SWRHL-90r
PLANT RADIOIODINE RELATIONSHIPS
A REVIEW
by
James C. McFarlane and Benjamin J. Mason
Radiological Research Program
Southwestern Radiological Health Laboratory
U. S. Department of Health, Education, and Welfare
Public Health Service
Environmental Health Service
July 1970
This study performed under a Memorandum of
Understanding (No. SF 54 373)
for the
U. S. ATOMIC ENERGY COMMISSION
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LEGAL NOTICE
This report was prepared as an account of Government sponsored
work. Neither the United States, nor the Atomic Energy Commission,
nor any person acting on behalf of the Commission:
A. makes any warranty or representation, expressed or implied,
with respect to the accuracy, completeness, or usefulness of the in-
formation contained in this report, or that the use of any information,
apparatus, method, or process disclosed in this report may not in-
fringe privately owned rights; or
B. assumes any liabilities with respect to the use of, or for damages
resulting from the use of any information, apparatus, method, or pro-
cess disclosed in this report.
As used in the above, "person acting on behalf of the Commission"
includes any employee or contractor of the Commission, or employee
of such contractor, to the extent that such employee or contractor of
the Commission, or employee of such contractor prepares, dissemi-
nates, or provides access to, any information pursuant to his employ-
ment or contract with the Commission, or his employment with such
contractor.
008
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SWRHL-90r
PLANT RADIO IODINE RELATIONSHIPS
A REVIEW
by
James C. McFarlane and Benjamin J. Mason
Radiological Research Program
Southwestern Radiological Health Laboratory
U. S. Department of Health, Education, and Welfare
Pub Iic Health Service
Environmental Health Service
Environmental Control Administration
Bureau of Radiological Health
July 1970
This study performed under a Memorandum of
Understanding (No. SF 54 373)
for the
, U. S. ATOMIC ENERGY COMMISSION
-------
TABLE OF CONTENTS
Page
LIST OF FIGURES ii
LIST OF TABLES . 111
I. INTRODUCTION 1
II. DEPOSITION 4
A. PHYSICS AND CHEMISTRY OF IODINE FALLOUT 5
B. HUMIDITY 12
C. SPECIES 14
1. Plant growth habit. 14
2. Leaf morphology. 20
III. ABSORPTION OF IODINE BY PLANTS 22
A. ROOTS 22
B. LEAVES 24
IV. HALF-LIFE OF IODINE ON VEGETATION . 25
A. GROWTH 25
B. ABSORBED IODINE LOSS 26
C. ADSORBED IODI.NE LOSS 26
V. SUMMARY AND CONCLUSIONS 29
BIBLIOGRAPHY 31
APPENDICES 42
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LIST OF FIGURES
Page
Figure 1. Physical Form of 131I in Air at Various
Distances from the Source. 11
Figure 2. Air Movement in Alfalfa Field. 15
Figure 3. Percent Water in an Alfalfa Stand
Between Irrigations, 18
Figure 4. Decay of 131I2 from Contaminated
Alfalfa Plants. 28
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LIST OF TABLES
Page
Table 1. Important Radioisotopes of Iodine
Produced by Fission. 5
Table 2. Variation in the Percent Water of Four
Plant Species on Three Different Collections. 18
Table 3. Distribution of 131I in Bean Plants Growing in
Na131I Contaminated Hoagland's Solution. 26
111
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I. INTRODUCTION
The discovery of atomic fission brought with it many important health
problems. 'One of these is the possible contamination of air, water,
food, and forage with radioactive iodine. Since radioibdine may reach
man in many ways, it is difficult to discuss all routes in one review.
This report, which is primarily a review of the literature, up to
January 1969, discusses only one intermediate in the passage-of
radioiodine from the source to man*s food—the plant.
The two objectives of this report are:
1. To bring together in one document a summary of plant-iodine
relationships.
2. To provide data to aid in the design of experiments to broaden
our present understanding of the contamination of plants with
radioiodine.
It seems logical to start the consideration of the iodine problem
with a brief look at the possible sources of radioiodine. Quantities
of radioiodine may be released to the environment in several ways.
Among the most important of these are nuclear explosions (both atmos-
pheric and cratering devices), nuclear reactor operations, and reactor
accidents. Under certain conditions, especially in nuclear facilities,
chronic contamination may exist. In 1959, the National Committee on
Radiation Protection (NCRPr84^ suggested that 9 .X 10"9 yCi/cc of air
was the maximum permissible concentration (MPC) of 131I allowable in
a nuclear facility. Although this level is very important in con-
nection with industrial operations, it is generally of no direct
consequence to the public.
Most releases of radioibdine to the environment are of short duration
lasting from a few minutes to a few days. Under these conditions one
1
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critical pathway of radioibdine is the air-forage-cow-mi Ik-man route/ '
In 1964, the Federal Radiation Council (FRC) ^ ' recognized the thyroid
as the critical organ for the radionuclides of iodine. Since milk is
one of the main vectors of radioidoine to the thyroid the FRC recommended
that children be considered the critical segment of the population.
The concern with milk as a route of 131-iodine to man is based on the
70 kg (fresh weight) of green alfalfa or grass per day, Radioiodine
is therefore important, not because of the amount deposited on the
vegetation but rather because of the efficient passage of radioiodine
through this food chain and its ultimate concentration in a child's
thyroid.
Although milk is considered as the main source of contamination, other
sources should be investigated. The following examples may best
illustrate the relationship of contamination by milk versus contami-^
nation from leafy vegetables. During a period of atmospheric testing
(1962), contamination in the milkshed of Salt Lake City was from
300 to 2000 pCi/liter. At the same time the concentration of leafy
tg\
lettuce reached 2800 pCi/kg fresh weight^ . To receive the same
amount of activity as that received from one liter of milk, consump-
tion of approximately 700 grams of leafy lettuce (not head lettuce)
would have been required. A large dinner salad contains approximately
100 grams of lettuce. Since it is more probable that a person would
drink one liter of milk each day than eat seven large dinner salads,
milk is clearly a more important source of radioiodine contamination.
However, it is clear that green vegetables may become contaminated and,
therefore, cannot be neglected as a contributing source to man's total
radionuclide intake. ,Thompson . suggested that as much as
20 to 40 percent of the possible 131I contamination may be attributed
to products other than milk in some non-urban adult population.
The same principles of plant contamination still exist, whether plants
are eaten by man or by a cow. There will certainly be differences in
-------
the quantity of plant material consumed and the method of food prepa-
ration, but the physiological and morphological principles which
control the uptake and retention of iodine by plants will apply in
both food crops and forage plants. The overall objective in studying
plant-iodine relationships is to allow predictions of possible
human ingestion of 131I. Two main questions are of concern in this
report.
.1. What is the rate of deposition?
(To describe deposition, it is necessary to understand
differences caused by species variation, environment,
and the form of contamination. To evaluate this, plant
morphological and physiological factors which control
the rate of deposition on and movement into the various
parts of plants must be understood.)
2. How long does the contamination remain?
(Variations caused by the chemical and physical states of
fallout and also the effect of various environmental
parameters such as wind, rain, temperature, and humidity
on the loss of iodine from plants must be evaluated,
Differences caused by plant morphology and physiological
parameters such as foliar absorption and translocation
must be considered in order to completely evaluate the
radioiodine-plant relationship. To be able to predict
the transfer rates of iodine to cows, it is necessary to
have some insight as to the chemcial .changes in the form
of the iodine which occurs in or on plant surfaces.)
