Environmental Engineering Reference
In-Depth Information
substantial atmospheric injection since 1981, the TSP Pu concentrations have decreased to <0.05 μBq
m
−3
in 1985. The annual average Pu in air concentration at ANL was <0.01 μBq m
−3
in 2001.
Lee et al. (1986) estimated the stratospheric mean residence time of 1.2 year while Holloway
and Hayes (1982) estimated a 71 day mean residence time for Pu aerosol fallout in the troposphere.
Both these pieces of information would suggest that all the fallout Pu was deposited on the surface
of the earth by 1985. The fact that Pu is still measurable in TSP, albeit with error terms of 100% or
more, leads to the conclusion that resuspension of material from the surface is now the controlling
mechanism for dispersion, as it is with U and Th.
Komosa and Chibowski (2002) reported on a weekly ground level aerosol data base in Lublin,
Poland initiated in 1993. Measurement of
239,240
Pu was performed radiochemically on 4000-8000 m
3
iltered air samples taken in 1993 to 1994 and 1998 to 1999. The
239,240
Pu was better correlated with
sample mass than volume indicating resuspension as a source. Their measurements are reported in
Table 22.3.
Kierepko et al. (2009) reported on the measurements of
239,240
Pu made in Krakow, Poland from
1990 to 2002 and Bialystok from 1996 to 2001. They reported a seasonal variation in the measured
air concentration and that the main source appeared to be from the North Sea. Their measurements
are shown in Table 22.3.
22.7 RESUSPENSION
Particle resuspension, while of importance, is an elusive process to quantify. Sehmel (1980) pub-
lished a deinitive review of the subject that gives an appreciation for the uncertainties associated
with this topic. Resuspension is a form of large-scale erosion describing the continual movement of
particles as a function of surface stresses. The stresses are saltation, surface creep and suspension,
while the transport means depends on the particle diameter wind speed and turbulence. Newman
et al. (1976) deined saltation as a process by which particles with diameters of 100-500 μm rise
or bounce in a layer close to the surface-air interface. Surface creep particles with diameters of
500-1000 μm slide or roll, pushed along the surface by wind stresses and the impact of saltation
particles. The smallest particles, <100 μm in diameter, move by suspension, following air motion.
The interplay of the stresses causes suspension particles to leave a surface when saltation par-
ticles impact the surface. Although particles <50 μm and particularly <10 μm in diameter are almost
impervious to wind erosion, when mixed with saltation particles, they become transportable by
suspension.
Modelers have not been able to predict resuspension factors for general situations. Langham
(1971) deined the resuspension factor (RF) as the ratio of the airborne pollutant concentration per
unit volume of air to the pollutant surface concentration per unit area on the surface. Thus, RF has
units of m
−1
. From Sehmel's (1980) review, the resuspension factors developed under experimental
conditions range from 10
−12
to 10
−2
m
−1
for wind resuspension and 10
−10
to 10
−2
m
−1
from human
activities. These factors were developed not only from wind erosion situations but also from agricul-
tural practices, vehicular and pedestrian trafic, and household chores. Similarly the resuspension
half-life ranges from days to years and is dependent on the situational parameters.
Golchert and Sedlet (1978) several resuspension studies were conducted at nuclear facilities and
the Nevada test site, but there appears to be only one study of the resuspension of U, Th, and Pu.
Golchert and Sedlet collected and analyzed TSP air ilters and soil samples at the off-site ANL
monitoring station. They assumed that the top one cm of soil was available for resuspension and
contained 925 μBq g
−1
of soil. The weight of the non-carbonaceous material remaining after dry
ashing the TSP ilter was taken to be resuspended soil. Resuspension factors for U, Th isotopes and
fallout Pu were calculated and are shown in Table 22.4. The RFs estimated in this study are inter-
nally consistent, despite the different sources, fallout and naturally occurring radionuclides. The Pu
was “aged” deposition, assuming most of the Pu was deposited in the 1960s. This would suggest that
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