Particle Resuspension Mechanisms1
Soils act as a highly effective sink for radionuclides depositing from the atmosphere.
Downwards migration rates of radionuclides in soils are commonly so low that the
distributions of many radionuclides in soils (particularly those of the actinides) are restricted
to the surface layers (Livens, 1985; Volchok, 1973; Cawse, 1980). Because of low soil-toplant transfer factors for actinides most of the plutonium associated with vegetation is due
to foliar contamination via atmospheric deposition and/or resuspended soil. Thus, the
processes of deposition and resuspension can have significant implications for ingestion
doses incurred via the terrestrial food chain pathway (Romney et al., 1970) as well as for
doses incurred through direct inhalation. Cesium and Plutonium are strongly adsorbed by
soil particles and their pattern of re-distribution and transport in the environment at large is
governed by soil erosion processes which are dominated by the actions of wind and water
(Hanson et al., 1981: Dreicer et al., 1984).
The resuspension of deposited or ground-surface material occurs through saltation,
suspension and surface creep and these mechanisms are particle-size dependent
(Bagnold 1941; Chepil1945).
Particles moving by saltation tend to have physical diameters of 50-500 µm and they are
small enough to move by direct wind action but large enough to have settling velocities
higher than the upward eddy velocity of the wind (Anspaugh at al., 1975). Saltating particles
move along a surface in a series of 'hops': when saltating particles hit the ground they
transfer momentum to other particles to initiate their suspension into the near-surface
airstream. Suspended particles have smaller aerodynamic sizes and have settling
velocities less than the turbulent eddy velocities of the wind. The resuspension and
transport of small particles are of concern because small suspended particles are able to
travel long distances to contaminate previously clean areas and may lead to elevated air
concentrations for an extended period afterwards (Paretzke & Garland, 1994). Secondly,
suspended particles less than 40-80 µm also contribute to inhalation dose (Healy et al.,
1966).
A third mechanism of resuspension is surface creep and this involves th e movement of
large particles up to 2 mm diameter along the ground (Anspaugh et al.. 1975). The
quantification and attribution of any of these individual mechanisms of resuspension to field
measurements, however, is difficult. Chamberlain (from Prupacher, 1987), nevertheless,
summarized some basic conditions favorable to soil erosion and subsequent particle
resuspension by wind, as follows:
1.effect of bombardment by saltating particles in resuspending smaller particles;
2.the existence of a most favorable particle size range (~30-100 µm) for resuspension;
3.a reduction in soil erosion with time at constant wind speed as the most erodible soil
fraction is removed:
4.the protective effect of soil moisture in binding soil grains together; and
5.the protective effect of vegetation which intercepts saltating grains thereby reducing the
drag of wind on soil which helps to bind the soil together.
Much of the work carried out on resuspension was carried out in arid and desert
environments with a deficiency of soil moisture (Sisal & Hsieth, 1966) and factors which
govern the initiation and sustainabllity of any of the above mechanisms of soil erosion and
subsequent wind-driven resuspension may not be strictly applicable to soils in the more
northern temperate latitudes (Garland, from Pruppacher, 1987).
A fourth mechanism of soil erosion and subsequent resuspension of particles relevant to
this work occurs via rain-soil splash. Dreicer et al. (1984) found that the impact of falling
raindrops resuspended soil particles to a maximum height of 40 cm above ground and· the
majority of resuspended particles were less than 125 µm diameter. These workers also
found a linear relationship between the intensities of rainfall from four natural rainfall events
and the resuspension of particle size less than 125 µm diameter up to a height of 40 cm
above the ground. This mechanism of resuspension, however, is episodic and difficult to
quantify under field conditions because of the range of environmental variables involved.
These include the initial surface moisture before the last rainfall (antecedent soil moisture),
soil type, vegetation cover, vegetation type, wind speed and particle size with activity
characteristics of the depositing or deposited nuclides.
On a local scale and with particular reference to nuclear fuel reprocessing plants, it is the
mechanical processes of resuspension which are more important than wind borne
processes of particle resuspension (Hakonson et al., 1980). Deposited material is reentrained and resuspended into the near-surface air from soils and concrete areas via a
number of mechanical processes including the use of ground-intrusion machinery such as
earth diggers, pile-driving equipment, vehicular tire movement and pedestrian traffic.
Agricultural activities, particularly plowing and tilling, are capable of raising soil-bound
plutonium and other nuclides within the near-surface air (Shinn et al., 1983) although the
distribution of activity with particle size will be a limiting factor with regard to inhalation
dose.
Inhalation of plutonium
A number of workers (Nicholson et al., 1989 :Garland, 1991) have noted that large particles
greater than 20 µm diameter are more easily resuspended than smaller particles. The least
resuspendible sizes are the less than 1 µm diameter particles (Gillette, 1977). The major
hazard from the resuspension of plutonium is the potential inhalation of contaminated
particles in specific physical size ranges (NEA, 1981; Nicholson, 1992). Particles up to 100
µm in diameter are generally thought of as being inhalable (Mark, 1995) with the less than 4
µm diameter size fraction defined as the respirable fraction (EN481: 5S EN481).
Inhaled particles deposit to either the nasopharyngeal regions, the tracheo-bronchial region
or the pulmonary or alveolar regions of the lungs . However, the fraction of inhaled particles
which is retained in the respiratory system and the depth to which the particles penetrate
before deposition is related to particle size (Brown et aI., 1950). Inhaled particles less than
0.5 µm aerodynamic diameter may be carried deep into the lung and if contaminated with
radionuclides will irradiate sensitive alveolar tissue (ICRP, 1975 Burkart, 1989). Particles
greater than 5 µm deposit out of the inhaled air stream via impaction or are intercepted by
mucus layers within the nasopharyngeal cavity. Inhalation of contaminated particles greater
than about 5 µm aerodynamic diameter is radiologically less damaging to lung tissue than
particles with an aerodynamic size range less than 1 µm because this size fraction is less
respirable.
1Environmental
Characterization Of Particulate-Associated Radioactivity Deposited
Close To The Sellafield Works In Great Britain, A thesis submitted for the degree of
Doctor of Philosophy, Imperial College of Science, Medicine and Technology, Ellis Induro
Evans, (October, 1997).