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II. DEPOSITION
The kinetics of iodine deposition on plants was first studied by
Chamberlain and Chadwick in 1953^ '. In their research they
recognized the need to express the amount of plant contamination
in relationship to the radioactive cloud. The term they defined (V )
was simply a ratio between the amount of activity deposited on a
horizontal surface per unit of time to the amount of activity in
a volume of contaminating air.
„ . . . deposited activity/cm -sec
Vg (cm/sec) = activity/cm' of air
The area of deposition in this equation is considered to be that area
of ground which may be completely or partially covered by vegetation.
The resulting units were the same as velocity (distance/time); there-
fore, the term was originally called "velocity of deposition." Since
that time, the term deposition velocity has been used by many investi-
gators to describe the contamination of plants by iodine. This
generalized equation is, however, useful only to the extent that the
parameters which control the transfer of iodine from air to a surface
are understood.
The transfer of radioactive fallout from the air to the surface of
plants and soils is a complex phenomenon. Although this phenomenon
is not well understood, it can best be studied by dividing it into
two fields on interest. The first is the study of the contaminant
from the time it is produced until it reaches the location of deposi-
tion. This area of study involves the fields of aerosol physics,
meteorology, and microclimatblogy. The second area of interest 1s the
study of those plant factors which have a modifying effect on the
environment and thereby influence the rate of deposition. An investi-
gation of these factors involves the study of plant morphology and
physiology.
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A. Physics and Chemistry of Iodine Fallout.
To understand iodine contamination of plants, it is important to
first understand something about the manner in.which iodine is
released and how it reacts in the atmosphere. All methods of iodine
release to the atmosphere involve high temperatures (in nuclear
detonations) or powerful solvents (in reactor fuel reprocessing)
and result in the evolution of isotopes of iodine as gasest43.
Although the fission process produces some 131-iodine directly,
the majority of the 131-iodine is derived from the precursors,
131-antimony and 131-telluriunr '. When fission products are
released to the atmosphere, the ingrowth of 131-iodine into the
cloud is rapid because the two precursors mentioned above have
Othei
,(30)
half-lives of only 23- and 25- minutes respectively^ . Other
isotopes of iodine are also produced in the fission chain'
An inspection of Table 1 shows that 132I through 135I are produced
in greater quantities than 131I. A look at the half-lives
quickly shows why they are generally thought to be of secondary
importance in the contamination of milk. Iodine-133 with a half-
life of nearly one day, is present in such large amounts that it
cannot be overlooked as a possible health hazard. However, most
attention has been given to 131I since its half-life is longer
and, after a short time, it is the main isotope of iodine found
in fal]out.
*
Table |. Important Radioisotopes of Iodine Produced by Fission.
Nuclide Half-life Fission Yield (%)
131I 8.05 days 2.9
132I 2.26 hours 4.4
133I 20.9 hours 6,5
13l*I 54 minutes 8.0
issi 6.75 hours 6J .
*Adapted from Bolles and Ballou, 1956;(7) Radiological Health Hand-
book, 1960 (30),
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Hhen a nuclear cratering device is exploded, a cloud is created which
includes fission products, activation'products, and part of the inert
material surrounding the detonation site, Partiiculate fallout is largely
the result of the attachment of radioactive hue!ides to small particles
of the inert or carrier material. The attachment of any particular nuclide
is determined by physical and chemical properties of both the particle
Tgg)
and the contaminant. Shleien suggested that attachment of nuclides
to particles of different sizes may vary with respect to the half-lives
of their parents. To visualize this it is necessary to consider two
processes,
1. Size fractionation (the size distribution of fallout particles
diminishes as the time or distance from the detonation increases).
2. Radioactive decay (nuclides with short-lived precursors have
a faster ingrowth rate than those having long-lived precursors).
These factors combined with the differences in the half-life of the
various radioisotopes of iodine (see Table 1) had an effect upon the
ratios of 133I to 131I in the fallout from a test conducted at the
Nevada Test Site.
In April 1966, the Atomic Energy Commission's (AEC) Project Pin Stripe
accidentally released a small amount of contamination from an under-
ground test. An experiment (unpublished) conducted by this laboratory
showed that the ratio of 133T to 131I deposited on plants decreased as
the distance from the hot line Increased. Since the half-lives of the
precursors of 133I are much shorter than those of 131I, it is under-
standable that the former was more plentiful in the region close to the
cloud center.
Radioactive fallout consists of a complex mixture of radioactive gases
and various sized particles. The meteorological parameters that control
mixing, movement, and dispersion of a fallout-cloud determine when and
to what rate portions of th.e cloud reach the surface of the earth. At
the surface, micrometeorological factors control the deposition of the
radioactive contamination. The physics of the actual removal and retention
process of small particles and gases is complex and therefore not well
understood.
6
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(38)
In 1966, Fisher^ ' reviewed the subjects of deposition of iodine vapors
and cQntam.inaitid aerosols on plants. Hi suggested that the
atrodynamle factors whleh control deposition of both aerosols and vapors
at the air-ground interface are gravity, electrical forces.thermal
forces, Brownian motion* molecular diffusion-, and turbulence. Describ-
ing the deposition of particles, Chamberlain^ ' considered the primary
transport mechanism in the turbulent boundary layer to be eddy diffusion
with molecular diffusion being effective over the last few millimeters.
Outside the boundary layer the particles are subject to the well known
Stokes' relation for terminal velocity and as a result possess a
constant downward velocity. Considering the various parameters,
/ 00\
Fisherv ' developed a model for the deposition of vapors and aerosols
on leaves. Using this model,, he compared some of the reported values
to the theoretical predictions. He found that he was only accurate
to within a factor of two for aerosols and a factor of three for vapors.
His model holds that increased wind speed causes a corresponding
increase in the deposition velocity and that an increase in particle
size from 0 to 20 ym causes an increase in deposition velocity from
0.25 to 3.0.
The difference between the observed deposition velocity and that
predicted by Fisher may possibly be explained by understanding more
about the morphology and physiology of the plant surface. The velocity
of deposition and retention of particles on leaf surfaces is largely
dependent on the thickness of the boundary layer of air at the surface
(3 17 22}
of the plantv ' * '. The thickness of this boundary layer is
influenced both by physical factors and plant factors, Hhere the
boundary layer is thin, contaminants are deposited in greater abundance.
This could explain why iodine is often concentrated at the margins of
(20 54}
1 eavesv ' '. High velocity wind reduces the boundary layer to a
very thin film next to the leaf surface. In contrast, low velocity
winds allow the boundary layer to thicken. The morphology of the leaf
surface also determines the shape and extent of the boundary layer,
Epidermal hairs act the same as a windbreak in reducing wind speed
close to the surface. This reduction in speed causes the boundary
layer to thicken, i.e., a leaf which is covered with epidermal
-------
hairs has a thicker boundary layer than a smooth leaf. Thus, by
modifying the boundary layer, variations in morphology can be
responsible for variations in retention of airborne nuclides. The
physiology of the plant effects the boundary layer by changing the
rate of gaseous exchange, by changing the relative humidity adjacent
to the leaf surface, and by positioning the leaf in different
attitudes with relationship to the environment.
The rate of settling the deposition of iodine depends on the physical
(34V
form of the contaminationv '. One of the limiting factors in our
understanding of the physical form of the deposition is the difficulty
incurred in sampling the atmosphere in a•meaningful way.
