower can strip volatiles from the water, producing a spike in the concentrations of trihalomethanes. It is appropriate to conduct a building walkthrough in advance of any interior sampling events to identify potential background sources, eliminate them, and educate the occupants on those activities that should be avoided immediately before and during the sample collection.” “Many hydrocarbons (notably petroleum-based hydrocarbons) are readily degraded to carbon dioxide (CO2) in the presence of oxygen (O2) by ubiquitous soil microbes. Oxygen is supplied from the atmosphere by vapor diffusion and barometric pumping and as a dissolved solute in infiltrating rainwater. Aerobic degradation is a rapid process and frequently occurs in a relatively thin (a few feet thick) zone where the concentrations of O2 and hydrocarbons are most ideal for microbial processes. The bioattenuation of hydrocarbons can potentially reduce soil gas concentrations and vapor intrusion by several orders of magnitude. Like petroleum hydrocarbons, chlorinated solvents also can be biodegraded, but the process tends to occur under anaerobic conditions and is slower than aerobic degradation of petroleum hydrocarbons like benzene, toluene, ethylbenzene, and xylenes (BTEX). In both cases,

$Text Box: degradation occurs via oxidation-reduction reactions that are used by the microbes as an energy source.” On Page 18… “For vapor intrusion studies, the importance of biodegradation of chlorinated solvents is that additional compounds of interest are created, with obvious implications for selecting target compounds. These daughter compounds may be considered worse than the parent compound because of increased carcinogenicity.” ‘… permeability of subsurface materials can be highly variable. Conditions such as fractured geologic media and gravel lenses or channels may allow an atypical preferential soil gas flow through high-permeability pathways (in some cases opposite to the groundwater flow). If such a migration route connects a source directly to a building or allows higher levels of groundwater contamination to migrate under a building, vapor intrusion may be exacerbated. Most buildings have subsurface utility penetrations, so their presence alone is not considered “preferential.” For this guidance (consistent with the vapor intrusion pathway in general and the Johnson and Ettinger [J&E] model specifically), some increased component of soil gas flow into the building is usually required to consider the pathway to be preferential. Anthropogenic preferential pathways include building sumps or drainage pits (that can serve as conduits for soil gas to enter buildings) or subsurface$ $Text Box: utility conduits or drains (that intersect vapor sources or soil gas migration routes and a building foundation). Natural preferential pathways include vertically fractured bedrock where the fractures are interconnected and in direct contact with the building foundation and the vapor contaminant source. Interestingly, the Georgia Environmental Protection Division had a case where rodent tracks or tunnels up to a building foundation allowed vapors to migrate into a basement. In addition, investigators must consider that preferential pathways may not be apparent based on external building inspections. According to Henry Schuver (USEPA), some communities with unexpectedly high concentrations of indoor contaminants were observed during indoor surveys to have uncapped pipes through the basement floor connected to “dry wells” designed to dispose of fluids through openings surrounded by very permeable fill materials. Thus, they served as preferential pathways by allowing an unexpected amount of vapor to migrate into the structures.’ “Another example is waste lines without functioning vapor traps. Petroleum compounds often biodegrade before causing vapor intrusion problems. However, one community had elevated petroleum-related concentrations in indoor air traced back to homes connected to sewer lines without vapor traps. These untrapped sewer lines formed$

ITRC Vapor intrusion intro - continued

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