Human Exposure Pathways - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 493

The first type, i.e., assuming mixing in the air volume is defined by a specified
wind speed and mixing height, is used in the RBCA model (ASTM 2004 ) where the
dilution velocity is described by:

U air δ air
W

Q / C = (11.57)

in which

the wind speed above ground in the ambient mixing zone [m s − 1 ]

U air =

δ air = the ambient mixing zone height [m]

W = the dimension of the soil source zone parallel to the wind direction [m].

The ambient mixing zone is often taken to be the height of the receptor, i.e.,
the height above ground at which human beings inhale contaminated air, i.e., at the
height of the breathing zone.
Several different methods are used to estimate the mixing height and the
effective wind velocity for mixing. An approximation used by the Danish EPA
(Miljøstyrelsen 2002 ) is that at low wind velocities the mixing height is propor-
tional to the length of the contaminated area. The empirical relationship that the
mixing height is 0.08 times the length of the contaminated area. Thus, Eq. ( 11.58 )
can be simplified as follows:

Q / C = 0.08 U air (11.58)

This approach is also adopted in the Swedish model for Soil Quality Standards
(Naturvårdsverket 2009 ).
A more complex model for determining the dilution velocity is used in the
CSOIL and Vlier-Humaan models. The mean wind speed in the mixing layer is
calculated from measured wind speeds at a height of 10 m assuming a logarithmic
wind speed profile above the ground surface. The effect of obstacles on the ground
is considered by using a roughness length . The height of the mixing layer is calcu-
lated as a function of atmospheric stability, roughness length and the length of the
contaminated area.
For the UK CLEA model (Environmental Agency 2008 ) a different approach
is used, in which air dispersion factors are tabled for thirteen cities in the UK
for four different sizes of source areas (from 100 to 20,000 m 2 ) and two different
receptor heights corresponding to adults and children. The tabled values are derived
from calculations performed with the US EPA dispersion model AERMOD PRIME,
calculating annual average air concentrations using local meteorological data.

11.5.2.2 Calculation of Exposure

The calculating of exposure to contaminants by inhalation of vapours outdoors
differs, depending on the Toxicological Reference Value which is used for the
contaminant being studied. There are two main approaches: