The Influence of Transient Processes on Vapor Intrusion Processes

A three dimensional vapor intrusion model was used to investigate the amount of time required to reach steady-state vapor intrusion rates. The effect of pressure fluctuations on vapor intrusion rates was also investigated. These results were obtained for a homogenous soil conceptual site model with groundwater located at 25 feet below ground surface. The time to achieve steady state ranged from approximately 6-12 months depending on geology. However, the time required for crack concentrations to return to zero after a vapor intrusion source had been removed was on the order of years. Pressure fluctuations can result in concentration spikes when the building pressure becomes negative. For highly permeable soils, the instantaneous mass flow rate into the building can be 2 times greater than steady state mass flow rates due to the effect that pressure fluctuations have on soil gas concentration profiles. The results provide insight about managing and characterizing vapor intrusion risks.