Field data and numerical modeling: A multiple lines of evidence approach for assessing vapor intrusion exposure risks

Abstract

USEPA recommends a multiple lines of evidence approach to make informed decisions at vapor intrusion sites because the vapor intrusion pathway is notoriously difficult to characterize. Our study uses this approach by incorporating groundwater, soil gas, indoor air field measurements and numerical models to evaluate vapor intrusion exposure risks in a Metro-Boston neighborhood known to exhibit lower than anticipated indoor air concentrations based on groundwater concentrations. We collected and evaluated five rounds of field sampling data over the period of one year. Field data results show a steep gradient in soil gas concentrations near the groundwater surface; however as the depth decreases, soil gas concentration gradients also decrease. Together, the field data and the numerical model results suggest that a subsurface feature is limiting vapor transport into indoor air spaces at the study site and that groundwater concentrations are not appropriate indicators of vapor intrusion exposure risks in this neighborhood. This research also reveals the importance of including relevant physical models when evaluating vapor intrusion exposure risks using the multiple lines of evidence approach. Overall, the findings provide insight about how the multiple lines of evidence approach can be used to inform decisions by using field data collected using regulatory-relevant sampling techniques, and a well-established 3-D vapor intrusion model.

US residential building air exchange rates: new perspectives to improve decision making at vapor intrusion sites

“…In fact, this is an active area of research for the authors and others. Therefore, to gain the mass entry rate, the research herein used a VI modeling approach that has been widely published 28 and has been compared to field data 4 .…”

“…4 . Recently, the importance of building operation has gained recognition within the VI community 5–15 .…”

Three-dimensional vapor intrusion modeling approach that combines wind and stack effects on indoor, atmospheric, and subsurface domains

“…13 One of the first 3-D VI models incorporated a finite difference numerical code in which a continuity equation was coupled with a chemical transport equation to calculate the soil gas pressure, velocity and chemical concentration in soil and indoor air. 2, 5, 7 Later, Pennell et al 3 developed a similar 3-D VI model, but incorporated a commercially-available finite element numerical code.…”

Characterizing Dnapl Contamination and Vapor Intrusion in Dayton, Ohio

“…Characterization of VI contamination is often challenging at times. Therefore, the US EPA has recommended using a multiple lines of evidence approach of analyzing field data, modeling, and geological information of a site (Pennell et al, 2016). Figure 3 shows an illustration of some pathways VI might follow into residential homes.…”