Common use of Active Sub-Slab Depressurization (SSD) Clause in Contracts

Active Sub-Slab Depressurization (SSD). A generalized illustration of an active SSD system is shown on Figure 10. An active SSD system typically consists of a blower and sub-slab air intake piping system. The SSD system is operated continuously to create a slight vacuum beneath the concrete floor slab of the building. The induced vacuum beneath the building floor slab overcomes the lower pressure that is sometimes found inside buildings. Therefore, when the SSD system is in operation, soil gases generally cannot flow from beneath the floor slab into the building. Rather, at the location of any cracks on the floor, indoor air will be drawn from inside the building into the lower pressure zone beneath the floor slab, thereby mitigating the vapor intrusion process. An SSD system requires installation of a vent intake pipe in one or more central or other appropriately selected location(s) in the baserock layer beneath a concrete floor slab. As an alternative, a geocomposite drainage mat or other liner with lateral permeability can be installed beneath the building and used as the means for withdrawing air from beneath the entire floor area. The vent pipe or drainage mat is connected to a blower to continuously create ventilation and a slight vacuum beneath the floor slab. The vacuum level created beneath the floor must be at a level sufficient to overcome the anticipated vacuum level inside the building (see design guidance documents listed in Section 5.1.3.1). The air and soil gases withdrawn from beneath a building during SSD operation are exhausted to the atmosphere. The emissions from the SSD systems will be treated to remove VOCs to the extent required by the Bay Area Air Quality Management District (“BAAQMD”) based on the estimated VOC emission rate for each system.4 4 For an SSD system at a nearby residential property on a former GTE site in Mountain View, California, treatment of SSD emissions was not required by BAAQMD due to low TCE emission rates relative to BAAQMD standards (U.S. EPA, 2004).