IPA-600/4-77-037
July 1977
Environmental Monitoring Series
RESUSPENSION OF PLUTONIUM
FROM CONTAMINATED LAND SURFACES:
Meteorological Factors
Environmental Monitoring and Support Laborator
Office of Research and Development
U.S. Environmental Protection Agency
Las Vegas. Nevada 89114
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environ-
mental Protection Agency, have been grouped into five series. These five
broad categories were established to facilitate further development and
application of environmental technology. Elimination of traditional grouping
was consciously planned to foster technology transfer and a maximum
interface in related fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL MONITORING
series. This series describes research conducted to develop new or improved
methods and instrumentation for the identification and quantification of
environmental pollutants at the lowest conceivably significant
concentrations. It also includes studies to determine the ambient
concentrations of pollutants in the environment and/or the variance of
pollutants as a function of time or meteorological factors.
This document is available to the public through the National Technical
Information Service, Springfield, Virginia 22161.
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EPA-600/4-77-037
July 1977
RESUSPENSION OF PLUTONIUM FROM CONTAMINATED
LAND SURFACES: Meteorological Factors
by
P. N. Lem, J. V. Behar and F. N. Buck
Monitoring Systems Research and Development Division
Environmental Monitoring and Support Laboratory
Las Vegas, Nevada 89114
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
LAS VEGAS, NEVADA 89114
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DISCLAIMER
This report has been reviewed by the Environmental Monitoring and Support
Laboratory-Las Vegas, U.S. Environmental Protection Agency, and approved for
publication. Mention of trade names or commercial products does not consti-
tute endorsement or recommendation for use.
ii
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FOREWORD
Protection of the environment requires effective regulatory actions which
are based on sound technical and scientific information. This information must
include the quantitative description and linking of pollutant sources, trans-
port mechanisms, interactions, and resulting effects on man and his environ-
ment. Because of the complexities involved, assessment of specific pollutants
in the environment requires a total systems approach which transcends the media
of air, water, and land. The Environmental Monitoring and Support Laboratory-
Las Vegas contributes to the formation and enhancement of a sound integrated
monitoring data base through multidisciplinary, multimedia programs designed
to:
develop and optimize systems and strategies for
monitoring pollutants and their impact on the
environment
demonstrate new monitoring systems and technologies
by applying them to fulfill special monitoring needs
of the Agency's operating programs
This report presents a brief overview of studies of the resuspension of
material into the air. Particular attention was directed to meteorological
factors that affect the resuspension of plutonium from contaminated land sur-
faces. This review can serve as a concise introduction to resuspension studies
for the new researchers in this field. The Monitoring Systems Design and
Analysis Staff at the EMSL-LV may be contacted for further information on the
subject.
George B. Morgan
Director
Environmental Monitoring and Support Laboratory
Las Vegas
iii
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ABSTRACT
A literature review is presented in a discussion of the relevance of
meteorological factors on the resuspension of plutonium from contaminated land
surfaces. The physical processes of resuspension based on soil erosion work
are described. Some of the models developed to simulate the resuspension of
materials for predicting airborne concentrations are reviewed. The signifi-
cance of some of the parameters used in the different models is also disucssed.
The interplay of meteorological factors measured, discussed, or implied in the
literature reviewed as related to the resuspension process is discussed in the
final section.
IV
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CONTENTS
Page
Foreword iii
Abstract iv
List of Figures and Tables vi
I. Introduction 1
II. Physical Aspects of Particle Movement 2
Dynamics of Particle Movement Near the Ground Surface 4
Particle-Size Distribution 5
III. Resuspension Models 6
IV. Discussion of Important Parameters 11
Resuspension Factor 11
Resuspension Rate 12
Resuspension Ratio 12
Deposition Velocity 14
V. Meteorological Considerations 15
VI. Summary 18
VII. References 20
VIII. Bibliography 24
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LIST OF FIGURES AND TABLES
Number Page
FIGURES
1 Schematic diagram of resuspension model concepts, redrawn 8
from Horst, Droppo, and Elderkin (1974).
2 Resuspension rates and resuspension factors as a function 17
of wind speed. (Sehmel and Lloyd, 1975a).
TABLES
1 Factors Influencing Wind Erosion 3
2 Resuspension Rates 13
VI
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I. INTRODUCTION
Plutonium contaminants in the environment have been introduced primarily
from three types of sources: the atmospheric testing of nuclear weapons, the
controlled releases from facilities handling plutonium, and the unintentional
releases resulting from accidents. Of these three source types, the largest
contribution has been from past atmospheric nuclear tests which introduced an
estimated 300 kilocuries (kCi) of plutonium-239 based on world-wide deposi-
tion of strontium-90 and estimated strontium to plutonium production ratios
(Harley, 1971). The plutonium-238 deposition is estimated to be about 3 per-
cent of this plutonium-239 deposition.
Controlled releases of plutonium total well below that released in atmo-
spheric nuclear tests. As an example, controlled releases from the Rocky
Flats Plant in Colorado as of 1970 were at upper limit estimates of 41 milli-
curies (mCi) for airborne effluents and 91 mCi for liquid effluents (Hammond,
1971).
Unintentional plutonium releases, although limited, have occurred in
several types of accidents. Plutonium from nuclear weapons was dispersed in
two separate incidents: one near Palomares, Spain, and the other near Thule,
Greenland. Both accident sites were cleaned up by the physical removal of
almost all of the debris (Jordan, 1971). Uncontrolled releases have also
occurred in and around plutonium facilities. Such releases from the Rocky
Flats Plant were estimated by Krey and Hardy (1970) to be 2.6 curies (Ci) over
an area defined by a minimum reliable measurement contour of 3 millicuries per
square meters (mCi/m2) of plutonium-239. The plutonium-239 deposited outside
this measured area has a greater uncertainty and is estimated to be 3.2 Ci,
attributable to releases from the Rocky Flats Plant, based on cumulative fall-
out estimates from soil samples collected at a nearby location. A refinement
of this value is reported by Krey (1976) to be 3.4 ± 0.9 Ci of plutonium-239
and plutonium-240 from the Rocky Flats Plant which was deposited on public and
private lands.
