Handex Insights

How to Stop Vapor Intrusion with Sub Slab Depressurization

Written by Mike Rumen | April 19, 2016

construction workers working on road

Contamination from hazardous chemicals is common at older dry cleaning sites throughout the country. In the past, EPA regulations in regard to the use of chlorinated solvents, such as perchloroethylene (PCE) and Tetrachloroethylene (TCE) were not as stringent and the chemicals were often not disposed of properly. As a result,  they were allowed to seep through concrete and into the soil and groundwater. The EPA has recently categorized PCE as a carcinogen in humans and recognized it’s potential for ozone depletion, prompting stricter regulations.

Because air pressure in homes and other buildings is lower than that of the surrounding soil, chemical vapors have the tendency to be drawn back into the building over time and pose a significant health risk to the occupants. This means that when soil vapor becomes contaminated, it is crucial that steps are taken to remove the contamination, or, at least, prevent it from seeping into nearby buildings. One of the most common and effective methods of preventing subsurface vapors from seeping into homes and other buildings is sub slab depressurization (SSD).

How Does it Work?

SSD works by drilling through the concrete and into the soil and venting the gas to an outside area that does not threaten public health. It is widely considered to be the most practical way to mitigate vapor intrusion. When working with existing structures, active SSD involves drilling one or more holes in the concrete and removing a small amount of soil from beneath the slab to create suction. A set of pipes is then placed into the holes and connected to an exhaust fan or regenerative blower so that the vapors can be pulled to an outdoor area. When using a blower or fan, the system is considered an active SSDS.

In cases where vapor intrusion is not a major concern, a passive SSD system may be installed. Passive SSD systems do not rely on fans to draw the vapors out of the soil. Instead, the outdoor air currents create a strong enough pressure differential to draw the vapors out from underneath the building. However, these systems are not as predictable as they rely on changing outside conditions to create the negative pressure and only provide intermittent ventilation.

Operations Monitoring & Maintenance (OMM)

Once a sub slab depressurization system has been installed, an ongoing OMM program will need to be implemented to ensure the system is operating properly. An effective OMM program will consist of three main components:

  • Regular inspections
  • Diagnostic measurements
  • Indoor air (IA) sampling

During each inspection, a Vapor Intrusion System Inspection Checklist and Subsurface Depressurization Monitoring Form will need to be completed to document the results of each visit. In cases where system design does not allow for diagnostic measurements, IA sampling may be the only way to measure the system’s effectiveness.

Working With Handex

Handex provides OMM services to customers with one or more mitigation systems in place. Our experienced staff and consulting personnel are available to assist with everything from the design and construction all the way to monitoring and maintenance of sub slab depressurization systems. Contact us to learn more about how we can help you today!



Author Avatar
Written by Mike Rumen
Mike Rumen is a Senior Project Manager for Handex. He is responsible for several high profile public sector contracts. Mike has over 12 years of experience in the environmental remediation industry.

Topics: Vapor Intrusion