Human Exposure Pathways - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 496

11.5.4 Influence of Physical Properties
The soil properties are very important for determining the release of the contami-
nants from the soil. The diffusivity of vapours can differ several orders of magnitude
between a dry soil and a wet soil. Soil porosity may also be important, but to a more
limited extent. Also the depth of the contaminant in the soil, determining the length
of the diffusion path, will be of importance for the release. The meteorological con-
ditions, such as wind speed and atmospheric stability are very important for the
resulting dilution.
11.5.5 Influence of Human Behaviour
Volatile contaminants are present in outdoor air due to their widespread use.
Petrol stations, dry cleaners, and other commercial/industrial facilities are impor-
tant sources of volatile contaminants to outdoor air. Indoor air may also become
affected from outdoor air containing volatile contaminants.
The exposure times and also the height at which human beings inhale contam-
inated air, i.e., at the height of the breathing zone, will be determined by the type
of human activities occurring at the site. For example exposure may be higher in
playgrounds, picnic areas and parks. The type (and intensity) of human activity has
an important effect on breathing rates and the volume of air inhalation.
11.5.6 Reliability and Limitations
It is generally recognized that modelling of exposure through outdoor air inhalation
is relatively uncertain. The factors that cause uncertainty are explained in detail, in
the following text.
Modelling of vapour release to outdoor air is based on the assumption that the
different phases of the contaminant (vapour, dissolved and sorbed to soil) are in
chemical equilibrium. This may not be the case, for example when transport in the
soil is fast or when the contaminant is a constituent in free phase liquids, e.g., petrol,
oil or chlorinated solvents. The assumption of equilibrium may give an overestima-
tion of the release (e.g., vapour phase concentrations are often less than equilibrium
concentrations due to the loss to the atmosphere), and, hence, of the human health
risk involved. In some cases an underestimation may occur, e.g., when large parts
of the pore water or the soil are not available to the contaminant. Furthermore, any
degradation of the contaminant as it migrates through the soil or when it reached
the ambient atmosphere is generally not considered, which may overestimate the
concentrations of readily degradable contaminants.
The models for the inhalation of vapours outdoors aim at predicting an effec-
tive average value over a relatively long time scale. A number of the important
input parameters such as water saturation of the soil and wind speed, however, will
vary considerably on a very short timescale. Thus, the variation in both release and