The destruction of a SNAP-9A (System for Nuclear Auxiliary Power) during
atmospheric reentry was the source of an uncontrolled release of plutonium-238
from a power generator. This resulted in a world-wide distribution of an esti-
mated 17 kCi of plutonium-238 (Harley, 1971), rather than in a concentrated
ground deposit.
The objective of this paper is to discuss the relevance of meteorological
factors on the resuspension of plutonium from contaminated land surfaces. The
material reviewed is presented first in a description of the physical processes
of resuspension. A discussion follows on some of the models developed to
simulate resuspension./ with the intent of using these models as predictive
aids. Critical parameters used in various models are also discussed. Because
meteorology plays a major role in the resuspension process, most of the dis-
cussions found in the literature, whether dealing specifically with plutonium
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or with some other erodible material, contain some reference to the meteoro-
logical conditions under which observations were made. The interplay of
meteorological factors measured, discussed, or implied in the literature
reviewed as related to the resuspension process is discussed in the final
section.
A substantial contribution to the plutonium literature has emerged from
the work of the Nevada Applied Ecology Group (NAEG) of the U.S. Atomic Energy
Commission (AEC), predecessor to the U.S. Energy Research and Development Ad-
ministration (ERDA). The work of this group is centerd around the several
areas contaminated by plutonium and other transuranics in a desert environment
at the Nevada Test Site. The studies are well documented in the ERDA reports
(U.S. AEC, 1974, and U.S. ERDA, 1975). Reports of work in these documents re-
lated to the resuspension processes were considered in this current review.
II. PHYSICAL ASPECTS OF PARTICLE MOVEMENT
In order to gain some understanding of the complexities of the movement of
particles through the air, several topics are treated in this section about the
physics of the problem. Some of the influencing factors illustrating this
complexity are listed in Table 1. Chepil (1975a) lists these factors as asso-
ciated with air, ground, and soil. With a different emphasis, Hilst and
Nickola (1959) developed a list of parameters describing soil conditions and
meteorological factors and a list of secondary or derivative parameters char-
acterizing their interaction. These considerations are also listed in Table 1.
Two mechanisms are associated with the initiation of particle movement.
Direct action of air moving past a particle may exert enough force to accel-
erate the particle, causing it to roll along the surface or to be lifted up and
moved in the air stream. A second means of initiating particle movement is
through the impact of airborne particles with particles on the ground. If the
surface consists of fine, uniform, and similar particles, the binding forces
are cohesive and are usually highly resistant to disturbance by air movements.
Particles on a solid surface which have chemical and physical properties dif-
ferent from the base material have adhesive contact. For the resuspension of
particles to occur, the force on the particle must be equal to or greater than
the force holding the particle to the surface. This applies to both cohesive
and adhesive bonding. Corn and Stein (1967), in their study of the mechanisms
of dust resuspension, identified several factors that are known to influence
particle cohesion and adhesion. The strength of these bonds depends on parti-
cle material, size, and shape, surface roughness, relative humidity of the
ambient air, the presence of electrostatic charge, and the nature and physical
characteristics of the substrate (in the case of adhesive bonds).
Particles that are dislodged from the surface can move in three ways:
suspension, saltation and surface creep. Particles move in suspension when
upward wind eddies counteract free fall, allowing transport at the average
forward speed of the wind. These particles are generally less than 0.1 milli-
meter (mm) in diameter and are redeposited via rainout or gravity after the
wind subsides. Particles between approximately 0.1 mm and 0.5 mm in diameter
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TABLE 1. FACTORS INFLUENCING WIND EROSION
AIR RELATED FACTORS
Velocity
Turbulence
Density affected by
Temperature
Pressure
Humidity
Viscosity
METEOROLOGICAL FACTORS
Wind velocity distribution in the surface
layer
Mean wind speed
Wind direction
Frequency, period, and intensity of gusts
Verticle turbulence exchange
Moisture content on ground surface
Precipitation
Dew and frost
Drying action of the air
GROUND FEATURES
Structure affected by
Organic matter
Lime content
Texture
Specific gravity
Moisture
SURFACE PROPERTIES
Large-scale surface roughness
Mechanical turbulence
Overall sheltering
Small-scale surface roughness
Sheltering of individual particles
Area of erodible surface
Vegetative cover
Live vegetation
Plant residue
Cohesiveness of individual particles
Moisture of surface
Binding action of organic materials
SOIL CHARACTERISTICS
Roughness
Cover
Obstructions
Temperature
Topographic features
PARTICLE PROPERTIES
Particle-size frequency distribution
Ratio of erodible to nonerodible fractions
Particle density
Particle shape
Chepil (I945a)«
Hilst and Nlckc-la (19S9).
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move by a series of short bounces called saltation. Larger particles from
about 0.5 mm to 2 or 3 mm in diameter can roll and/or slide along the surface
in what is called surface creep. Bagnold (1941) notes that with average dune
sand, the suspension flow, even under a relatively strong wind, does not exceed
one twentieth of the flow in saltation and surface creep. In sand flow, salta-
tion accounts for three quarters of the movement. Chepil's 1945 studies with
various soils also showed that the greatest portion of the movement was by
particles in saltation. The relative proportion of each type of movement
varied greatly for different soils. Of the four soil types studied (Sceptre
heavy clay, Haverhill loam, Hatton fine sandy loam, and fine dune sand), be-
tween 55 and 72 percent of the weight of the soil was carried in saltation, 3
to 38 percent was carried in suspension, and 7 to 25 percent was carried in
surface creep.
DYNAMICS OF PARTICLE MOVEMENT NEAR THE GROUND SURFACE
Because particle movement occurs mainly by saltation, much knowledge has
been accumulated by studying this phenomenon. Saltation data were mainly
collected in soil erosion studies.
