Demonstration/Validation of More Cost-Effective Methods for Mitigating Radon and VOC Subsurface Vapor Intrusion to Indoor Air

Dr. Todd McAlary | Geosyntec Consultants, Inc.



A large number of Department of Defense (DoD) facilities have volatile organic compounds (VOCs) in the subsurface near or below occupied buildings, and mitigation is likely to be required at a large number of buildings because of conservative regulatory policies and guidance regarding subsurface vapor intrusion to indoor air (vapor intrusion). About one-third of the land area in the United States also has high levels of naturally-occurring radon. The objective of this project is to demonstrate and validate a more technically advanced process for the design and optimization of sub-slab venting (SSV) systems to reduce the capital and long-term costs of mitigating vapor intrusion for VOCs and radon.

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Technology Description

Active SSV or sub-slab depressurization (SSD) is the most common alternative for protecting against human health risks associated with vapor intrusion. The current standards of practice for design, installation, operation, and maintenance are essentially the same for VOCs and radon (e.g., ASTM E2121-12). These practices are based on technology that is decades old and depend primarily on a single performance metric (sub-slab vacuum) that is not an ideal metric because of limitations imposed by measurement sensitivity and baseline fluctuations in the cross-slab pressure differential. As a result, the current standard design results in poor energy efficiency and excessive cost. This project will demonstrate a test procedure analogous to a groundwater pumping test that provides five lines of evidence, including: (1) vacuum versus distance (manometer), (2) vacuum versus time (data logger/pressure transducer), (3) travel time versus distance in the sub-slab region (tracer test), (4) VOC vapor concentration versus volume removed (photoionization detector and selected laboratory analysis), and (5) mass flux (flow x concentration). These lines of evidence can be used to solve uniquely for sub-slab transmissivity, cross-slab leakance, and thickness of the permeable zone beneath the slab, which are the key inputs for mathematical modeling of the subslab venting system. A simple spreadsheet model will be provided and more complex numerical models will be developed for validation of the optimal mitigation system design. This process will be demonstrated at four test buildings. Each mitigation system will be installed and operated under a range of conditions for a period of 1 year to provide field data to demonstrate and validate the design process. Each sub-slab vapor pumping test also provides data regarding the subsurface distribution of vapor concentrations within a radius of about 30 to 100 feet from the monitoring point, which saves resources that would otherwise be applied to characterizing the nature and extent of vapors below a building. The cost of conventional mitigation systems will be compared, and the project will be considered successful if the new design process reduces overall costs by a factor of 4 to 10.

ER-201322 figure (July 2016)

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This test procedure can be performed quickly and efficiently for sub-slab gas flow and can be used with mathematical modeling and selective performance monitoring to achieve a more cost-effective mitigation system for vapor intrusion. The potential savings to DoD are estimated to be in the range of $100M to $1,000M. (Anticipated Project Completion - 2018)

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McAlary, T., T. Gallinatti, G. Thrupp, W. Wertz, D. Mali, and H. Dawson. 2018. Fluid Flow Model for Predicting the Intrusion Rate of Subsurface Contaminant Vapors into Buildings. Environmental Science & Technology, 52(15):8438–8445.

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Points of Contact

Principal Investigator

Dr. Todd McAlary

Geosyntec Consultants, Inc.

Phone: 416-637-8747

Fax: 647-775-1501

Program Manager

Environmental Restoration