Particulate contaminants are generally classified by one of three
methods: graded filter.casade impactor, or photographic methods,
Two types of filter systems are used for sizing particles. One type
uses filters with several different pore sizes placed in series.
Microfilters are very efficient for this type of determination since
they are essentially 100 percent efficient for particles larger than
their pore size. They are available in ten porosity grades from
(co\
0.01 to 5 ynr . Another system uses only one type filter and
measures the depth to which various sized particles penetrate into
this filter. Silverman^ ' used a polyester filter to collect and
classify particles from 0.002 to 0.35 ym diameter. One limitation
of both of these systems is the fact that the filter characteristics
change with time because of the particles which are collected on or
in the filter material. A particle may only partially cover a pore;
therefore, limiting the size of the particle which will pass through
that pore. Especially when the air has a high dust load, the
efficiency of the filter changes and thus particle sizing becomes
less accurate as the operation progresses.
Inertial particle or cascade collectors make use of the fact that
particles moving in anairstream tend to follow along their original
direction when the airstream is deflected by an obstacle. Impactors
collect the particle on the surface of the deflector. Impingers use
8
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the same principle but collect the particle in a liquid. By having
a series of impactors with a different airspeed at each step in the
sampler, particles of different sizes will be collected, Cascade
impactors are generally not efficient for parttcles less than 1 ym.
The final method of measuring fallout particles is via microscopic
measurement. Particles for these measurements are often collected
on planar surfaces and sized visually by the aid of a photograph taken
through the use of either a light or an electron microscope. Although
this is a very time-consuming and laborious task, there is one advantage
in this method over the previous two. In deposition studies we are
interested in the distribution of deposited or settled particles rather
than the total in the air sample. Since a planar collector resembles
the surface of a plant more than a higfuvoliime air sample, the ratio
of various~sized particles-on such a plate would be more representa-
tive of the distribution on a plant surface. (A plant is not, however,
simply a planar surface, therefore, some difficulties arise in inter-
preting this type of information.)
Detection of iodine gas is equally as difficult as detection of the
particulate materials and there is, perhaps, even more possibility of
error in this determination. The most commonly used method of classi-
fying fallout into its two major fractions (particulate vs gas) is
done by collecting the particles on an inert prefilter and the gases,
which pass the prefilter, on activated charcoal.
Since activated charcoal is nearly 100 percent efficient for the col-
lection of most of the gaseous iodines, the resolution of this system
is largely dependent upon the efficiency of the prefilter for removing
the particulate material from the air sampled. Small particles are
perhaps the biggest source of error in this system. To have a system
which collects all submicroiTsized particles on the prefilter generally
i • ' • ...
requires some type of graded filter system. Without this there is a
good possibility that much of the activity seen on the charcoal filter
is in reality very small particles rather than gaseous iodine,
Other possible sources of error are related to the adsorption of gaseous
iodine onto the prefilter and also to the possible revolatilization of
-------
some of the iodine from the charcoal during the period of time between
sampling and the time the filter is collected.
Kuhn^63' gives information on methods for analyzing air for very small
particles.
Keeping the above limitations of collection systems in mind, the follow-
ing material will examine the data in which particulate and gaseous
iodine has been investigated.
(33}
Eggleton, et al. , ' classified world-wide fallout from the Russian
atmospheric testing of 1961. This study, which used filters to size
the fallout, covered a three-month period following the tests. As
a prefilter they used a high-efficiency asbestos filter followed by
brass gauze for elemental iodine and charcoal-impregnated filter paper
for removal of certain other compounds of iodine. This filter system
was backed by a one-inch bed of activated charcoal. These authors
found that an average of 75 percent of the 131I contamination was in
particulate form.
Megaw^ ' contaminated the inside of a reactor shell with radioiodine
in order to study simulated conditions of a reactor rupture. He found
that from 40 to 80 percent of the iodine released had become attached
to particulate material within the first hour. A large amount had also
become attached to the containment vessel walls. Much of the particulate
iodine can be accounted for by very small particles called Aitken
nuclei^ '. These are particles of about 10 nanometers in diameter
which are formed in all combustion processes and are therefore generally
present in air., Aitken nuclei are so small that they are sometimes
thought to move in a manner similar to gases. Even though this may be
partly true, there are some differences which should be recognized.
Under the influence of gravity the Aitken nuclei has a settling velocity
which is different from that of a gas. The important difference is the
fact that the Aitken nuclei consolidate to form larger particles as
they are carried from the point of release. These particles and their
aggregates therefore do not respond to changes in meteorological
conditions in the same manner as a gas.
10
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The studies of Perkins' nave $hown that a large and varying fraction
(from 10 to 90 percent) of the radioiodine in fallout is in the gaseous
form. He tried to characterize the chemical form of the gaseous iodine
and found that very little, if any, exists in. the elemental or HI form.
However, the gaseous compounds of 131I in air were not identified. Up
to 20 miles, there was an increase in the amount of particulate iodine
contamination in a cloud with increased distance from the release
(Figure 1).
C
OJ
u
0)
o.
(U
o
•r-
•tJ
as
Q.
40
30
20
10 J
•i—»-
5 10 15 20 25 30
Distance from source
(miles)
Figure 1. Physical Form of 131I in Air at Various Distances
from the Source ^0).
Nishita^ has stated:
"The chemical and physical properties of fallout depend on
the energy yield of the nuclear device, the degree of inter-
section of the fireball with the ground surface, the mineral
composition of the ground surface, and the structural
material surrounding the device. Nuclear devices detonated
on or near the surface of the ground have been found to
yield predominantly siliceous fallout particles because
of the incorporation of soil into the fireball. Particles
from detonations at higher elevation more nearly reflect
the incorporation of the structural materials surrounding
the device. A large fraction (>50%) of the close-in fall-
out from nuclear devices detonated on steel towers at.the
Nevada Test Site was attracted to magnets and was red-
brown in color suggesting the formation of magnetite. By
comparison, the devices that were not detonated on steel
towers produced fewer magnetic particles (<10%). Devices
11
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fired at the Pacific Proving Ground produced fluffy conglom*
erates of CaCOs from coral and crystalline NaCl particles
from sea water, Thus, the chemical composition of fallout
may vary considerably depending upon the conditions of
detonation."
It seems obvious that the ratio of gaseous to particulate contamina-
tion and the size distribution of particles is dependent on the
amount of particulate material in the cloud and the distance or
time from release. Shlelerr99' found that airborne fresh fission
material occurred on larger particles and that the size of the
contaminated particles decreased with time. Most of the older
fission products were associated with particles having diameters
less than about 1.75 ym.
B. Humidity.
Humidity has been recognized as a factor which influences deposi-
tion of iodine* . Cline and Hungate* ' observed that moist
leaves of a plant species accumulated up to 2.2 times as much
i3i!2 as did dry leaves of equal area. The same was true with
moist paper but there was only a very slight increase in accumula-
tion noted for moist soil. Barry and Chamberlain offered two
possible explanations to account1for these observations. "It may be
that humidity was responsible for regulating the size of the stomatal
aperture. Alternately, adsorption of iodine on the external
surface of the leaf may have been in some way facilitated by
Conditions of high humidity."
Let us consider transpiration, not because of any suspected rela-
tionship with iodine diffusion, but rather because it might give
us some information on which to form a hypothesis. The rate of
transpiration through open stomates is regulated by the gradient
between the vapor pressures of water inside and outside of a leaf.
In the substomatal cavity the humidity remains very close to
saturation. The vapor pressure gradient is therefore determined
by the temperature of both leaf and air and the relative humidity
of the air. When the gradient decreases, the rate of transpiration
also decreases until at 100 percent relative humidity (RH)
transpiration stops.
12
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One would intuitively think that transpiration would also be
greatly influenced by the degree of stomate opening. This is,
however, not the case. Diffusion through the stomate is not
correlated to the area of the individual stomate but to the
(6?}
circumference of the opening1 . This means that small changes
in the amount of opening and closing are of little importance
in restricting diffusion.
With this in mind, let us consider how humidity might alter the
diffusion rate. Atmospheric humidity does not directly affect
the degree of stomatal opening. Under high atmospheric humidity
the gradient of pressure potentials between the atmosphere and
the substomatal cavity is decreased and the rate of transpiration
would therefore be suppressed. This would cause a decrease in
the rate of water loss and could, under conditions of water
deficits, cause a decrease in plant water stress. Under such
conditions greater tugor would result in the guard cells and
the stomatal aperture would open larger. This effect would
only occur under special conditions and would not be a general
response to increased atmospheric humidity. In addition to this,
it is not logical to presume that humidity may change the con-
centration of iodine in the air. A more probable explanation
would be related to the chemical properties of iodine in a humid
environment and the physics of deposition and retention of particles
and vapors on a moist surface.
The importance of humidity in the deposition of iodine on plant
surfaces is still undefined. There is no data available which
would allow a statistical evaluation of the effect of various
amounts of atmospheric moisture upon deposition. The effect of
humidity on retention and absorption of foliar-applied iodine is
likewise not understood. Since humidity has been documented as
one of the important factors in determining the rate of contamina-
tion, attention should be give to quantitating this effect.
13
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Species,
The bas.is of plant classification'is the anatomy of the flower.
There are other differences, however, which are of greater con-
sequence in the contamination of plants by radioiodine. Each
species of plant can also be characterized by leaf differences
such as size, shape, surface, number of leaves, and their orienta-
tion on the stem. Plant leaves are special organs which function
mainly to absorp radiant energy and exchange gases with the
environment. Both of these functions require leaves to present
a considerable surface to the atmosphere. Herein lies the key
to the importance of plants in the passage of iodine to man.
Because of their large area and ability to exchange gases, leaves
are efficient collectors of radioactive fallout. Let us there-
fore consider some of the differences in leaves as they relate
to the collection of radioiodine.
1. Plant growth habit.
The fact that species differ in their collection ability
for radionuclides is exemplified by the data of Gorhanr^
(29)
and Davisv . These authors, working with 90Sr from
fallout, showed that mosses and lichens accumulate far
more of this radionuclide than do vascular plants. This
difference was considered to result from the differences
in the growth habit of these plants. Similar results
can be expected with other radionuclides.
Preliminary data obtained at this laboratory indicate that
131-iodine from fallout is retained differently by
different species of plants. A dense stand of alfalfa
retains more iodine than a more open stand of sudan grass.
This again is considered to be a result of both growth
habit and leaf morphology.
14
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Natural desert vegetation such as sagebrush tends to have an open (less
leaf area per unit of space occupied by the plant) character, thus the
air is free to move through the plant and expose more of the leaf
surface area to the contaminant than would be encountered in a dense
plant such as alfalfa.
An alfalfa field has only the top few inches of the plants situated
in an area of great air movement. The lower leaves are mostly
protected from wind and air movement. Figure 2 shows the profile
of air movement in a stand of alfalfa growing on a research farm
managed by the Southwestern Radio!ogical Health Laboratory for the
U. S. Atomic Energy Commission. * ' It is obvious that the lower
leaves are in a different environment, therefore, have a different
exposure to contamination than the upper leaves.
to
O)
=J
d)
Q.
CO
-o
c
100
90
80
70
60
50
40
30
20
10
0
1 [ I I ' ! L-H
I I TXT I
0 10 20 30 40 50 60 70 80 90 100
Height (cm)
Figure 2. Air Movement in Alfalfa Field.
(122)
15
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(27)
Cline, Wilson, and Hungate^ .' observed thait rye plants collected
gaseous iodtnemostly in the middle portion of the plant, The top
collected 39 percent, the middle 49 percent, and the bottom 18 percent,
They also observed that tnts resulted from the seed heads causing the
tops of the plants to hang down so that the middle of the stem was
(3)
actually the highest and most exposed portion. Barry and Chamberlain
observed that the smaller leaves at the top of the stem were often
(13)
associated with higher absorption than the lower leaves. Bunchv '
reported greater absorption of iodine on the upper parts of grass leaves.
From the above observations, it seems apparent that the amount of con-
tact with the contaminated air appears to be greatest at the top of
a plant and that this is the area of greatest contamination.
An important point to remember fn evaluating contamination of plants
by radioactive fallout (particularly iodine) is the method of
expression. Since the leaf is the plant part in greatest contact
with the environment, it is the plant organ on which deposition is
ultimately dependent. The number, size, and type of leaves presented
to a radioactive cloud are therefore the most important plant
factors in) determining the rate of accumulation or deposition.
(13)
Under laboratory conditions, Bunchv ' showed that the deposition
velocity rises logarithmically with an increase in the density of
vegetation. Gifford^ ' showed that vegetation, specifically sage-
brush and grass, collected more contamination per area of ground
(27)
cover than either bare soil or flat plate collectors. Cline^ '
observed the deposition velocity on bare soil and found it to be
approximately half that found on living plants. Plant cover presents
a larger collection area than a bare soil or a planar fallout
collector. This difference could account for the difference in
observed deposition velocities between plants and soils. By
reexamining the equation commonly used for deposition velocity, we
find that it is based upon the ground surface area. The observations
cited above make it obvious that the rate of deposition on plants
should be based in some way upon the effecttve collection area of
the particular plant involved. The leaf area of a plant is a
16
-------
difficult parameter to obtain. The effective collection area (includes
all epidermal surfaces of the leaf and stem, all protrusions, the
effective gas exchange area created by the stomates, and all surfaces
of bark and other dry material on the plant) is even more difficult to
measure than leaf area. There must be, between the ultimate and the
obvious, some parameter which will allow us to gain a better under-
standing of the deposition of radioactivity upon plants.
Some authors have based contamination data on the fresh weight of the
sample. It has been found in this laboratory that the fresh weight
of forage samples can vary as much as ten percent due to the time
between the collection and the weighing of the sample. Placing the
samples in sealed plastic bags can certainly reduce the loss of moisture
but cannot eliminate it.
Other sources of variation in the moisture content of vegetation are
the time since the last irrigation or rainfall and the relative humidity
of the air. Figure 3 shows the change in the percent moisture of an
alfalfa field between periods of irrigation. If the loss of contamina-
tion from alfalfa were studied using for a basis of expression the fresh
weight of alfalfajan erroneous interpretation of the data could easily
result. Also comparison between species can be confounded by differences
in their moisture percentages. Table 2 shows the variation of the
moisture content of four species collected at the same site at the same
time.
In order to overcome the above mentioned difficulties resulting from
the use of fresh weight measurements or those resulting from the use
of ground surface area, some authors have used the sample dry weight
as the basis for expressing vegetation contamination data. This method
has the advantages of 1) reflecting the amount of material and 2) avoid-
ing the problems of moisture variation associated with fresh weight
methods.
