Human Health Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 219

H ∗ C a = C pw (5.3)

where K d is the distribution coefficient between soil solid phase and pore water
(L/M) and H (the dimensionless Henry coefficient (-)) is the distribution coefficient
between soil air and pore water.
Alternatively, the distribution of contaminants over soil phases can be determined
on the basis of the fugacity theory of Mackay ( 2001 ). Fugacity is the tendency of a
contaminant to escape from the medium in which it is present and has the dimen-
sions of pressure (M L − 1 T − 2 ). At low concentrations fugacity is proportional to

concentration, as follows:

·

Z ∗ f

C = (5.4)

where Z is the fugacity capacity ( T 2 / L 2 ) and f is fugacity ( M T − 2 ).

For air, Z equals 1/ R ∗ T ,forwater Z equals 1/ H and for solid phases Z equals
K d ∗ ρ b / H . In a multi-compartmental system at equilibrium, the fugacities are equal
and concentrations in the soil compartments can be calculated from the Z values.
The fugacity approach was initially applied to organic contaminants distributing
over all phases. If the distribution of non-volatile contaminants were to be modelled,
modifications to the equations would be needed.

L − 1 · ·

5.3.3.3 Contaminant Transfer

The second step in exposure modelling is the prediction of the transfer of the
contaminants from the mobile phase of the soil to contact media, such as:

•

air (volatile phase), outdoor and indoor;

• air (particles);

• water (groundwater, surface water, drinking water);

• household dust;

• vegetables;

• meat;

• eggs;

• milk.

Accumulation in cattle can be considered as transfer or as exposure, depending
on the protection targets considered (human health or the cattle).
In the final step, contact by humans with the contaminants in these compartments
is quantified, taking into account site characteristics and resident properties.

5.3.3.4 Major Exposure Pathways

Formulae for the following exposure pathways have been included in almost all of
the existing exposure models (Carlon and Swartjes 2007 ; McKone 1993 ):