Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 421

coefficients, so the diffusive flux in the aqueous phase will generally be negligi-
ble (except for compounds with very low Henry's Law Constants, where the mass
fraction in the gas phase is very small). Thus, effective diffusion coefficients in
soil tend to be high, although very dry soil can also result in low effective dif-
fusion coefficients by direct mineral sorption of contaminants to the solid phase
(McAlary 1989 ). Under nearly water-saturated conditions, such as within the capil-
lary fringe, the effective diffusion coefficient may be quite low. As a consequence,
transport through the capillary fringe is often the limiting factor in vapor transport
from groundwater into indoor spaces. Contaminants with very low Henry's Law
Constants (<0.001) may also have very low effective diffusion coefficients.

10.3.2 Biodegradation

Several vapor intrusion studies (DeVaull et al. 2002 ; Fischer et al. 1996 ;Hers
et al. 2000 ; Lahvis et al. 1999 ; Ostendorf and Kampbell 1991 ; Roggemans 1998 ;
Roggemans et al. 2001 ) have demonstrated that the aerobic biodegradation of hydro-
carbon vapors can be significant in the vadose zone. These studies reached these
conclusions through:

• field investigations examining soil gas concentration profiles of petroleum hydro-

carbons and indicators of biological activity (oxygen and carbon dioxide);

• a comparison of modeled and measured vapor intrusion attenuation factors for

petroleum and non-petroleum contaminants;

• modeling studies to characterize the potential impact of biodegradation on the

indoor air concentrations.

There is uncertainty regarding whether and to what extent the presence of a
building inhibits the supply of oxygen and therefore, whether degradation below
a building is similar to degradation beside a building. Few studies to date have gen-
erated data specifically to assess the extent of degradation directly beneath buildings
(Luo et al. 2009 ; McAlary et al. 2007 ).
DeVaull et al. ( 1997 ) listed conditional criteria for aerobic biodegradation of
aromatic hydrocarbons in soil. In order for biodegradation to occur, sufficient hydro-
carbon, oxygen, nutrients, moisture, and microbial populations must be present.
Typically, sufficient oxygen is the limiting criteria since sufficient microbes, soil
moisture, and nutrients are present at most sites. Biodegradation of contaminant
vapors can potentially occur by anaerobic processes where oxygen has been con-
sumed or by cometabolic processes where there are appropriate mixtures of primary
metabolites and cometabolites, however, both of these processes tend to be much
less significant than aerobic metabolism and have not been demonstrated for vadose
zone vapor transport.
The rate and extent of aerobic degradation varies from site to site (Roggemans
et al. 2001 ), such that it is difficult to predict the degree of degradation a-priori,