The method used by this laboratory^ ' is to keep the sample in a sealed
plastic bag from collection until weighing. If the sample cannot be
weighed immediately it is kept in an ice chest or refrigerator until
17
-------
Table 2. Variation in the Percent Water of Four Plant Species on Three
Different Collections.(122)
Date
27 March
22 April
4 June
*
Arar .
55.3 ± 4.1
62.0 ± 2.0
53.0 ± 2.0
Percent Water *
Epne"*" Orhy*
48.5 ± 5.0 16,0 ± 2.6
49,7 ± 1.5 41.7 ± 7,6
44.3 ± 1.1 51.7 ± 2.1
Sihy*
59.0 ± 5.2
63.3 ± 4.6
54.6 ± 1,6
* = Artemisia arbuscula (black sagebrush)
J = Ephedra nevadensis (Mormon tea)
* = Orhyzopsis hymenoides (Indian rice grass)
= Sitanion hystrix (squirrel tail grass)
cu 80..Irrigation
10
•r~
-P
c
03
70,
o
-------
the time of weighing. After weighing, the samples are counted then
returned to the principal investigator who is responsible for drying
the sample to a constant weight and reweighing the sample. (It has
been found that drying at 75°C for 24 hours or longer will provide
this condition.)
The two main disadvantages of the dry weight method are the
increased costs due to extra handling and failure of this method
to account for the dilution of the activity resulting from plant
growth during the time of an experiment or study.
At this time, basing data on a dry weight basis appears to be the
best available method. The ideal however would be to have a
rapid means of measuring the actual surface area of the plant that
is exposed to the contamination: Efforts are underway in this
laboratory to try to develop a suitable method to determine such
a measurement.
Also, It must be remembered that plants change in response to
climatic variation; therefore, the crown density, leaf morphology,
and physiology will not always remain constant. Crown density, for
instance, may be much different on plants of the same species at
different locations, at different times of the year, or at different
periods of development. Leaf morphology may change at different
sites or different seasons; i.e., some leaves shed epidermal hairs
or cur] in response to drought. Likewise in an area of ample
moisture, leaves are generally larger and more succulent than in
arid regions. Time of day, wind, and available water cause changes
in plant physiological functions. Transpiration, gaseous exchange,
and leaf positioning are changed in response to the environment.
Because of these variables, contamination of plants will always
be a complex subject that will only be understood as each factor
is evaluated.
19
-------
2. Leaf morphology,
The purpose of this paper Is not to review leaf morphology but
simply make it clear that there are many differences in leaves,
some of which may be very important in the collection and
retention of radioibdine from fallout. A review of the anatomy
and morphology of leaves can be found in "Plant Anatomy" by
/oc \
Esau^ . There has been very little information reported con-
cerning the effects of different leaf types in relationship to
(54)
iodine contamination. In 1963, Hungate, et al,» ' reported
on an experiment where plants of different leaf characteristics
had been contaminated by the effluent gases of a simulated
reactor disaster. Two of the species used in this field test
were of extreme difference in leaf morphology. The hairy leaves
of geranium had no more iodine contamination per unit area of
leaf surface than the smooth leaves of Peperomia. Contrary to
•(94)
this, Romneyv reported the results of Project Teapot where
he observed that hairy plants accumulated more fallout than
smooth plants.
Creosote bush, Larrea divaricata, provides an example of another
plant characteristic that effects the retention of radionuclides.
The* leaves of this species are covered with an exudate which gives
the leaf a sticky surface. This material acts as a trap for much
of the particulate contamination which makes contact with the
lea,f. A comparison made in this laboratory revealed that the
leaves of this plant are as sensitive a particle collector as
are planchets which have been covered with an alkyd resin.
The use of plants as an indicator of the presence of radioactive
contamination has an advantage.over planchets in that it is not
necessary to pre-place the collector in the expected path of the
contamination cloud. Due to the stickiness of the leaves, the
Southwestern Radiological Health Laboratory utilizes creosote
bush, when available, to determine the location of the deposition
20
-------
"hot line" which results from those tests conducted on the
United States Atomic Energy Commission** Nevada Test Site which
release radiation to the environment around the test area, Samples
of vegetation are collected and counted for gamma activity.
Although some plant species have been regarded as better collectors
(54 94)
of fallout than others and some authorsv . ' . have reported
differences between species (casually observed) in their ability
to retain radioactivity, the effect of leaf morphology upon
collection or retention of fallout has not been evaluated con-
clusively. This is an area of needed investigation.
21
-------
III. ABSORPTION OF IODINE BY PLANTS
A. Roots
Independent of the advent of nuclear weapons and the threat of
fallout contamination, "iodine in plants has been observed and
reported (Orr, et al.£86\ 1948; Bohn^, 1917; Campbell and
(15V
Youngv ',1949; South Carolina Agricultural Experiment
Station^10^, 1929; Chilean Iodine Educational Bureau, Inc.^24',
1948, (23h960; Vogel^115^, 1934; and Maihotra^66^, 1931).
Selders^97;, 1954, was the first to report the effect of different
substrate conditions on the uptake of iodine. He experimented
with plants both in soil and in hydroponic cultures and came to
the following general conclusions:
1. The percent iodine in the plant tissue responded directly
to the levels in the substrates. At a level of six micro-
grams of iodine (as KI) per gram of soil the plant concen-
trations reached a maximum of aprroximately twelve times
that found in the substrate.
2. Iodine uptake increased by a factor of four when the pH was
changed from seven to four. This was probably due to the
effect of the hydrogen ion upon the cell membrane rather
than any change in solubility or availability of iodine.
3. Th« four species tested absorbed iodine in different amounts.
The order was bean>tomato>barley>Russian thistle.
4. Iodine was only slightly translocated from the site of
original deposition. Some iodine was lost from the roots,
apparently, being translocated to the primary leaves rather
than to the culture solution,
Iodine is regarded as a physiologically non-essential element for
most plants. It is possible that iodine may partially substitute
for chlorine in some plant functions. However, at concentrations
22
-------
above 1 yg/ml it produces toxic symptoms in plants and therefore interfers
with observations^ '. llhler^11^' found that below toxic levels iodine
absorption and translocation were independent of both photoperiod and
transpiration. In 1965, Uhler^^' reported on relationships of iodine
uptake and different metabolic inhibitors. Iodine uptake was found
to be independent of transpiration and followed a different pathway than
cations under the same conditions. Uptake was also temperature dependent
and was decreased by metabolic inhibitors. This evidence suggests that
iodine absorption by roots is dependent on a source of energy and is
therefore an active uptake process.
The point of maximum uptake of iodine in roots appears to be within
a few millimeters of the root apex and is not enhanced by the presence
of root hairs*44/. jn the studies reported in this section, iodine
was present in the substrate in the iodide form. Once inside of the
plant, the majority of iodine remained in the iodide state. Of the
organic compounds of iodine which have been observed in plant tissues,
three have been identified as ami no acids. They are 3:5-di-iodotyrosine,
3:5:3-tri-iodothyronine and 3:5-di-iodothyronine^ .
Iodine contamination is not considered a soil or root problem because
of its short half-life. However, it is soluble in many forms, is
able to percolate into the soil, and (as discussed above) is con-
centrated by plants when in the root substrate. The limiting factor
in this route of plant contamination is the reactivity of iodine with
the organic and clay components of the soil^^ . Iodine is mostly
held in the top few centimeters of the soil even against large amounts
of leaching water. By the time radioiodine can reach the root zone,
be absorbed into plants, and translocated to the leaves, it is of
little consequence as a radioactive element.
