Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 433

10.4.2.4 Numerical Models
A three-dimensional (3-D) numerical model for vapor intrusion was recently devel-
oped at Arizona State University (Abreu and Johnson 2005 , 2006 ), which is the
most comprehensive model for vapor intrusion available to date. This model has
been used to simulate a number of common scenarios to improve the general under-
standing of the processes affecting subsurface vapor transport mechanisms (Abreu
in press; Abreu et al. 2008 ). Pennell et al. ( 2008 ) and Bozkurt et al. ( 2008 )alsohave
developed a 3-D model using a commercially available generic numerical code and
have used the model to investigate the influence of heterogeneous soils on vapor
intrusion. 3-D models allow simulations of much more complex geometries than
one-dimensional models, and therefore provide additional insight into the expected
behavior of vapors under a wider range of conditions.
Subsurface vapor transport (without explicit simulations of buildings) can also
be simulated with Air3D (Joss and Baehr 1995 ), VapourT (Mendoza and Frind
1990a , b ), CompFlow Bio (Yu et al. 2009 ) and a multiphase model by Sleep
and Sykes ( 1989 ). These models were designed to simulate subsurface contam-
inant transport through multi-phase or vapor transport, and are flexible enough
to simulate heterogeneous soils, complex geometry, advection and diffusion, and
other mechanisms that may help to elucidate the important mechanisms involved
in vapor intrusion. These models are more complicated and require consider-
able effort to use, so they have had limited application to vapor intrusion studies
to date.
10.5 Sampling and Analysis
The sampling and analysis necessary for site-specific assessment of subsurface
vapor intrusion is challenging, because of the low quality standards, multiple alter-
native sources of vapors at these low concentration levels, and the potential for
sampling bias and variability. This chapter will discuss these challenges in detail,
and discuss approaches for managing them.
10.5.1 Sampling and Analysis Challenges
Indoor air sampling and analysis may initially seem like the first choice for assess-
ing subsurface vapor intrusion to indoor air. However, the results are almost always
difficult to interpret because of background contributions from consumer products,
building materials, and even outdoor air sources. Several contaminants have indoor
air quality standards that are lower than typical reporting limits for conventional lab-
oratory methods of analysis, in which case, indoor air quality monitoring may not
be a viable line of evidence and other lines of evidence will be required. Indoor air
quality can also be variable in time and space, depending on wind, barometric pres-
sure, occupant's activities and heating or air conditioning operations, and outdoor