Risk Assessment Tool for Chlorinated Vapor Intrusion Based on a Two-Dimensional Analytical Model Involving Vertical Heterogeneity

At contaminated sites, it is common to observe volatile organic compounds rising from subsurface sources and migrating into overlying buildings through cracks or other openings present in foundation slabs and basement walls. Such process, called vapor intrusion, is usually the most critical exposure pathway at sites contaminated by chlorinated solvents. In this study we present a chlorinated vapor intrusion tool implemented in Microsoft (R) Exce (R) using Visual Basic for Applications and integrated within a graphical interface that helps users to visualize two-dimensional (2D) soil gas concentration profiles and indoor air concentration in scenarios involving subsurface vertical heterogeneity. Vertical heterogeneity in soil gas concentration can be induced by a variable soil moisture profile caused by layering (geological barriers) or by the presence of the capillary fringe. This tool can be used in the risk assessment procedure to assess expected indoor air concentrations in conjunction with other lines of evidence, such as the evaluation of the 2D soil gas concentration profiles below and beyond the building footprint. Moreover, this tool allows users to predict indoor air concentration by employing either the traditional perimeter crack entry pathway or the empirical subslab-to-indoor attenuation factor. After a brief description of the developed tool, we show some practical applications to highlight the potential benefits in using this tool compared with the U.S. Environmental Protection Agency (U.S. EPA) tool that implements a simplified form of the Johnson and Ettinger model. Based on our testing, we found that the two-layer approach (capillary fringe and vadose zone) employed in the U.S. EPA tool, can lead to an overestimation of subslab vapor concentrations by more than an order of magnitude.