23
-------
The portion of iodine which reaches the edible portion of forage
plants has had time to decay and much of the radioactivity is
lost. Soils contamination therefore is not considered to be a
serious problem in the transport of radioiodine to man.
B. Leaves
Iodine from fallout has been observed to be absorbed into leaves.
The extent of this absorption in comparison to that simply adsorbed
to the surface is important because of the possibility of decon-
taminating the plants by removing the latter in some cultural
operation such as irrigating. Hungate^ >55' and Selders^9°'
followed the penetration of iodine from three different sources
into leaf mesophyll. They found that when leaves were exposed
to elemental 131I, 35 to 40 percent penetrated to the mesophyll.
When leaves were dipped in a solution of Na131I no penetration
was observed. Plants contaminated by the effluent from a
simulated reactor accident had only 10 percent of the 131I in the
mesophyll of the leaf.
One would expect that iodine in the form of a gas could enter
plants via the stomates. Meyer, et al., '"' listed the size of
fully opened stomates of 14 different plant species. Assuming
an elliptical shape, the calculated area of the stomate opening
ranges from 17 to 294 square micrometers. The size of gas mole-
cules is assumed to be considerably smaller than this because
gaseou^ water molecules, which are relatively complex, are known
to readily pass through the stomate opening. Aitken nuclei and
even larger particles up to perhaps 0.5 micrometer in diameter
are also small enough to enter the stomate opening.
24
-------
IV, HALM.IFE OF IODINE ON VEGETATION
/
The motivating force behind most investigations of radioiodine has
been the desire to minimize the hazard of this pollutant (one of
the most prevalent in fresh fission fallout). The degree of plant
contamination at the time of ingestion by cows is the primary factor
determining the amount of radioiodine which appears in milk. The
amount of contamination a cow receives is determined by the amount
of deposition minus the amount lost before consumption. An under-
standing of the loss of iodine from plants is therefore equally as
important as an understanding of its deposition. Iodine-^ 131 decays
to 131Xe (stable) by both 3 and y emission with a radioactive half-
life of 8.05 days. Activity is also lost by other processes. The
effective half-life (T .-) on plants is therefore defined as the
cumulative effect of both radioactive decay and all other loss pro-
cesses. Reported values of T .... (Appendix B) range from 3.5 to
6.5 days. Plants may lose contamination by three methods—dilution
by plant growth, loss of physiologically incorporated iodine, and
physical loss of surface-attached iodine. For a review of effective
iod-
(21)
half-life of iodine on plants refer to Thompson^108' and Chamberlain
and Chadwiek
A. Growth
Loss of contamination by growth is rather straightforward. A
given amount of contamination is simply diluted as plants increase
in size and weight. If contamination were expressed on the basis
of ground covered by vegetation, this effect of dilution would be
masked. In some respects this masking may be desirable, but since
a cow is interested in a quantity of feed and not on the area required
to produce the feed, it seems only logical that data expression
must be on a weight basis. The effect of plant growth is obviously
most important during periods of rapid growth.
25
-------
As an example, consider an alfalfa field one week before cutting.
Under good conditions, an alfalfa field may produce IJg to 2 tons
per acre in a five-week period. If the field were contaminated
by 131-iodine and cut and fed one week later, the contamination
would be decreased by 45 percent due to radioactive decay and
another 20 percent due to dilution by plant growth. It is
obvious that under some curcumstances this type of loss may be
of considerable importance.
B. Absorbed Iodine Loss
Translocation of iodine in plants away from the site of absorption
has been studied by Fowden, 40' Hungate et al., ' and Selders
and Rediske, 97' and found to be very slow. Experiments in this
laboratory have shown that after 72 hours in hydroponic cultures
containing Na131I the distribution of 131I in bean plants is as
shown in Table 3.
Table 3. Distribution of 131I in Bean Plants Growing in
Na131I Contaminated Hoagland's Solution.
Root
Stem .
Leaf
Fruit
96.5$*
' 2.1%'
1.1$
0.2$
*Percent based on dry weight of tissue.
Based on these observations we would conclude that the transloca-
tion of absorbed iodine is very slow and that it is only of minor
importance in determining the effective half-life of iodine on
plants.
Adsorbed Iodine Loss
(21)
Chamberlain and Chadwickv ' commented on five methods in which
iodine may be lost from vegetation, four of which describe
adsorbed iodine loss. Their conclusions are based on reported
literature and are as follows:
26-
-------
1. Rain water is of secondary importance in washing iodine from
plants.
2. Volatilization has been reported as both important and
unimportant as a mechanism of iodine loss from plants. The
importance of this is still uncertain.
3. Translocation to other parts of the plant is not rapid nor
important as far as loss of iodine is concerned.
4. Dieback may cause some parts of the plant to escape sampling.
5. Plants may shed and regenerate parts of their cuticle.
There have been many differences in the reported half-lives of 131I
on vegetation (Appendix B). Some of these are undoubtedly a result
of differences in the mode of expressing results. Other differences
may be due to environmental variables and the chemical or physical
form of the contamination. There is no clear definition of the
relative .importance of the environment upon the loss of the contamina-
tion. Different forms of iodine (particulate, gaseous, or liquid)
seem to be attached to or absorbed in plants with varying degrees of
affinity under different environmental conditions. Therefore, it
seems logical that the loss of the iodine would also be dependent on
different forces. Hhether these forces act cumulatively or indepen-
dently and what the major mechanism of loss is, remains a very complex
and challenging problem. The full explanation for the relationship
between radioactive decay and effective half-life is, at this time,
not understood.
Straub^ ' reported that when cows were taken off 131I contaminated
feed the effective decay occurred in two phases. The first portion
had a half-life of 16 hours to two days. Later the effective half-
life leveled off to about seven days. Experiments in this laboratory
with contaminated dry aerosols indicate the same type of decay scheme
in plants. Immediately following contamination, 131I is lost from
plants rapidly. The contamination is so loosely attached that it can
be blown off by wind or washed off by rain or irrigation. After
27
-------
a short period of time the contaminants become bound to the surface
and incorporated into the plant in such a way that they are dtslodged
only very slightly by changes in the environment. A moderate amount
of experimentation in this lab indicates that the species of plant
and the physical and chemical form of the contaminant determines the
rate of fixation as well as the resultant loss rate. We have observed
that this first period lasts from one to four days and has a T -* of
from one to three days. After the end of the first phase, 131I is
lost from the plants much more slowly, i.e., T ^ * five to seven days,
Decay in this portion of the curve is more a result of plant growth
and radioactive decay of 131I than the loss of contaminant. Figure 4
shows the decay of gaseous elemental 131I on alfalfa plants,
Figure 4. Decay of 131I2 from Contaminated Alfalfa Plants,
(Each point is the mean of four observations.)
(122)
28
-------
V. SUMMARY AND CONCLUSIONS
The term deposition velocity has been widely used to describe iodine
contamination of plants. The physics of iodine transfer from air to
plants is a complex problem and involves both chemistry and physics
of the fallout, various forces which cause movement of the particles
in the atmosphere, and the behavior and extent of the boundary layer
(38)
which surrounds each leaf surface. In 1966, Fisher^ ' developed a
model which can be used to predict the deposition velocity to within
a factor of three. Increases in wind speed or particle size cause
a corresponding increase in deposition velocity. It has been recog-
nized that different plant species collect contamination at different
rates. These differences have not been clearly defined, but before
a complete understanding of deposition velocity can be attained,
differences in deposition on various species must be understood in
relationship to their physiology and leaf morphology. Some of the
factors which should be evaluated are differences in deposition
under light and dark conditions, differences caused by variations
in leaf morphology, and differences caused by change in humidity.
