Before redeveloping industrial and commercial properties, it is important to consider the legal, financial, and environmental risks
associated with vapor intrusion. The migration of contaminants, whether natural (e.g. radon) or associated with contamination (e.g., gasoline or solvents), into indoor air is called vapor intrusion and it can create significant potential legal risk for developers and property owners. With increased attention to indoor air quality, changes to regulatory standards for “acceptable” indoor air, and the expanding list of contaminants of concern, redevelopment projects on former industrial and commercial properties should consider volatile contaminants and their potential negative impact on indoor air quality.
Radon mitigation systems have been around a long time. Naturally occurring radon in the soil beneath a building, accumulates in the air between the soil particles, also known as soil gas. The presence of a warm building over relatively cool soil, exacerbated in the winter, leads to a negative pressure that draws air from beneath the building into the living space. If radon is in the air, the radon migrates into the living space.
When elevated radon is detected in a living space the solution generally involves coring a hole in the floor, installing a pipe and applying a vacuum to exhaust the contaminant to an exterior location above the roofline. In practice the design and installation of radon systems is more sophisticated, but by creating a negative pressure beneath the floor the radon-air can be diverted from the living space, eliminating the risk.
The same principles apply to mitigating vapor intrusion from known or potential industrial contaminants associated with releases of oil or hazardous materials. Sub-slab depressurization systems (SSDS) are an essential mitigation tool to assure that volatile organic compounds (VOCs) present in contaminated soil or groundwater do not migrate with soil gas into overlying buildings.
SSDS design is typically a multi-step process including the delineation of the environmentally impacted area, evaluation of the soil properties to determine how easily air flows through the soil, and determining if there are any sub-slab structures that could alter air flow. Subsurface structures, such as footings and foundation walls, inhibit air flow. From this information an SSDS can be designed and installed beneath the floor to divert unhealthy subsurface air to the exterior of the structure
For several years, installation of systems to control soil gas has been an element of new-construction projects consistent with standards developed and updated by ANSI/AARST (American National Standards Institute / American Association of Radon Scientist) in 2018. The integration of SSDS, also called Soil Gas Control Systems (SGCS) into redevelopment projects is becoming the norm.
The concepts of SGCS and SSDS are similar and revolve around what the standard refers to as the soil gas collection plenum. Simply, the plenum is a three-dimensional enclosure for collecting radon and other soil gasses from under slabs. Think of the plenum sides and top as being a building’s foundation walls and concrete floor. Different sections of a building might represent different plenums just as there are different rooms in a house. Typically, a polyethylene vapor barrier or liner is placed beneath the floor to act as a barrier. Beneath that is a gas permeable layer that might include porous stone and a network of pipes to collect soil gas which sits on top of the natural soil or fill material.
In new construction, understanding the plenums and designing a SGCS/SSDS is relatively straight forward because one knows the location of all of the subsurface structures that might affect soil gas migration. The ANSI/AARST standard includes guidelines describing how to construct the liner system, what stone to use and what the piping layout should look like depending on the size and dimensions of the plenum. But redevelopment projects pose a challenge because of the unknowns beneath a concrete slab. Structural footings or remnant foundations may represent barriers to air flow (i.e., plenum walls) whereas buried conduits might enhance air flow. Utility penetrations in the floor (e.g., drains) can represent preferential pathways to vapor migration.
Construction of SGCS/SSDS during redevelopment projects requires a hybrid approach. The first step is most often a pilot test to determine subsurface conditions and identify the limits of the various plenums. It is not uncommon for a large open floor space to consist of multiple plenums because a building was constructed in phases. Once the plenums are defined the SGCS/SSDS can be designed.
A design for one area might vary significantly from that of another depending upon existing conditions beneath the building and proposed use of the space. Some buildings may be amenable to a network of horizontal pipes that all connect at one collection point. However, cutting concrete can be expensive and if cost is a factor, it may be better to design a system that sits above the existing floor.
As redevelopment continues to convert vacant industrial buildings into modern residential units, developers need to pay attention to the risks associated with potential indoor air contaminants. With increased attention to indoor air quality, changes to regulatory standards and a growing list of contaminants of concern, the installation of systems to control soil gas migration is a smart decision.
A member of the REBA Environmental Law Section, Sam Butcher is a licensed site professional and principal at Loureiro Engineering where he works on all aspects of property redevelopment projects. Sam’s email address is email@example.com.