SYMPOSIA PAPER Published: 24 December 2013

Dynamic Subsurface Explosive Vapor Concentrations: Observations and Implications


Conventional vapor intrusion characterization efforts can be challenging due to background indoor air constituents, preferential subsurface migration pathways, sampling access, and collection method limitations. While it has been recognized that indoor air concentrations are dynamic, until recently, it was assumed by many practitioners that subsurface concentrations did not vary widely over time. Newly developed continuous monitoring platforms have been deployed to monitor subsurface concentrations of methane, carbon dioxide, oxygen, hydrogen sulfide, total volatile organic constituents, and atmospheric pressure. These systems have been integrated with telemetry, geographical information systems, and geostatistical algorithms for automatically generating two- and three-dimensional contour images and time-stamped renderings and playback loops of sensor attributes, and multivariate analyses through a cloud-based remote project management platform. The objectives at several selected sites included continuous monitoring of vapor concentrations and related physical parameters to understand explosion risks over space and time and to then design a long-term risk-reduction strategy. High-frequency data collection, processing, and automated visualization have resulted in greater understanding of natural processes, such as dynamic contaminant vapor intrusion risk conditions potentially influenced by localized barometric pumping. For instance, contemporaneous changes in methane, oxygen, and atmospheric pressure values suggest there is interplay and that vapor intrusion risk may not be constant. As a result, conventional single event and composite assessment technologies may not be capable of determining worst-case risk scenarios in all cases, possibly leading to misrepresentation of receptor and explosion risks. While dynamic risk levels have been observed in several initial continuous-monitoring applications, questions remain regarding whether these situations represent special cases and how best to determine when continuous monitoring should be required. Results from a selected case study will be presented and implications derived.

Author Information

Kram, Mark, L.
Groundswell Technologies, Inc., Santa Barbara, CA, US
Morris, Peter
Ion Science Ltd., Fowlmere, Cambridge, GB
Everett, Lorne, G.
REA II, L. Everett & Associates, LLC, Santa Barbara, CA, US
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Developed by Committee: G18
Pages: 20–31
DOI: 10.1520/STP157020130018
ISBN-EB: 978-0-8031-7586-0
ISBN-13: 978-0-8031-7585-3