Field and laboratory measurements of the concentration of windborne par-
ticles made by Chepil (1945 a-e, 1950 a,b, 1951 a,b) indicate that with salta-
tion movement, soil was carried at a height of less than 1 meter (m) above the
ground. For the several soil types investigated, more than 90 percent of the
soil was transported below the height of 30 centimeters (cm). Individual
particles which have been set into motion will strike other stationary par-
ticles and either rise almost vertically in the initial stage of the movement
in saltation, or eject other particles upward. These upward-moving particles
are drawn into the air stream and are carried along at the same horizontal
velocity while they begin to descend. Upon impact, they either rebound and
continue saltation movement, or they transfer their momentum to other particles
which are ejected and carried in the air stream in saltation or bounce along in
surface creep. The continued saltation movement is sustained by this impact-
ejection mechanism and not necessarily by the force of the airstream. Bagnold
(1941) concludes this on the basis of momentum transfer from the airstream to
the ejected particles. The equivalent of a steady counterforce or drag which
resists the flow of air, causes the wind velocity near the surface to diminish
to a level below the fluid threshold velocity, preventing further pickup by the
direct action of the wind.
Particles fine enough to be moved in suspension are usually undisturbed by
wind forces while they are on the ground. Chepil (1945a) found that quartz
particles less than 0.05 mm in diameter could not be moved by wind velocities
as high as 16.5 m/s (37 miles per hour) passing over the surface at a 15 cm (6
inches) height. However, when coarser grains ranging up to 0.5 mm in diameter
were included as a mixture, the smaller particles easily moved into suspension.
The larger particles in the mix are subject to wind forces and moved in salta-
tion, which in turn initiated the movement of the finer particles into sus-
pension. In a similar manner, grains in surface creep, which are too heavy to
be moved by direct air currents, are moved during impact with smaller grains
moving in saltation.
Newly deposited material behaves differently from the surface soil layer
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in its reaction to wind forces. However, as the surface is subjected to
weathering and aging, the deposited material becomes an integral part of the
surface soil and this surface behaves according to the concepts developed for
soil erosion.
Relationships among some of the factors associated with soil erosion have
been developed empirically. The surface soil layer consists of a mixture of
particle sizes and a minimum wind speed is required- to initiate soil movement.
This minimum speed is associated with a predominant particle size range and was
found to be unaffected by surface roughness (Chepil, 1945b). The rate of soil
flow, however, varies inversely with surface roughness. The erodibility, as
gauged by different measures, varied as the square-root of the apparent density
of the erodible grain or aggregates. (The apparent density is the density of
the removed material as opposed to the density of the soil bed). Under speci-
fied conditions of wind velocity and surface roughness, and given size and
proportion of non-erodible fractions, erodibility varied inversely as the
square-root of the equivalent diameter of the erodible fractions. Chepil
(1951a) defined the equivalent diameter as "the diameter of an imaginary quartz
grain having an apparent density of 2.65 and an erodibility equal to that of a
discrete soil grain or aggregate of some particular diameter and apparent den-
sity." The cohesive force of adsorbed water films surrounding the soil par-
ticles also affect erodibility. The rate of soil movement varied with changes
in the density of the air, the drag velocity, and the degree of wind gustiness.
PARTICLE-SIZE DISTRIBUTION
Trevino (1972) notes that among the parameters which most influence the
motion of soil by wind are the space and time variation of the soil particle-
size distribution. The soil particle-size distribution also influences the
particle-size distribution of the particles removed by various processes such
as resuspension, as reported by Slinn (1973a). He stresses the importance of
reporting simultaneous measurements of both size distributions (airborne and
surface soil) in future resuspension studies.
Gillette et al. (1972) similarly concluded that the size distribution of
aerosols resuspended from a ground deposit of soil is very similar to the size
distribution of the soil itself. For a variety of erosive field conditions for
particles in the size range of 0.3 to 0.6 mm, a common power law was suggested
for both the aerosol particle-size distribution and the size distribution of
the soil.
In considering particles of plutonium, some caution must be exercised.
Although it is currently unknown what fraction of the plutonium in resuspension
consists of unattached tracer particles and what fraction is attached to host
soil particles, it is suspected that a large portion of plutonium material can
be attached to resuspended soil particles. In a particle-size study using
impactor samplers, Sehmel (1975c) found plutonium on individual soil particles
to be much smaller than the impactor size diameter of the fraction collected.
The plutonium present, assumed to be PuC>2, had particle diameters of less than
0.25 mm.
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III. RESUSPENSION MODELS
There have been many studies on the resuspension of material previously
deposited on a natural surface. Some of the works have provided the basis for
the development of empirical models used for estimating the average concen-
tration of airborne material. Other models have been developed from theoret-
ical considerations of the movement process.
Sehmel and others (Sehmel and Orgill, 1973; Sehmel and Lloyd, 1974b and
1975b) have reported work directed towards developing general models for pre-
dicting particle resuspension from different environments. Based on field
measurements and data from other investigators, and on reported monitoring
program data, Sehmel et al., assembled information applicable to resuspension
models for the environment surrounding the Rocky Flats Plant in Colorado. As
a first approximation, plutonium resuspension was assumed to be similar to the
physics of soil erosion. In soil erosion, the amount of material removed is
proportional to the cube of the air velocity when the air velocity is above a
threshold velocity. In .the case of plutonium resuspension, it is postulated
that the amount of material resuspended is a function of the air velocity to
some unknown power when the air velocity is above some unknown threshold ve-
locity. Using data collected at a specific sampling station during the 6-month
period of July 1970 to January 1971, the wind-caused plutonium resuspension in
femtocuries per cubic meter (fCi/m3> was found to be:
X = 0.45CU)2-1
In the preceding formula, u is the air velocity in miles per hour averaged over
a 1-hour period. The average airborne concentration for a 1-hour period is a
function of the dominant winds — in this case, westerly and southwesterly.
