Estimation of Generic Subslab Attenuation Factors for Vapor Intrusion Investigations

Abstract

Generic indoor air:subslab soil gas attenuation factors (SSAFs) are important for rapid screening of potential vapor intrusion risks in buildings that overlie soil and groundwater contaminated with volatile chemicals. Insufficiently conservative SSAFs can allow high‐risk sites to be prematurely excluded from further investigation. Excessively conservative SSAFs can lead to costly, time‐consuming, and often inconclusive actions at an inordinate number of low‐risk sites. This paper reviews two of the most commonly used approaches to develop SSAFs: (1) comparison of paired, indoor air and subslab soil gas data in empirical databases and (2) comparison of estimated subslab vapor entry rates and indoor air exchange rates (IAERs). Potential error associated with databases includes interference from indoor and outdoor sources, reliance on data from basements, and seasonal variability. Heterogeneity in subsurface vapor plumes combined with uncertainty regarding vapor entry points calls into question the representativeness of limited subslab data and diminishes the technical defensibility of SSAFs extracted from databases. The use of reasonably conservative vapor entry rates and IAERs offers a more technically defensible approach for the development of generic SSAF values for screening. Consideration of seasonal variability in building leakage rates, air exchange rates, and interpolated vapor entry rates allows for the development of generic SSAFs at both local and regional scales. Limitations include applicability of the default IAERs and vapor entry rates to site‐specific vapor intrusion investigations and uncertainty regarding applicability of generic SSAFs to assess potential short‐term (e.g., intraday) variability of impacts to indoor air.

Improving Risk‐Based Screening at Vapor Intrusion Sites in California

“…USEPA's default AF has been the subject of much debate (Yao et al 2013(Yao et al , 2018Brewer et al 2014;Ettinger et al 2018). The argument stems, in part, from the fact that the AF was largely based on vapor data collected from single-family residences with basement foundations (16% unfinished), several of which were located in states, such as Colorado and New York, where VI can be enhanced by stack effects from heating of indoor air during winter months.…”

“…To date, AFs and associated SSSLs have been based on either model estimates (Johnson and Ettinger 1991; Brewer et al 2014) or empirical studies of subsurface and indoor air vapor data (USEPA 2012). The USEPA (2012) study involved a statistical analysis of C IA and C SOURCE data collected at numerous, predominantly chlorinated, VI sites across the United States.…”

Vapor Intrusion Investigations and Decision-Making: A Critical Review

“…Researchers have identified a number of limitations with the AF approach used to calculate subsurface screening concentrations. First, when background VOC sources contribute to the VOC concentration in indoor air, the empirical attenuation factor can be biased high (i.e., underestimate true attenuation). − If this bias is not adequately controlled when analyzing large empirical AF data sets, then the resulting upper-bound values used for screening AFs will also be biased high . Second, the individual empirical AFs included in large data sets are typically based on a single indoor air and a single subsurface sample.…”

Sampling strategies in the assessment of long‐term exposures to toxic substances in air

“…Establishing mean exposure levels is difficult due to the large spatial and temporal variations of VOC concentrations from VI. For example, spatial variability in large buildings can lead to subsurface concentrations ranging four orders of magnitude (Brewer, Nagashima, Rigby, Schmidt, & O'Neill, ). Folkes et al () show historical VI sources with average monthly concentrations that varied by nearly a factor of nine times in 12 months.…”