The Vapor Intrusion Process Sample Clauses

The Vapor Intrusion Process. The vapor intrusion process occurs through several chemical transport processes, as summarized below. A generalized illustration of the vapor intrusion process is shown on Figure 8. The vapor intrusion process begins when VOCs in soil or groundwater volatilize into soil gas in the subsurface. The degree to which VOCs volatilize into soil gas depends on the chemical properties, i.e., VOCs with higher vapor pressures, lower water solubilities, and less tendency to adsorb to soil particles tend to partition into soil gas more readily than other VOCs. Chlorinated solvents such as those found in groundwater at the NRP readily partition into soil gas. Once in the soil gas, VOCs may migrate upwards or laterally by both diffusion and convection. In general, VOCs diffuse more readily in drier, granular soil than in wetter, clayey and silty soil. Diffusion is a relatively slow transport process as compared to convection, which occurs when soil gases containing the VOCs are drawn to the surface by pressure gradients. Pressure gradients can be caused by barometric pressure changes, as well as the reduced pressure that occurs inside many buildings, as discussed below. After VOCs in soil gas migrate to the area directly beneath a building (e.g., the baserock beneath the floor slab), vapor intrusion into the building can occur. Soil gases containing VOCs may migrate into the building by diffusing through cracks in the floor. Soil gases may also be swept into the building through cracks in the floor by convective flow, driven by a lower pressure inside the building. Lower pressures inside of buildings are sometimes referred to as the “stack effect”. The stack effect can be caused by: • warmer air inside the building, which tends to rise and draw in air from the lower parts of the building; • wind, which tends to impart a lower pressure inside the building; • appliance exhausts, which tend to draw air into the building and lower the interior pressure; and • active ventilation systems that exhaust outside the building and induce a slight negative pressure inside the building. Considering the mechanisms of vapor intrusion, vapor intrusion prevention or mitigation tends to be based on (a) eliminating soil gas flow into the building by creating either a lower pressure (slight vacuum) beneath the floor of the building, or a higher pressure inside the building, (b) preventing VOCs from migrating to the area beneath the building floor, using barriers or source removal, and/or (c)...