The constants were determined by a least squares analysis of the predicted and
measured weekly average concentrations. Sehmel et al.., note that although
theoretical predictions suggest airborne concentration to be proportional to
(u)2'" compared to -the experimental (u)2'1, modeling for other time periods and
sampling areas is required before any model generalization of this exponent is
warranted. It is also noted that this model predicted higher average concen-
trations than the measured weekly average concentrations.
The majority of measurements of airborne resuspended material reported in
the literature has been normalized by surface measurements which are presumed
to be the source of the resuspended material. This normalization is expressed
as a resuspension factor:
_ resuspended air concentration (activity/m3)
surface deposition Cactivity/m2}
Values of K reported in the literature were tabulated in reports by
Mishima (1964) and Stewart (1967). These values show a range of several
orders of magnitude. Anspaugh et al., (1974) have suggested a model for pre-
dicting the airborne concentrations of resuspended contaminants over long
periods of time by making the resuspension factor a function of time. A time-
dependent resuspension factor was first formulated by Kathrens (1968), and
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again by Langham (1971)/ as follows:
K(t) = K exp(-At)
o
For this expression, Kathrens1 value of half-time (I/A) was 45 days and
Langham's estimate was 35 days. Anspaugh et al., (1974) point out that for
several weeks after deposition, the above expressions adequately agreed with
observations. For aged deposits, the predicted concentrations of resuspended
materials were seriously underestimated. Accordingly, Anspaugh et al., have
proposed an alternate expression for a time-dependent resuspension factor which
should be applicable to reentrainment processes around aged sources. The
following constraints were imposed on the formulation:
1. The apparent half-time of decrease during the first 10 weeks
should approximate a value of 5 weeks and should nearly
double during the next 30 weeks.
-14 -1
2. The initial resuspension factor should be 10 m
3. The resuspension factor 17 years after the contaminating
event should approximate lO'^m"1 (based on the longest
period of post-deposition measurements of plutonium
resuspension).
The Anspaugh et al., model is as follows:
K(t) = 10~ exp(-0.15 days ~ /t) + 10~ [m~ ]
In the above, t = time in days. Note that this model was derived to
simulate experimental measurements, and, as Anspaugh et al., point out, the
model contains no fundamental understanding of the resuspension process. It
was derived for the prediction of long-term averages of airborne concentra-
tions, but has an initial value which is sufficiently high (10"1*) to account
for unusual disturbances. Anspaugh et al., point out that the model assumes
resuspension to be a local phenomenon with the air concentration dropping off
rapidly downwind of the deposited source. This is consistent with experimental
observations.
The time-dependent resuspension factor is used to determine the integrated
air activity over some specified time period. The formula is written as
follows:
A = vA /K(t)dt
o
The deposition velocity is v (normally written as v^) and AQ is the ini-
tial integrated air activity measured in activity-time per volume. The product
vA is the ground deposition.
Horst et al., (1974, 1975) have suggested concepts of a resuspension model
where there is a separation of the actual resuspension from the atmospheric
dilution and transport of the airborne material. Attention is given to the
total ground concentration components consisting of a portion available for
resuspension and one that becomes unavailable through soil fixation. Figure 1,
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redrawn from Horst et al., (1974), is a schematic diagram of these concepts.
Resuspension and soil fixation are assumed to be proportional to the concen-
tration of available material on the ground through a resuspension rate A and
a fixation rate a while the deposition velocity v relates the deposition of
material to the primary air concentration. An initial time-independent resus-
pension factor K can be defined in terms of the available concentrations. The
available ground concentration G and the total ground concentration S are
derived to be as follows:
= G exptv^K -A-ct)t
o do
and
S -
(v* -A-a)
d o
[a
At t = o, the time of initial surface contamination, the available ground
concentration is the total ground concentration (i.e., G = S ).
o o
TOTAL AIR CONCENTRATION
PRIMARY AIR CONC. X0 RESUSPENDED AIR CONC, x
T
DEPOSITION. V .X RESUSPENSION. AG REDEPOSITION. V X
do i i d r
AVAILABLE GROUND CONCENTRATION. G
SOIL FIXATION, a G
FIXED GROUND CONCENTRATION
TOTAL GROUND CONC. S
Figure 1. Schematic diagram of resuspension model concepts, redrawri from
Horst, Droppo, and Elderkin (1974).
Horst et al., noted the consistency of their formulation with observations.
For example, the exponential decay of the available ground concentration de-
pends on the sum of the resuspension and fixation rates less the redeposition
rate.
Two situations were considered in this formulation. The first situation
applies to soils showing small total ground concentration losses. In the
8
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second situation, the material is distributed uniformly over an area beyond the
transport distance of the surface-air exchange process and resuspension must
balance redeposition.
It has been shown for the first situation that less than 10 percent of the
material was lost by resuspension based on observations at a particular site
over an extended period. When a uniform primary air concentration x is pro-
duced by a continuous trace level release of material to the atmosphere, Horst
et al., found that after several half-lives of decay of the available ground
concentration, the ratio of the resuspended air concentration x to the primary
air concentration was as follows:
xr
— = v,K /a
XQ do
This ratio could range from 5 to 5 x 10^.
In the case where resuspension balances redeposition, the ratio of resus-
pended air concentration to primary air concentration becomes:
XT-
— = A/a
The measured range of this ratio is between 0.05 and 0.5.
In a later report, Horst and Elderkin (1975) extended the model to account
for radioactive decay of the hazardous material and allowed a lower limit for
the resuspension factor. After testing this revised model, Horst and Elderkin
concluded that the evaluation of the hazard of environmental plutonium release
is affected only by a source which emits continuously for an extremely long
period.
Some preliminary efforts by Slinn (1973a) to develop initial resuspension
models were made from two standpoints. The vertical transport of materials
dislodged was first described using the convective diffusion equation. An ex-
pression was then derived, which is subject to several assumptions and boundary
conditions, for the number of particles per unit volume of some radius r
located at some heights as follows:
f(r,z) =
In the preceding formula, v is the magnitude of the particle-setting velocity,
u# is the friction velocity, and C is an unknown parameter. Slinn notes sev-
eral limitations of this approach and points out the poor predictive ability
of the formula when compared with plotted data from the literature. In a later
report, Slinn (1975) suggests that the best available procedure for obtaining
the vertical flow of particles is the use of experimental data to estimate the
horizontal flux as a function of altitude. He cites as an example Chepil's
work which shows how the weight of dust in a unit volume of air varies with
height.
