June 30, 2020

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Todd McAlary Published an Article on Vapor Intrusion Mitigation Systems in the Journal Science of the Total Environment

Todd McAlary, Ph.D., P.Eng., P.G. (Ontario) authored a paper entitled "Mathematical analysis and flux-based radius of influence for radon/VOC vapor intrusion mitigation systems" in the journal Science of the Total Environment.

His coauthors were William Wertz, Darius Mali, and Paul Nicholson, also of Geosyntec.

Todd McAlary is a Practice Leader for Vapor Intrusion Services at Geosyntec with more than 30 years of international consulting experience focused on the evaluation of contaminant fate and transport in soil and groundwater.

The article provides a new equation to assess the radius of influence (ROI) of SSD systems based on a mass flux balance. Mathematical analysis of 121 pneumatic test data sets is included to demonstrate statistically the use of a 2-layer spreadsheet model that provides multiple lines of evidence for assessing the ROI, including static vacuum, velocity, travel time, flux and proportion of flow across the floor slab as a function of radial distance from a suction point. This constitutes a major step forward for diagnostic testing for vapor intrusion and radon mitigation systems and will both increase the confidence and reduce the cost of mitigation systems compared to conventional design methods. The spreadsheet model will be posted for free download.

Science of the Total Environment is an international multi-disciplinary journal for publication of novel, hypothesis-driven and high-impact research on the total environment, which interfaces the atmosphere, lithosphere, hydrosphere, biosphere, and anthroposphere.


Volatile organic compounds (VOCs) and radon progeny pose potential health risks to occupants of certain buildings via subsurface vapor intrusion (VI) to indoor air. VI mitigation is usually performed using systems that extract gas from below the building, and the system performance is typically evaluated by measuring the distribution of applied vacuum below the floor. This article provides a new approach to assessing the radius of influence (ROI) for subslab venting systems based on mass flux instead of static vacuum distribution and includes an analysis of 121 pneumatic tests performed at 65 different suction points in 16 different buildings. The mathematical model represents a two-layer system with horizontal radial flow through transmissive material below the floor slab and vertical flow through discontinuities in the floor slab (which is simplified to approximate an equivalent porous medium). The analysis includes comparisons of the flux-based ROI to values calculated using the two-layer model for 1) vacuum, 2) velocity, and 3) travel time, which may be useful as alternative performance metrics for mitigation systems.

More Information

Learn more about the article: https://authors.elsevier.com/a/1bGNBB8ccoGhl
Spreadsheet model available: on the ESTCP website
Learn more about the journal: https://www.journals.elsevier.com/science-of-the-total-environment
For consultation regarding Vapor Intrusion Mitigation Systems, contact Todd McAlary at This email address is being protected from spambots. You need JavaScript enabled to view it..
Learn more about Todd McAlary at: https://www.geosyntec.com/people/todd-mcalary
Learn more about William Wertz at: https://www.linkedin.com/in/william-wertz-3b1b1b57/
Learn more about Darius Mali at: https://www.linkedin.com/in/darius-mali-p-eng-c-nrpp-b6657267/
Learn more about Paul Nicholson at: https://www.linkedin.com/in/paul-nicholson-/