Mass Flux Characterization for Vapor Intrusion Assessment

Dr. Helen Dawson | Geosyntec Consultants, Inc.

ER-201503

Objective

The objective of this project is to improve the Department of Defense’s (DoD’s) environmental risk management capabilities by reducing the uncertainty associated with spatial and temporal variability in the concentrations of volatile organic compounds (VOCs) at vapor intrusion sites. That uncertainty causes regulatory agencies to make conservative risk management decisions, which in turn forces DoD to incur additional, often unnecessary, costs for mitigation and additional data collection. The technology to be demonstrated relies on measuring vapor phase mass flux instead of concentration because mass flux is less susceptible to variability. This approach also minimizes the complexity associated with interpreting the relative contribution of vapor sources inside the building (background sources), which is an issue for current vapor intrusion assessment methods. The mass flux monitoring provides an improved estimate of vapor intrusion potential, and the results can be used to provide more representative estimates of the indoor air VOC concentrations associated with vapor intrusion.

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Technology Description

Three independent techniques for measuring flux from the subsurface into a building will be tested. The first method is based on measuring vertical soil concentration gradients and soil moisture and calculating the mass flux using a form of Fick’s Law. The second method is based on measuring the building air exchange rate and indoor air concentrations under conditions of forced depressurization. The third method is based on capturing the contaminant vapor from below the building using a sub-slab venting system and measuring the air flow rate and corresponding vapor concentrations. The underlying assumptions are that upward diffusion from a source below the building is the rate limiting process associated with vapor intrusion, that the sub-slab venting system captures the available mass flux, and that in the absence of sub-slab venting the available mass flux enters the building. Theoretically all three methods should provide equivalent results. Differences among the three can be used to determine the state of equilibrium or whether preferential pathways are present. All three methods employ controls that aim to minimize temporal variability. The second and third methods also minimize spatial variability by integrating the vapor flux over the building footprint and volume. Testing will be performed at several buildings of different sizes, including two commercial buildings and a dozen residential buildings, in three to four seasons.

ER-201503 Flux Graphic

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Benefits

This project is expected to lead to the development of a more effective, more efficient, and less expensive process for assessment, monitoring, and management of vapor intrusion related risks, including providing a metric for termination of mitigation. The DoD has a large number of buildings near subsurface VOCs that will likely require assessment. As a ballpark estimate, a savings of $50,000 per building for 500 buildings would add up to $25 million. (Anticipated Project Completion - 2019)

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Points of Contact

Principal Investigator

Dr. Helen Dawson

Geosyntec Consultants, Inc.

Phone: 703-533-3148

Program Manager

Environmental Restoration

SERDP and ESTCP

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