Slinn's 1973a report also examines resuspension from a statistical ap-
proach, but concluded that this approach was even less valuable than via the
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convective diffusion equation. One of the major difficulties is that many of
the statistical properties are unknown. However, even if these properties were
better known, Slinn acknowledges that the statistical analysis approach is not
without serious difficulties.
From another strictly theoretical viewpoint, Trevino (1972) uses power-
spectral analysis of random wind-speed fluctuations to develop a mathematical
model of wind-induced motion of ground deposits. Spectral analysis is con-
cerned with the splitting of a time series into different frequency components,
and, as applied to the wind, allows an inclusion of its effects on the resus-
pension process.
Trevino's (1972) formulation of the total loss at any coordinate at any
time is as follows:
The variable h.(t,t') defines the infinitesimal loss at the time by some
removal process P. (saltation or suspension) due to a unit impulse or gust of
wind applied at time t'. The wind which initiates the corresponding P. motion
is f. Trevino observes that since f is a random function of time and as such
has an indeterminate form, the integral equation above cannot be solved in its
present form. Trevino applies the method of power-spectral analysis to rewrite
the equation in a form amenable to solution as:
|2(|)f(a))da)
P. is understood to be the total mean-square contamination loss and <(>£ is
the frequency spectrum of the wind. |H^(t,(u) | is the absolute value of the
contamination loss at the time t by P. due to a sinusoidal wind of frequency u.
Some known methods of determining H and are described by Trevino. Note that
Trevino's model accounts for the random nature of the wind, and furthermore,
the system under consideration is time-dependent.
Another theoretical model was developed by Amato (1971, 1976) which math-
ematically describes the prediction of long-range, time and space redistri-
bution of surface contamination by wind in a one-dimensional flow pattern. By
considering a series of continuous intervals, Amato writes a first-order
differential equation to describe the contamination concentration as a function
of time within each interval. A Button-Chamberlain diffusion model was used
for air concentration computations. In Amato's 1976 report, the model was
extended and theoretical resuspension ratios were calculated. According to
Amato, the calculated ratios indicate "the steady state effects of a continuous
source on the relative amount of downwind air concentrations moving directly,
or by means of resuspension."
In Travis' (1976) development of the redistribution of deposited material,
he assumes contaminants and host material have the same properties. Horizontal
flux formulation is based on soil erosion work of Bagnold and Chepil and the
vertical flux component is based on Gillette's work which Travis referenced.
Vertical emissions are discrete and periodic; they diffuse downwind and are
10
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characterized by a three-dimensional Gaussian diffusion model. The potential
hazard from wind-eroding toxic materials is determined through application of
the model.
IV. DISCUSSION OF IMPORTANT PARAMETERS
*
Many parameters used in developing characterizations of materials movement
in the atmosphere are critical in the resuspension model developments. Some of
these parameters are discussed here in more detail relative to their impor-
tance, limitations, and measurability.
RESUSPENSION FACTOR
In proposing a time dependent resuspension factor for calculating airborne
concentrations of resuspended material, Anspaugh et al., (1974) were aware of
the limitations of this parameter. For example, relating air activity measured
over a certain area to ground deposition measurements at the same point is not
entirely justified when data show resuspension factors in low deposition areas
are greater than in areas of relatively higher deposition. Furthermore, the
resuspension factor neglects many important variables such as wind velocity,
surface roughness, the physical and chemical characteristics of the soil
surface, and vegetation cover.
-2 -13 -1
Measured values of the resuspension factor range from 10 to 10 m
The large values were measured for indoor air above mechanically disturbed fine
material on a newly painted concrete floor, while the lower range was from
measurements of natural turbulence of aged plutonium on desert soil. Although
this wide range may limit the value of the resuspension factor as a predictive
aid, Horst et al., (1974), Anspaugh (1974), and others point out that this
approach does compensate for one important source of variability in measured
air concentrations by normalizing these to the potential source strength.
However, Sehmel and Lloyd (1975a). Mishima (1964), Stewart (1967), and others
state that the amount of airborne material would tend to depend upon the extent
and level of contamination upwind rather than upon local surface contamination.
It is observed that despite the shortcomings of the resuspension factor,
investigators continue to report results in terms of this parameter. One way
to improve this parameter, as reported by Slinn (1975), is to relate soil
erodibility to the resuspension factor. Chepil (1950a,b), Woodruff and Siddo-
way (1965), and others/ have worked to develop a general functional relation-
ship which defines the rate of soil loss in terms of influencing variables.
Slinn transforms this formulation with the aid of a Gaussian plume model
evaluated at a distance downwind of the field so that the product o o ap-
proximates the area of the contaminated area to yield the resuspension factor.
This is stated as follows:
-9 -1
3.5 x 1Q cm-s rE
1 ton acre-lmo-1 (2ir u6)
E is the erodibility or soil removal rate. Slinn identifies the parameter r as
the fraction of the horizontal flux which travels at the height of the sampler.
11
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The mean wind speed at the sampler height is u and 6 is the depth of penetra-
tion of the contamination into the soil.
RESUSPENSION RATE
The resuspension model concepts of Horst et al., (1974) show the flux of
material from the ground to the air to be proportional to the available ground
concentration through a resuspension rate X(s~l). However, only a few field
measurements of this parameter have been reported in the literature. Sehmel
and Lloyd (1974a, 1975a) have conducted several field experiments to determine
resuspension rates using tracer particles. For ZnS particles on asphalt, the
range of values was reported on the order of 5 x 10"9 to 60 x io~9 fraction
removal per second in wind speeds of 0.9 to 4 m/s(2 to 9 mph). Calcium molyb-
date deposited on sandy soil had resuspension rates on the order of 2 x io~ s~
in the wind range of 1.3 to 3.6 m/s (3 to 8 mph), and upwards to 220 x lO"1^"1
in 5.8 to 20 m/s winds (13 to 45 mph). A summary of measurements by Sehmel et
al., is presented in Table 2.