Iodine is considered as a non-essential element for plant growth.
Absorption of iodine through the roots has been shown to be dependent
on the iodine concentration, pH, and plant species. Although it
has not been documented, it appears that iodine absorption is by
an active uptake mechanism. At levels above 1 yg/ml in a culture
solution, iodine was found to be toxic to plants. After being
absorbed by a plant tissue, iodine is only very slowly translocated.
Contamination of plants through the roots by fallout iodine has been
shown to be of little importance in the total contamination of plants.
Foliar absorption, on the other hand, provides a route whereby iodine
can be fixed against most decontamination measures. Foliar absorption
of iodine by plants is largely dependent on the chemical and physical
form of the contaminant. Very small particles and gases can enter
29
-------
the plant through the stomata. Dissolved iodine enters through the
cuticle mainly in the areas of ectodesmata. The rate of iodine
absorption by plants from different forms of contaminant is a field
which warrants added investigation,
The effective half-life of iodine on plant tissues includes both
radioactive decay and other loss processes. Effective decay rates
have been reported from 3.5 to 6.3 days. There are three methods of
loss which need to be considered in evaluating half-life data.
1. Apparent loss caused by dilution resulting from plant growth.
2. Loss from the exterior surface. Suggested methods of loss
are particle removal, volatilization, and cuticle flaking.
3. Loss from inside plants. There is very little translocation
of iodine but iodine can possibly, escape via transpiration
or other gaseous exchanges.
The significance of these three routes has not been evaluated but
must be understood in order to accurately predict effective half-
lives. Experiments done in this lab show that the effective decay
rate of iodine from plants occurs in two phases. At first the loss
rate is rapid, presumably caused by the loss of surface contamination.
Later the loss rate is less with dilution by growth and loss of
absorbed iodine becoming the principal processes.
Various authors working with radioiodine have reported their results
on the basis of wet weight, dry weight, leaf area, and the total
ground area covered by plants. This paper presents evidence to show
that areas of ground cover and wet weight are poor bases for data
expression. Dry weight, although not perfect, is a much sounder basis
for expressing contamination data than either of the others. Pre-
dictions of contamination cannot accurately be made until we under-
stand the differences between species and these will never be
understood unless there is a standard method for data expression.
30
-------
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41
-------
APPENDICES
Page
APPENDIX A. Deposition Velocity of 131I. 43
APPENDIX B. Effective Half-life of 131I. 45
APPENDIX C. Half-life of 131I on Vegetation Contaminated
with Sedan Fallout. 47
C-l. Groom Valley, Nevada 47
C-2. Penoyer Valley, Nevada 48
C-3. Railroad Valley, Nevada 49
C-4. Currant, Nevada 50
42
-------
APPENDIX A. Deposition Velocity of 131I.
CO
Dep. Vel.
V = cm/sec
O.Olt
0.82
0.30
0.11
0.25
0.21
0.23
1.53 ± 0.59
2.2
2.8
0.6 ± 0.22
0.55
0.59
0.52
0.35
0.33
1.91 ± 0.32
2.65 ± 0.50
Contam-
inant
CH3I
V
V
V
V
V
V
V
Species
Grass
Grass
Grass
Grass
Grass
Grass
Grass
Grass
Grass
Grass
Grass
Sece
Sece
Artr
Brte
Dry soil
Grass
Grass
Density
of cover Wind RH
gm/m m/sec %
300W 15* 33
5.4* 34
'
46d 7.1*
129d ± 36
65d ± 15
129d + 19
75d ± 14
500W 4.20**
200W 3.72**
Temp Refer-
°C ence
17.8 3
9.3 3
104
104 .
46
46
46
39
73
50
49
27
27
27
27
27
10 20
18 20
Remarks
No 131I was detected
Winds cale accident N
Windscale accident S
on samples
. England
. England
S.L. accident 1 KM from release
8.5 KM from releasee
67 KM from release
Grass in trays
Grass 13 cm high
Sunny
Cloudy
-------
APPENDIX A. Deposition Velocity of 131I. (Continued)
Dep.
V
1.79
3.75
1.72
0.5
1.0
2.0
3.0
0.5
1.0
0.2
5.56
Vel.
cm/sec
± 0.21
± 0.19
± 0.27
± 2.80
Contam-
inant
131j
V
V
V
*lp
10 y
V
20y
2.5y
V
lu
IP
Species
Grass
Grass
Grass
Grass
Grass
Grass
Grass
Grass
Sece
Artr
Artr
Density
of cove/
gm/m
260W
420W
420W
Wi nd RH Temp
m/sec • % °C
4.42** 20
3.10** 21
1.38** 16
4.47***
4.47***
4,47***
4.47***
Stable***
Stable***
2.5***
5.68***
Refer-
ence
20
20
20
40
40
40
40
101
101
57
57
Remarks
Sunny
Sunny
Nearly dark
60-70 cm tall
60-70 cm tall
60-70 cm tall
60-70 cm tall
Zinc sulfide crystals
20.4 cm leaf blade (58% top,
29% bottom, 13% soil)
Strong inversion
Fluorescent particles
t = calculated Vq
± = 1 standard deviation
V = vapor
Sece = Seeale ceroeal (Rye grass)
Artr = Avtemesia tridentata (Big sagebrush)
Brte = Brorms teotorum (Cheat grass)
w = density based on vegetation wet weight
d = density based on vegetation dry weight
*=wind measured at 4 m
**rwind measured at 1 m
*** = height of measurement not indicated
-------
APPENDIX B. Effective Half-life of 131I.
en
Teff
Days
3.
5.
6.
5
5.
4.
3.
4.
2,
7.
8.
6.
9.
4.
5.
5
1
5
to
0
1
1
0
9
0
4
8
4
9
0
6
± 0
± P
± 0
± 0
± 0
± 0
± 0
± 0
Contam-
inant
1311
.3
.5
.6
.3
.3*
.5*
.4*
.8*
V
V
V
V
V
V
V
V
V
SD
SD
SD
SD
V
V
Species
Agde
Pasture
grass
Pasture
grass
Grass
Mixture
grass &
forbs
Sece
Agde
Artr
Brte
Artr
Atco
Atco
Artr
Pasture
Grass
Density of
Vegetation Method of
g/m2 Expression
46 Wet weight
Area
300 Area
Area
Area
129d ±3.0 Area
6 ±1.4 Area
12 ± 1.8 Area
7 ±1.3 Area
Dry weight
Dry weight
Dry weight
Refer*-
ence
49
13
13
20
76
27
27
27
27
112
112
112
112
8
8
Remarks
CERT 1 wind 7.1 m/sec at 4 m 21.1°C
5.2%/day = rate of grass growth -
13 cm high
CERT 2
9.3°C 34% RH, wind
Based on five trial
Rain 65 mm on 18th
on T ff
Gil
Clipped 1.5 inches
Leaves only
Groom Valley 27 mi
5.4 m/sec CERT 7
s
day. No effect
from GZ
Penoyer Valley 44 mi from GZ
Railroad Valley 70
mi from GZ
Currant Valley 70 mi from GZ
Winds cale accident
-------
APPENDIX B. Effective Half-life of 131I. (Continued)
Teff
Days
5.8
4.7
5.5
i i
Contam-
inant
131j
WF
SD
SD
IT i ._ j j ~
Species
Chilton
grass
Artr
Av desert
plants
Density of
Vegetation Method of
g/m Expression
Refer-
ence
89
70
70
Remarks
Russian series in 1961
Groom and Currant Valleys
All plants averaged—Groom, Currant,
Penoyer, Railroad Valleys
i - j. stcmuaru ueviauiun fiio\
* = half-life was computed from data published by Turner & Martin 2Hsee Appendix C)
V = vapor
SD = Sedan debris
WF = World-wide fallout
Agde = Agropyron desertorum
Sece = Seoale oeTceal
Artr = Artemesia tridentata
Brte = Bromus tectorwn
Atco = Atriplex oonfertifolia
d = density based on dry weight
Area = ground covered by plant sample
-------
APPENDIX C. Half-life of 131I on Vegetation Contaminated with Sedan Fallout*.
