More frequently, development is occurring at sites having subsurface contamination. When volatile or semi-volatile contaminants are present, the potential for migration of those compounds to interior spaces must be considered and addressed. Two general types of mitigation measures have been utilized extensively in California over the last several years. These include membranes and ventilation systems installed beneath building floor slabs. These two mitigation measures are often combined to provide a higher level of safety and redundancy.

Some of the most common types of membranes used today for solvent or Volatile Organic Compound (VOC) mitigation are spray-applied products, such as CETCO's LIQUID BOOT® spray applied membrane. Spray applied membranes are typically comprised of proprietary mixtures of rubberized asphalt applied to a geotextile substrate. This type of membrane generally has an advantage over sheet products with respect to the ease with which membrane seams, perimeters and penetrations can be sealed. Long-term diffusion tests have been conducted on LIQUID BOOT® membranes at VOC levels in excess of 100,000 μg/m³ without any indication of membrane degradation.

Typical diffusion testing results and field performance indicate LIQUID BOOT® membranes provide an effective barrier against subsurface vapors at concentrations far in excess of those that are typically encountered. Membrane and chemical specific diffusion testing results allow VOC migration rates, potential interior air concentrations, and associated risk levels to be quantified for a proposed structure using standard contaminant transport models and risk assessment procedures.

Sub-slab ventilation systems are routinely utilized in conjunction with membranes to further reduce the potential for vapor transmission to interior areas and to provide a greater level of system redundancy. There are three general types of sub-slab ventilation systems in common use today: passive ventilation systems, active ventilation systems and de-pressurization systems.

Passive ventilation most common
Passive ventilation systems consist of a network of perforated horizontally orientated vent lines installed beneath the floor slab of a building. The vent lines are typically embedded within a permeable sand or gravel blanket. The perforated vent lines are connected to vertical vent risers that extend through the walls of the structure to outlets above the roof. The system works by preventing the development of excess pressure beneath the floor slab and by purging VOC vapors from the sand or gravel blanket beneath the slab. Passive ventilation systems represent the most common type of system utilized in conjunction with membranes for vapor mitigation purposes in California. They are typically less expensive to install than other alternatives, and once installed, require little or no maintenance.

Active ventilation systems generally have the same components as passive systems along with a blower component. The sub-slab vent piping with active systems often consists of two separate networks of perforated piping—an extraction network and a fresh air recharge network—each with their own vent risers. Air is removed from the extraction piping network by the blower and exhausted to the atmosphere through a vent riser at roof level.

Air is drawn into separate fresh air recharge vent risers at the roof level and distributed to perforated fresh air recharge piping beneath the floor slab. In the design of this type of system, an effort is typically made to induce only nominal negative pressures in the sub-slab vent lines. This reduces the potential for VOCs to be drawn from the soil into the sand or gravel blanket as well as the need for any associated emissions monitoring and/or treatment.

De-pressurization systems are generally configured similar to active systems without the fresh air recharge components. They are intended to reduce the potential for VOC migration to interior areas by reducing the air pressure within the sub-slab sand or gravel blanket to a level significantly below that which exists in the interior of the building. This is done by using a blower to extract air from a vent riser that is connected to the perforated sub-slab vent piping network, typically without the provision for fresh air recharge into the system. This can induce relatively high negative pressures beneath the floor slab of the building.

The combination of a sub-slab membrane and passive sub-slab ventilation system, such as the LIQUID BOOT® Gas Vapor Membrane and Vent System, has been found to be highly effective in controlling vapor migration to interior building areas. This type of system is relatively easy to install, provides a wide margin of safety at most sites, and a high level of redundancy. Buildings incorporating this type of system can be safely constructed and occupied at sites with subsurface contamination where development was not previously possible.

Glenn Tofani is president, GeoKinetics, Inc., Irvine, Calif.