The significance of the resuspension rate is that it accounts for how
rapidly material is removed from the surface and transported downwind. Sehmel
et al., (1974) found this parameter to increase rapidly with wind speed. Above
3.6 m/s, the resuspension rate was estimated to increase with wind speed to the
6.5 power.
Although there are only a few measurements of resuspension rates reported
in the literature, the measurements of Sehmel et al., summarized in Table 2,
indicate some apparent relationships between resuspension rates and resuspen-
sion factors. Sehmel cautions, however, that there are insufficient data to
draw general conclusions on these relationships and the data only suggest a
comparability of resuspension factors between 10~10 and 10~9m~1 to resuspension
rates between 10~1C' and 10~8 fraction resuspended per second.
Further efforts to understand the limitations of the resuspension rate
parameter is reported to Sehmel (1975a) via some laboratory experiments using
10 ym diameter monodisperse uranine particles. The resuspension rate from a
smooth surface was measured under varying conditions. As a function of time,
the parameter decreased non-exponentially at a constant friction velocity. At
increasing air flow rates, the parameter increased. The values ranged from
10~6 to 10~3 fraction resuspended per second. Sehmel's comparison with field
data obtained elsewhere suggests a decrease in resuspension rates with in-
creasing surface roughness.
RESUSPENSION RATIO
Another approach to evaluating the relative significance of resuspended
material was introduced by Amato (1976) in calculations of theoretical resus-
pension ratios. As described previously, Amato's derivation is based on a
number of simplifying assumptions and considers the transport mechanism of
material into and out of a series of continuous intervals. The resuspension
ratio for each interval is calculated as the ratio of the portion of the air
concentration attributed to resuspension upwind to the portion of the air con-
centration due to contaminants arriving at the interval directly from the
source. Within a specified range of values for the dependent parameters,
12
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TABLE 2. RESUSPENSION RATES
Particles
ZnS
ZnS
Particle Data
Not given
Not given
Surface
Asphalt
Asphalt
Wind Speed
m/s (mph)
0.9- 4 (2-9)
1.3- 6.2 (3-14)
Average Resuspension
Rate Range — fraction
removed/second
5 x 10~9 to 6 x 10"
-5 -<+
1 x 10 to 7 x 10
Reference
2
2
ZnS
*TD =4.1 g/cc
MMD = 5 urn
NMD = 2 ym
Not given
Not given
Uranine 10 m diameter
CaMo^
(Disturbed surface
by walk through)
Asphalt
(Disturbed surface by
vehicular traffic)
Sandy soil
(lightly vegetated)
Sandy soil
(lightly vegetated)
Smooth surface
(laboratory experiment)
Not given
Vehicle speed
ranged from
2.2-22 (5-50)
10~5 to 10~2
1.3- 3.6 (3-8) 2.0 x 10
~10
5.8-20 (13-45) 2.2 x 10
~8
16.5-18.3 (37-41)
-6 -3
10 to 10
*TD = Theoretical Diameter; MMD = Mass Median Diameter; NMD = Number Median Diameter.
1. Sehmel (1972a).
2. Sehmel (1972c).
3. 3ehmel and Lloyd (1974a).
4. Sehmel and Lloyd (1975a).
-------
Amato's calculations show the following behavior:
"(1) The steady-state surface contamination decreases
with both increasing distance from the source and
increasing wind speed.
(2) The largest values of the resuspension ratio, R,
occur near ground level during low wind speeds and
stable meteorological conditions, and when the effects
of weathering are minimal.
(3) The resuspension ratio increases with increasing
distance from the source."
These observations applied to downwind distances up to 700 meters.
Horst (1975a) made some extended calculations on resuspension ratios using
a surface flux model. In Amato's calculations, the Button-Chamberlain equation
accounts for dry deposition by substituting the original source-term with an
effective depleted source-term. Horst1s formulation diminishes only the por-
tion of the plume adjacent to the surface. Estimates of surface air concentra-
tions and, hence, the deposition flux are lower by factors of 2 to 4 for mod-
erately strong deposition by assuming surface depletion rather than source
depletion. The resuspension ratio derived by Horst is a function only of the
ratio of deposition velocity to wind velocity. Horst's resuspension ratios
concur with Amato's calculations and were calculated out to 105 meters down-
wind.
DEPOSITION VELOCITY
The removal rate of airborne material is an important factor in modeling
the resuspension process. Several of the models described earlier make some
assumptions of the value of this removal rate which is expressed as a deposi-
tion velocity. The ratio of the amount deposited per cm2 of surface per second
to the airborne particle concentration per cm^ at 1 meter above the surface has
units of length divided by time for the deposition velocity. One of the dif-
ficulties in specifying the value of this parameter is that the value will vary
depending on particle characteristics, surface characteristics, wind speed, and
other meteorological conditions.
Van der Hoven (1968) summarized from the literature results of field
experiments for the determination of deposition velocities for 131I. These
were found to range over one order of magnitude. The summarized data suggest
that chemically active materials are deposited more readily than inactive ma-
terials, and that the presence of vegetation increases removal rates. Sehmel
and Schwendiman (1971a,b) suggest that this latter effect is caused by particle
interception by rough surfaces and by increased eddy diffusivity. Their
studies, using uranine particles, indicate dependence of deposition velocity on
the type of deposition surface, the particle diameter (2 to 28 ym used in their
experiments), and the friction velocity (11 to 44 cm/s). Controlled laboratory
measurements for the deposition velocity ranged from 0.06 to 12 cm/s for two
different surfaces. These measurements also indicated no correlation between
the deposition velocity and the average air velocity.