Half-life was computed for samples collected at each point even though there was no replication in the
sampling. Variation between points was so great that grouping of points was considered invalid. Half-
lives for each area were computed by taking the mean of all points and compounding the errors.
C-l. Groom Valley, Nevada
Point 2
Time- Days
5
10
15
20
25
30
60
T ff
eff
103
125
60.5
101
542
35.0
2.9
9.3 ± 1.2
346
Activity (pCi/g of vegetation)
22433
14134
5330
3305
1223
500
59.9
6.3 ± 0.6
873
629
410
85.9
97.8
27.3
6.9
7.5 ± 1.1
19369
11622
6089
738
302
506
15.6
5.3 ± .7
7
8063
351t
2090
1605
578
399
33.6
7.1 ± .5
8
9082
2040
389
428
252
15.9
6.3 ± .8
_
fJote—Mean half-life 7.0 ± .3 /
*Adapted from Turner and Martin, 1963V112>
tOmitted from analysis
-------
CO
APPENDIX C. Half-life of 131I on Vegetation Contaminated with Sedan Fallout.
C-2. Penoyer Valley, iNevada
Point 1
Time- Days
5
10
15
20
25
31
61
Teff
3656
1409
989
466
219
31.8
8.7 ± .8
4 5
Activity (pCi/g of
681.5
360
260
124
56.8
4.9
8,0 ± .6
467
265
60.9
54.6
40.0
3.6
8.0 ± .9
7
vegetation)
746.5
2299t
189
111
144
102
7.1
9.0 ± 1.0
9
800
402.5
104
152
198
50.0
8,0 ± 2.4
20
1301.5
567.5
196
196
197
12.9
8.4 ± 1,0
Note—Mean half-life 8.4 ± .5
tOmitted from analysis
*Adapted from Turner and Martin, 1963 ^ '
-------
APPENDIX C. Half-life of 131I on Vegetation Contaminated with Sedan Fallout/
C-3. Railroad Valley, Nevada
Point 2
Time-Days
5
10
15
20
25
30
T ff
eff
985
475.5
228
86.9
90.1
5.3 ± .8
3 4
Activity (pCi/g of
353
188
85.9
74.1
44.6
6.8 ± .8
752
242.5
129
151
70.5
22.7
5.8 ± .9
6
vegetation)
675
422
267
143
65.1
7.9 ± .6
7
608
389
242
91
99.6
87.3
8,2 ± 1.4
Note—Mean half-life 6.8 ± .4
*Adapted from Turner and Martin. 1963. (112)
-------
01
o
APPENDIX C. Half-life of i3ij on Vegetation Contaminated with Sedan Fallout.*
C-4. Currant, Nevada
Point
Time-Days
5
12
16
21
26
31 ...
62
Teff
2
Activity
596
142
63
29
21
27.3
4.3
. 9.1 ± 1.8
3
(pCi/g of vegetation)
99.2
111
49
35
22.3
27.3
3.7
11.4 ± 1.4
4
653
70
48
14
10.5
104
1.6
7.6 ± 1.9
5
324
225
53
60
44.6
24.6
4.3
9.4 ± 1.2
Note—Mean half-life 9.4 ± .8
*Adapted from Turner and Martin 1963. (112)
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DISTRIBUTION
1 - 20 SWRHL, Las Vegas, Nevada
21 Robert E. Miller, Manager, NVOO/AEC, Las Vegas, Nevada
22 .R. H. Thaigott, Test Manager, NVOO/AEC, Las Vegas, Nevada
23 Henry G. Vermillion, NVOO/AEC, Las Vegas, Nevada
24 Chief, NOB/DASA, NVOO/AEC, Las Vegas, Nevada
25 Robert;R. Loux,. NVOO/AEC, Las Vegas, Nevada
26 D. .W. Hendricks, NVOO/AEC, Las Vegas, Nevada
27 Mail & Records, NVOO/AEC, Las Vegas, Nevada
28 Martin.B. Biles, DOS, USAEC, Washington, D. C.
29 Director, DMA, USAEC, Washington, D. C.
30 John S. Kelly, DPNE, USAEC, Washington, D. C.
31 Daniel ;W. Wilson, Div. of Biology & Medicine, USAEC, Washington, D. C.
32 Philip Allen, ARL/ESSA, NVOO/AEC, Las Vegas, Nevada
33 Gilbert Ferber, ARL/ESSA, Silver Springs, Maryland
34 - 35 Charles L. Weaver, BRH, PHS, Rockville, Maryland
36 J. C. Villforth, Director, BRH, PHS, Rockville, Maryland
37 John.G. Bailey, BRH, PHS, Rockvilie, Maryland
38 Regional Representative, BRH, PHS, Region IX, San Francisco, Calif.
39 Bernd Kahn, BRH, Chief of Rad. Eng. Lab.,'Cincinnati, Ohio
40 Northeastern Radiological Health Laboratory, Winchester, Mass.
41 Southeastern Radiological Health Laboratory, Montgomery, Ala.
42 W. C. King, LRL, Mercury, Nevada
43 ;D. Hamil, Technical Library, AEC/NVOO, Las Vegas, Nevada
44 Harry.L. Reynolds, LRL, Livermore, California
45 Roger Batzel, LRL, Livermore, California
46 Ed Fleming, LRL, Livermore, California
47 Wm. .E. Ogle, LASL, Los Alamos, New Mexico
48 Harry.S. Jordan, LASL, Los Alamos, New Mexico
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49 Arden Bicker, REECo,'Mercury, Nevada
50 Clinton.S, Maupin, REECo, Mercury, Nevada
51 Byron Murphey, Sandta Corp., Albuquerque, New Mexico
52 R. H. Wilson, University of Rochester, New York
53 R. S, Davidson, Battelle Memorial Institute, Columbus, Ohio
54 - 55 DTIE, USAEC, Oak Ridge, Tennessee
56 Paul T. Tueller, U. of Nev., Reno, Nevada
57 Charles Hanson, U. S. Fish & Wildlife Service, Las Vegas, Nevada
58 V. R. Bohman, U. of Nev.,.Reno, Nevada
59 H. M. Kilpatrick, U, of Nev., Reno, Nevada
60 Director, Nevada Fish &-Game, Reno, Nevada
61 CETO, Ecology Studies, Mercury, Nevada
62 Dr. Arthur Wallace, Lab. of Nuclear Med. & Radiation Biology,
U. of Cal., Los Angeles, California 90007
63 Dr. Wade Berry, Dept. of Vegetable Crops, U. of Cal.,
Riverside, California 92507
64 - 65 William Link, BRH Library, Rockville, Maryland
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