Human Exposure Pathways - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 489

For sites with buildings, intrusion of vapours into buildings and the exposure
due to inhalation of indoor air will usually be the dominating exposure pathway.
This exposure pathway is treated in detail in McAlary et al. ( Chapter 11 of this
topic). However, for sites without buildings and especially sites where large areas
are contaminated with volatile contaminants, exposure due to inhalation of vapours
in outdoor air may become of importance. In this section an overview is given of
how the exposure due to inhalation of vapours in outdoor air can be calculated,
including the calculation of outdoor air concentration and exposure.

11.5.1 Conceptual Model

Volatile soil contaminants will be distributed between soil particles, soil air and
pore water. In the case of severe contamination also a free phase may be present.
The transport of volatile contaminants through the soil can occur by diffusion or by
advective transport. Diffusion in soil air will be the dominating process for volatile
contaminants that to a large degree are present in soil air (contaminants with large
Henry's Law constant). In soils with very high water saturation, diffusion of con-
taminants in vapour phase is very slow. However, dissolved contaminants may also
diffuse in the pore water and may thus be transported towards the soil surface from
where it can evaporate. This can be an important exposure pathway for contaminants
with small Henry's Law constants. A common approximation is that the contami-
nant concentration in soil air and pore water is in equilibrium and that diffusion in
the soil air and pore water can occur in parallel.
An advective flow can be caused by gas generation in the soil, thermal gradients
in the soil or pressure differences in the atmosphere. In climates with a high evapo-
ration rate, capillary rise can also cause an upward flow of water in the soil column,
causing advective transport to the surface of contaminants in the soil water.
Vapours released from the soil surface will mix with the ambient air resulting in
dilution. The degree of dilution will depend on wind speed, atmospheric stability
and the roughness of the ground surface, but also on the size of the contaminated
site and the height above ground at which human beings inhale contaminated air,
i.e., at the height of the breathing zone.

11.5.2 Description of Models

The exposure due to inhalation of vapours in outdoor air is included in several Risk
Assessment models. The starting point for the Risk Assessment may be the contam-
inant concentration in the soil, in the groundwater or in the soil air. Here we will
focus on modelling approaches taking concentration in the soil as a starting point.
The mathematical model for exposure due to inhalation of vapours outdoors
contains the following steps:

• calculation of outdoor air concentration;

• calculation of exposure.