T72
MARTIN
interdependent. The variations within and between different study areas
and the twofold difference between estimates of f, based on '*/I con-
centrations in plant samples and in the stomach contents of rabbits are
good examples of the kinds of disparities to be anticipated.
Although it is quite tempting to use such models to predict the
biological consequences of close-in fallout, there are many good
reasons for proceeding with caution. Some of the uncertainties, possible sources of error, and other obstacles to the achievementof this
goal are described and discussed below.
From our analyses, the relation between initial gamma dose rates,
Ry, and initial concentrations of radionuclides on fallout-contaminated
plants,
Pj,
appears to take the form of a linear regression formula.
There are several reasons for viewing this apparently simple relation
with suspicion. For example, Guillou”? has noted that the probable
accuracy of Ry, estimates based on aerial-survey data is no greater
than +50%. Consequently the regression coefficients given in Table 3
could be in error by +50%, and errors associated with extrapolations
beyond the standard regression formula limits could be even greater.
Because of nuclide fractionation. and downwind variations in the
particle-size composition of fallout, one might not expect to find significant correlations between gross gamma dose rates and concentrations of specific radionuclides on plants or soils in fallout fields resulting from surface or low-altitude nuclear detonations. Previous
studies '*7»78 along the hot lines of fallout from balloon- and towersupported detonations have shown correlations between plant and
animal contamination and the distribution of fallout particles <44 yu in
diameter.* In several cases, the highest percentages of fallout < 44 y in
diameter and the highest levels of plant and animal contamination were
found at intermediate distances from ground zero and hence at inter-
mediate levels of gross gamma dose rate. Under these conditions, it
would not be possible to predict plant contamination levels by means of
linear regression formulas involving Ry as the independent variable.
As shown in Table 4, it is also necessary to consider those factors
which influence a plant’s ability to intercept and retain the fallout par-
ticles deposited on it. Other things being equal, it may be assumed that
plants with dense foliage composed of oily, resinous, or pubescent
leaves should have higher f, values than plants with sparse foliage
composed of smooth or waxy leaves; but more data are needed to determine f, values for a variety of wild and cultivated plant species in
relation to specific morphological features, the particle-size com-
position of fallout, and various deposition rates.
*Autoradiographs and microscopic examinations have shown that virtually
all the radioactive particles on plant foliage in the Sedan fallout field were <50 u
in diameter and that about half of those measured were < 20 y in diameter.