This study was motivated by reports from a number of vapor intrusion (VI)-impacted sites that the measured indoor concentrations of chemicals of concern could vary by orders of magnitude. The sites in question included two that were impacted by operations at military bases (Hill Air Force Base in Layton, UT and Naval Air Station North Island near San Diego, CA) and one studied by the U.S. Environmental Protection Agency in Indianapolis, IN, where the impacts came from nearby former commercial establishments. The objective of this study was to apply numerical modeling to develop an improved understanding of the VI processes at these sites. The goal was also to demonstrate that such modeling tools offer insights that are difficult to develop without such formal analysis, establishing the value of mathematical modeling as a “line of evidence” in analyzing VI scenarios. A further objective of this work was to demonstrate that the tools needed for such numerical analysis are now at a level of development that no particular expertise in numerical analysis is required in order to make effective use of the tools.
The present work has emphasized that use of “ordinary” commercially available finite element solver packages, implemented on desktop computers, can provide powerful insights into the complicated processes involved in VI. The choice was made to use a user-friendly and widely available platform, COMSOL. The goal was to show in a step-by-step way how these tools can be brought to bear in addressing vapor intrusion problems.
The model has been successfully implemented on several different computing platforms. Both the hardware and software costs are modest (a few thousand dollars), requiring only ordinary desktop machines with a bit of enhanced memory and finite element software.
This report describes earlier VI modeling efforts and then describes how a finite element-based numerical model can be set up. The model is used to discuss what has been learned about the transient processes in VI at the sites that motivated this work. Key conclusions are:
- Orders of magnitude variations in indoor air contaminant concentrations are only possible when there is a confluence of particular site conditions, but variations of one order of magnitude are to be expected given natural variations in air exchange rates and indoor depressurization.
- The contaminant “entry pathway” determines whether there is significant sensitivity to indoor air pressure changes; when the resistance to sub-slab airflow is significant, the variation of indoor air pressure has a small effect on VI.
A reliable, easily implementable model of a site of VI concern will help guide field investigations. Currently, many investigations are conservative, and by their nature, intrusive to potentially impacted residents. Any tool that will allow more targeted investigation of a more limited scope would be beneficial. In addition, at the present time, regulators do not have a good sense of how large the variations in contaminant vapor exposure concentrations might be, and cannot develop health-protective guidelines based upon predictions of such variation. This model would provide such information. (Project Completion - 2020)
McAlary, T., T. McHugh, B. Eklund, C. Lutes, E. Suuberg, H. Hayes, K.G. Pennell, D. Folkes, H. Dawson, R. Truesdale, L. Beckley, and C. Holto. 2016. Comments and Corrections to: “The Emperor’s Old Clothes: An Inconvenient Truth About Currently Accepted Vapor Intrusion Assessment Methods,” and “Emperor’s Old Clothes Revisited,” Two Recent Editorials by Mark Kram, Groundwater Monitoring and Remediation, 36(3):84-87. doi.org/10.1111/gwmr.12166
Ström, J G.V., Y. Guo, Y. Yao, and E.M. Suuberg. 2019. Factors Affecting Temporal Variations in Vapor Intrusion-induced Indoor Air Contaminant Concentrations. Building and Environment, 161:106196. doi.org/10.1016/j.buildenv.2019.106196
Yao, Y., J. Zuo, J. Luo, Q. Chen, J. Ström, and E.M. Suuberg. 2020. An Examination of the Building Pressure Cycling Technique as a Tool in Vapor Intrusion Investigations with Analytical Simulations. Journal of Hazardous Materials, 389:121195. doi.org/10.1016/j.jhazmat.2019.121915
Yao, Y., Y. Xiao, J. Luo G. Wang J. Ström, and E.M. Suuberg. 2020. High-frequency Fluctuations of Indoor Pressure: A Potential Driving Force for Vapor Intrusion in Urban Areas. Science of the Total Environment, 710:136309. doi.org/10.1016/j.scitotenv.2019.136309
Ström, J.G.V. 2020. Understanding the Dynamics of Vapor Intrusion Processes (Ph.D. Thesis). Brown University, School of Engineering.