Page 19 of the PAERAB's Spring 2011 Community Update Newsletter

Text Box:

Text Box: soil gas from the space immediately under a slab or basement. It is distinct from shallow (e.g., 5 feet below ground surface [bgs]) soil gas samples that are collected outside and either next to (near slab) or some distance (exterior) from a slab. Subslab soil gas concentrations, by themselves, do not necessarily indicate the extent to which vapor intrusion is occurring or, if it is occurring, whether vapor intrusion represents a health risk. However, if subslab soil gas concentrations are more than 1,000–10,000 times the target indoor air levels, the probability of unacceptable vapor intrusion is likely sufficient to warrant proactive mitigation without further investigation”. On Page 35… “Measuring indoor air is the most direct approach, but may not be the simplest. It is intrusive, and background contaminant sources in the building must be considered. Many commonly used household products contain some of the same target COCs (e.g., tetrachloroethene [PCE] from dry-cleaned clothes, trichloroethane [TCA] from degreasing cleaners). In some areas, especially urban centers, outdoor ambient concentrations (e.g., benzene) may exceed allowable indoor risk based levels, further complicating this approach. When doing indoor air sampling, a household inventory should be performed to find sources of indoor air contaminants. In

Text Box: addition the occupants should be instructed on appropriate actions that should be taken prior to and during the sampling event. Indoor air sampling may still be the method of choice if the COC is not one commonly found in household products or ambient air (e.g., 1,1-DCE, carbon tetrachloride). Agencies may find that residents prefer indoor air sampling over other investigative methods, and the data can be directly compared to screening values rather than having to be predicted or modeled”. “There are several approaches to investigating crawl spaces that involve collecting either air or soil gas samples using both active and passive methods. Typically, crawl-space air samples are collected following protocols similar to those for indoor air samples for a period of up to 24 hours. Ambient air can confound analytical results because of the uncertainty of outdoor air contribution to the crawl space. Soil gas collection can also be performed in the crawl space by inserting the probe rod or sampling tube horizontally through the crawl-space access ports or vertically through the overlying structure. Additionally, flux chambers can be used to determine the surface flux of the contaminant that may be moving into the crawl space. There are little data available to assess the attenuation from crawl space to indoor air”. “When indoor air sampling is performed as part of a vapor intrusion study, coincident outdoor ambient air samples should also be collected. Ambient air samples are important to characterize site-specific outdoor air contaminants. Depending on building air exchange rates,

Text Box: contaminants from outdoor ambient air may make up a large percentage of indoor air contaminants. Ambient air typically contains numerous VOCs, such as benzene, PCE, and TCE, that often exceed indoor air risk-based screening levels. Some compounds, however, that are not typically found in ambient air (e.g., vinyl chloride or 1,1-DCE) may be a concern for vapor intrusion studies. For residential structures, outdoor air samples should be collected from a representative upwind location, away from wind obstructions such as trees and buildings. The typical intake would be at about 3–5 feet off the ground (at the approximate midpoint of the ground level of the building) and about 5–15 feet away from the building. For commercial structures, outdoor air samples should be collected in representative locations for the intakes of the building heating, ventilating, and air conditioning (HVAC) systems”. On Page 37… “...buildings with a specific design feature may be more susceptible to vapor intrusion and warrant closer attention or proactive mitigation. The building does not necessarily need to be located over the most highly contaminated area”. “USEPA’s draft VI guidance (2002b) suggests selecting the worst-case building(s)—those most likely to be impacted by vapor intrusion—for the initial

ITRC Vapor intrusion intro - continued

CONTINUED ON P. 20