14
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In a later study, Sehmel, Sutter, and Dana (1973) found that the deposi-
tion velocity increased with air velocity for a constant particle diameter. A
minimum deposition velocity is associated with a particle diameter in the range
of 0.1 ym caused by a shift in the importance of eddy diffusivity and Brownian
diffusion in the transport process. Measurements by Sehmel and Horst (1972)
show that the minimum deposition velocities are nearly independent of atmo-
spheric stability.
In these several controlled laboratory studies, the measured deposition
velocities are directed toward predicting other values which can be used in
atmospheric transport models. There is good evidence from the above studies
that data obtained from wind tunnel studies can be applied to atmospheric cal-
culations .
To determine deposition velocities of resuspended particles, Sehmel
(1972a,b,c) conducted field experiments using ZnS particles deposited on as-
phalt surfaces. The range of values were a minimum of 0.4 cm/s to a maximum of
17 cm/s. The particle size characteristics in relation to the deposition
velocity were not determined.
V. METEOROLOGICAL CONSIDERATIONS
Meteorological factors which influence the resuspension of material from
ground deposits are wind characteristics, and moisture as it affects ground
surface conditions. The amount of material that can be carried in the air
currents is dependent on the density, velocity, and viscosity of the air. The
force exerted on the particles is directly proportional to the density and
viscosity of the air and varies with the square of its velocity (Chepil,
1945c). In common natural conditions, the viscosity of the air is independent
of atmospheric pressure and has a minor variation with temperature. Tempera-
ture, pressure, and humidity determine the density of the air. Moist air
consists of dry air and water vapor and is shown from gas law relationships to
be lighter than dry air at the same temperature and pressure. Thus, the
erosive force of moist air currents is lower than that of dry wind.
Fixation of material can occur at a deposition site and it has been shown
that fixed material will be dispersed in a surface layer several centimeters
deep. Stewart (1967) reported on experimental results obtained in field tests
which indicated continuous changes in surface layer characteristics such that
a simple relationship between resuspended material and wind speed had limited
validity. For example, in Healy and Fuquay's 1959 study using a zinc sulphide
particulate on a variety of different surfaces, the amount of airborne material
was found to be a function of the square of the wind speed. To have this
simple u2 or u3 relationship, Stewart suggests that the contaminant must be
very finely-divided submicron particles which are insoluable in water, and the
surface must be in equilibrium with only a minor degree of soil movement
occurring. The host material on which contaminants adhere may not have the
ideal characteristics to show an obvious correlation between resuspended mate-
rial and various wind parameters. In Volchok's (1971) report on elevated
Plutonium levels in surface air near the Rocky Flats Plant, there was a
15
-------
qualitative indication of some correlation between the plutonium air concentra-
tion and some wind parameters, such as mean wind speed, peak gusts, mean weekly
gusts, and number of hours in the sampling period which the wind exceeded
various speeds. Volchok's example of concentration versus wind speed for one
week sampling periods showed reasonable correlation (r = 0.83) for the fall
data and a poor correlation (r = 0.18) for the summer data. The better corre-
lation in the fall data was suggested to be attributable to the higher average
wind speeds experienced in that period and, further, suggested a threshold
average wind speed above which good linear correlation is evident.
Gross measurements, such as airborne dust concentrations, have been made
using different combinations of particle size and wind speed. For several of
these combinations, a nonlinear relationship between airborne dust concentra-
tions and increasing wind speed is inferred. Data from Sehmel (1975b) show
these dust concentrations range in proportionality from 0.6 to 3.2 power of
wind speed. For example, above 4 m/s (9 mph), a cubic relationship held. An
order of magnitude increase in particle concentration is observed in different
particle size ranges for increases in wind speed from 1.3 to 9 m/s (3 to 20
mph). According to Shinn et al., (1974), the dust concentration can sometimes
vary with the sixth power of the wind speed.
Improvement in Trevino's (1972) model takes into account the random wind
effects on particle transport and its effects on deposition and emphasizes two
important meteorological dependencies: soil transport and particle deposition.
Soil transport by wind is dependent on the wind velocity distribution with
height, and suspended particle deposition is dependent on the time variation of
the mean wind speed. Chepil (1951b) cautioned, however, that the effects of
surface roughness on the velocity distribution above the ground must be known
for wind velocity measurements to be meaningful. For this reason, the drag
velocity is suggested as a better indicator of the force exerted by the wind at
ground level. The drag velocity V^ is formulated as follows:
v
V, =
* 5.75 log(z/k)
In the above formula, v is the wind velocity at any height z, and k is the
height at which the extrapolated wind velocity is zero. If the velocity is
plotted against the logarithm of height, the drag velocity determines the slope
of the wind velocity distribution. A relationship of the relative amount of
erosion to the drag velocity has been shown by Chepil (1951b) to be a power
relationship. Chepil indicates that the power relationship is not completely
uniform but varies with soil conditions and surface roughness, as well as other
factors. However, in some wind tunnel and open field tests with both erosive
and nonerosive soil fractions, Chepil (1945) showed that the rate of soil
movement varied with the cube of the drag velocity and the degree of gustiness
of the wind. He further pointed out that the thermodynamic relationship among
the air parameters indicates only a minor effect of air density changes on
erodibility. For example, a 10°C decrease in temperature increases air density
which in turn increases the wind force, but only by a small percentage. An
increase in pressure would have a similar, minor effect.
Among the several parameters which characterize the resuspension process,
16
-------
some correlation with meteorological factors is evident. Resuspension rates
were found to increase with increasing wind speed (Sehmel and Lloyd, 1975b)-
For respirable particles, the resuspension rate increased from 2.0 * 10~
fraction resuspended per second with a wind speed interval of 1.3 to 3.6 m/s to
2.2 x 10~8 for 5.8 to 20.1 m/s. The tracer material in these tests was depos-
ited on a lightly vegetated area of about a 23-m radius. For undisturbed
soils, the resuspension rate is a nonlinear function of the wind speed, in-
creasing by the 6.5 power for wind speeds greater than 3.6 m/s. In areas where
mechanical disturbances due to human activities occur, a similar increase in
resuspension can be expected with a greater availability of contamination for
resuspension.
