Soil Vapor Intrusion
Soil vapor intrusion is the process by which volatile chemicals move from a sub-surface source into the indoor air of overlying buildings. Soil vapor, or soil gas, is the air found in the pore spaces between soil particles. Because of a difference in pressure, soil vapor can enter buildings through cracks in slabs or basement floors and walls, through openings around sump pumps, or where pipes and electrical wires go through the foundation. Heating, ventilation or air-conditioning systems may also create a negative pressure that can draw soil vapor into the building. This intrusion is similar to how radon gas seeps into buildings.
Soil vapor can become contaminated when chemicals evaporate from subsurface sources and combine with the soil vapor. Chemicals that readily evaporate are called "volatile chemicals," which include the volatile organic compounds benzene, toluene, ethylbenzene and xylene, which are identified by their initials as BTEX. Subsurface sources of volatile chemicals may include contaminated soil and groundwater or buried wastes. If soil vapor is contaminated and enters a building as described above, indoor air quality may be affected.
