Vapor transport from soil and groundwater: Numerical modeling approach

Three-dimensional numerical models can be used to predict the effect of various site features and processes on vapor intrusion, and indicate where critical field data needs exist. In this way, numerical vapor intrusion models can be viewed as one of many tools available to vapor intrusion risk assessors who are tasked with characterizing and mitigating vapor intrusion risks for current and future site uses. To date, some vapor intrusion models have been developed as screening tools for identifying whether additional investigation of vapor intrusion is warranted. Screening models are often one-dimensional models for which analytical solutions are more easily obtained as compared to full three-dimensional (3-D) models, which require numerical approaches for solution because of their complexity. Because screening models are easily solved, they are routinely used to provide guidance on whether investigation of vapor intrusion risks is or is not warranted; however, it is generally acknowledged that analytical screening models capture only a few of the elements that can play a role in determining whether vapor intrusion does or does not pose a risk at a particular site. To overcome the limitations inherent in use of simple screening models and to provide additional insight into the vapor intrusion process at a given site, more sophisticated 3-D numerical models have been developed, which are capable of producing 3-D soil gas concentration profiles while considering various site features (Abreu and Johnson 2005, Pennell et al. 2009). This chapter discusses current vapor intrusion numerical modeling efforts with regard to approach, limitations and future directions. In addition, it highlights situations when models can be used to improve vapor intrusion risk assessments. It is important to note that this is an area of active research, and increasingly greater levels of complexity and realism are being built into the models, while at the same time, efforts at field validation continue.