Sehmel and Lloyd (1975a) also reported similar behavior of the resuspen-
sion factor. This might have been expected based on earlier work relating the
resuspension factor to resuspension rates. The parallel behavior of these two
parameters is illustrated by Sehmel and Lloyd (1975a) in Figure 2.
uj a
«• in
M« IU
RESUSPENSION
RATE —
FRACTION/S
SLOPE 6.5
RESUSPENSION FACTOR AT
1.8m HEIGHT, m-1
DATA POINTS PLOTTED AT LOWER
LIMIT OF WIND SPEED INTERVAL
WIND SPEED, m/s
Figure 2. Resuspension rates and resuspension factors as a function of wind
speed. Sehmel and Lloyd (1975a).
Amato (1976), in introducing the resuspension ratio, also presented data
on the meteorological effects on this ratio. He noted that the largest values
were observed near ground level during low wind speeds and under stable meteor-
ological conditions, and when past weather least affected the surface layer.
Other meteorological parameters can affect suspended particles. Wagman et
al., (1967) reported a trend toward increasing sulfate particle size with
increased relative humidity at several urban areas. However, they caution that
a theoretical humidity dependence for comparison with their results cannot be
17
-------
inferred because of certain conditions associated with their measurements.
Specifically, the system studied consisted of heterogenous particles of un-
known composition and structure which contained varying amounts of sulfate.
Wagman et al., (1967) further reported poor correlation of sulfate par-
ticle size with absolute humidity. However, when atmospheric sulfate concen-
trations were considered, good correlation was found with absolute humidity
whereas poor correlation with relative humidity was evident.
In Hagen and Woodruff's 1973 work, visibility could not be correlated with
various powers of wind speed. They suggested that particulate concentrations
can be estimated from visibility measurements during dust storms when low
humidity and particle-size distributions are relatively constant.
While it was noted that the very important parameter, deposition velocity,
correlated well with friction velocity, Sehmel and Schwendiman (1971b) found
that in general, the average air velocity does not correlate with the deposi-
tion velocity. In the case of a constant particle diameter, v does increase
with air velocity. It was further noted that a minimum deposition velocity is
characteristic of a given particle diameter. Sehmel and Horst (1972) concluded
that the minimum deposition velocities are nearly independent of atmospheric
stability.
The vertical stability of the atmosphere does determine dust devil activ-
ity, a meteorological phenomenon described by Sinclair (1969) as thermal up-
drafts initiated by dry convective currents and which later develop into a
vortex of sufficient intensity to pick up surface debris. While dust devils
may only occur in a limited geographical area, their potential for transporting
toxic or hazardous materials resuspended from contaminated surfaces in these
geographical areas could be a critical problem. The visible portion of the
dust devil rarely exceed 600 m (2,000 feet) (Sinclair, 1969), but a thermal
plume may extend upward to 5,000 to 6,000 m (15,000 to 18,000 feet) mean sea
level (Sinclair, 1973). An estimated 2,700 metric tons of desert dust and sand
per 100 km2 area may be picked up and transported downwind by dust devils over
an average season (based on observations by Sinclair at two 100 square miles
area sites). Very fine particles may be suspended in the upper portion of the
vortex and could conceivably be transported large distances by general circula-
tion in the troposphere.
VI. SUMMARY
The resuspension of material from ground deposits is a complex process.
While a considerable body of work has been reported on studies related to the
resuspension process, a satisfactory mathematical formulation of the process is
not evident. Deficiency is evident in some of the physical parameters which
have proven to be important in the characterization of materials movement in
the atmosphere. For example, only a limited number of measurements have been
reported on the resuspension rate parameter.
There appears to be some understanding of the meteorological factors which
18
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influence the resuspension of material from ground deposits. The effects of
wind and moisture on credibility depend on the particle size and density of the
erodible material. Some correlation with meteorological factors is evident
amoving the several parameters which characterize the resuspension process. This
too depends on the particle size and density of the material for which the
parameter measurements were made.
Some studies have been made about the effect of diffusion of plutonium
through the soil (Horst et al., 1974 and Horst and Blderkin, 1975). However,
measurements are needed to better characterize this diffusion process.
While the primary focus of this review was on the meteorological factors
affecting the resuspension process, some discussion on resuspension models and
parameters was included. It is beyond the scope of this review to discuss the
assets and liabilities of various diffusion models or to compare the advantages
and/or disadvantages of and relationships among the several resuspension
parameters.
19
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/4-77-037
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
RESUSPENSION OF PLUTONIUM FROM CONTAMINATED LAND
SURFACES: Meteorological Factors
5. REPORT DATE
July 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
P. N. Lem, J. V. Behar and F. N. Buck
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Environmental Monitoring and Support Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Las Vegas, Nevada 89114
1O. PROGRAM ELEMENT NO.
1FA628
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency—Las Vegas, NV
Office of Research and Development
Environmental Monitoring and Support Laboratory
Las Vegas, Nevada 89114
13. TYPE OF REPORT AND PERIOD COVERED
Final Report
14. SPONSORING AGENCY CODE
EPA/600/07
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A literature review is presented in a discussion of the relevance of
meteorological factors on the resuspension of plutonium from contaminated land
surfaces. The physical processes of resuspension based on soil erosion work
are described. Some of the models developed to simulate the resuspension of
materials for predicting airborne concentrations are reviewed. The significance
of some of the parameters used in the different models is also discussed. The
interplay of meteorological factors measured, discussed, or implied in the
literature reviewed as related to the resuspension process is discussed in the
final section.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COS AT I Field/Group
*Plutonium
*Soil Erosion
Radioactivity
Dust Particles
Wind Erosion
Particle Size Distribution
Resuspension Process
Transport Modeling
Particle Dispersion
Meteorological
Influences
04B
18H
20H
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
36
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
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