Use of VOC Mass Flux to Estimate Vapor Intrusion Impacts

摘要

Estimating the potential impacts of volatile organic compound (VOC) vapor intrusion into buildings often relies on the use of prescribed, generic concentration ratios between indoor air and subsurface media, commonly referred to as "attenuation factors." The use of prescribed attenuation factors, which are lumped, unitless parameters, fails to account for site-specific mass transfer processes that are ultimately responsible for realizing vapor intrusion. This paper will show how site-specific estimates of mass flux can improve vapor intrusion assessments. Because mass flux better reflects actual mass transfer processes, its use can help identify and enhance understanding of sites that pose vapor intrusion risks. Through several project examples, we will show how generic attenuation factors developed primarily based on data from residential structures can be misleading in evaluating vapor intrusion, particularly for large buildings equipped with HVAC air handling units. As an alternative approach, several direct and indirect methods to estimate mass flux will be illustrated to help support vapor intrusion evaluations. Methods to be discussed will include: ⅰ) estimating mass transfer across the vadose and shallow saturated zone supported by vertical profiling, ⅱ) estimating diffusive flux through the building floor slab based on subslab gas concentrations, and ⅲ) using HVAC system data to estimate mass flux though the building envelope via the combination of advection and diffusion. These methods can provide independent, quantitative estimates of mass flux that offer multiple lines of evidence for evaluating vapor intrusion and the potential magnitude of impacts to indoor air quality. Finally, we will also demonstrate how the use of mass flux estimates can support the design and implementation of efficient mitigation strategies.