May 13, 2021

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Todd McAlary Authored Article on Subslab Depressurization Versus Subslab Ventilation for Journal Groundwater Monitoring & Remediation

Todd McAlary, Ph.D., P.Eng., P.G., (Toronto) authored an article entitled "Subslab Depressurization Versus Subslab Ventilation: Insights from Recent Research" for publication in Groundwater Monitoring & Remediation on April 9, 2021.

Todd McAlary is a Senior Principal Engineer with more than 30 years of international consulting experience focused on the evaluation of contaminant fate and transport in soil and groundwater. Todd specializes in assessing and mitigating the migration of volatile organic compound (VOC) vapor from the sub-surface environment into buildings and in the assessment of human health risks associated with inhalation exposure.

Since its inception in 1981, Groundwater Monitoring & Remediation (GWMR) has been a resource for researchers and practitioners in the field. It is a quarterly journal that offers the best in application oriented, peer-reviewed papers together with insightful articles from the practitioner's perspective. Each issue features papers containing cutting-edge information on treatment technology, columns by industry experts, news briefs, and equipment news. GWMR plays a unique role in advancing the practice of the groundwater monitoring and remediation field by providing forward-thinking research with practical solutions.

The National Groundwater Association is a community of groundwater professionals working together to advance groundwater knowledge and the success of its members through education and outreach, advocacy, cooperation and information exchange, and enhancement of professional practices.


Many vapor intrusion (VI) mitigation systems involve some form of gas extraction from below the concrete floor slab of a building to create a static vacuum below the slab that meets or exceeds a value specified in a guidance document or standard. This also results in some degree of ventilation below the floor slab, which reduces vapor concentrations and achieves some level of mass removal of target chemicals. The relative contribution of vacuum and ventilation to protecting building occupants is generally not quantified. Recent research completed under ESTCP Project ER‐201322 yields some new insights using new lines of evidence. The results warrant changes in the guidance documents and standards of practice to promote and enable VI mitigation system designs that vary according to the transmissivity of the material below the floor slab and the rate of vertical leakance of indoor air across the floor slab and incorporate additional lines of evidence including the mass removal rate.

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