802
TURNER
From a theoretical standpoint it is more fruitful to consider the
contamination of milk by ‘I as the last link in a chain of related events
and to attempt to develop a step-by-step representation of the process
by means of a compartment model. Such a model could be arranged in
three compartments: iodine on cattle forage, iodine in cattle, and io-
dine in milk (a fourth compartment, the ground surface, could also be
added if desired). But would such a model perform adequately when
applied to a real environmental situation? The general idea might be
evaluated with the use of a more simple model, e.g., one involving
only two compartments.
PROCEDURE
Following the Sedan shot of July 1962, the model approach was
tested in terms of the time-specific relation of ‘I on desert vegetation
to
that
in the thyroids of jackrabbits consuming this vegeta-
tion.!3,!5 The deterministic model used has been discussed previously
by French'® and French and Van Middlesworth" and is repeated here
in slightly modified form for convenience:
A=
IDF
Ag —A,
(ert _ e*Et)
where A = amount of "I in thyroid of consumer
I= amount of “I per gram of vegetation on day of contamination
D = number of grams of vegetation consumed per day
F =fraction of ingested {I reaching thyroid of consumer
Ap = physical decay constant of '*'1, 0.0865 (In 2/half-life in days)
Ag = effective decay constant of ‘I in thyroid of consumer
t = time in days after the contamination event
When t is defined as the number of days following the introduction of
1317 into the environment, the model predicts the amount of ‘*!I in the
thyroid of a consumer at time t in termsof a specified initial level of
contamination of food, I.
The values of the other parameters involved were either measured directly during the month following the Sedan shot or based on
existing experimental results. The rate of disappearanceof !*!I from
vegetation is not accurately expressed by \,, as shown in the preceding
equation but is more rapid because of loss from plants by means other
than radioactive decay. The effective half-life of '°4I on plants was es-
timated to be 5.5 days on the basis of periodic analyses of vegetation
samples.
The initial amount of this nuclide per gram of vegetation, I, was
also estimated from measurements of plant material, Bulk samples
collected in the field and the air-dried stomach contents of jack-