FOOD-CHAIN KINETICS OF RADIONUCLIDES
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Also needed are data to indicate the effects on f, of plant density
(e.g., g/sq ft) and other phytosociological characteristics of vegetation
such as species composition, community structure, and phenology. In
desert-shrub communities, for example, shrubs are widely spaced, and
it is probably safe to assume that the f, value for plants in a given area
is independent of plant density. In pasture or forest, where plants are
more crowded and where some may grow in the shade of others, f, may
decrease as density increases. Some evidence of this relation in regard
to 3!1 has been cited recently by Straub and Fooks.*!
Many of these factors (fractionation, particle-size composition and
distribution, and retention factors, a, = f,) are considered in the fallout
model proposed by Miller.®? If suitable plant data were available for a
given situation, Miller’s model would surely provide a theoretically
sounder basis than unqualified linear regression formulas for the
prediction of the initial concentrations of radionuclides on plants at
different locations in a close-in fallout field. If one wanted to use the
food-chain model described earlier (Eqs. 3 to 6) to make predictions of
Py, Pe, Br, and At, a fallout model such as Miller’s should be substituted for Eq. 3.
After the deposition of fallout, the concentration of a given radionuclide on fallout-contaminated plants can be expected to decline ata
rate significantly faster than would be predicted on the basis of its
radioactive half-life. Our estimates of effective half-lives on plants in
the Sedan fallout field, 18 days for *Sr and 5.0 to 5.5 days for 1°11,
indicate environmental half-lives (i.e., half-time rates of loss due to
all causes other than radioactive decay) of approximately 28 days for
8Sr and 15 days for '1]. Since there was little or no rain in the area of
the Sedan fallout field during the period of this study, the environmental
half-life of ®sr on plants can be attributed primarily to wind action that
removed particles from foliage or foliage from plants. The shorter
environmental half-life of ‘I on plants may reflect the combinedef-
fects of wind action and sublimation./!’
Other studies have indicated that our estimates of environmental
and effective half-lives may not be applicable to other situations. For
example, Bartlett et al.’ Sprayed solutions of different fission products
on grass that was then exposed to both wind and rain for periods up to
60 days. Their results indicated an average environmental half-life of
about 14 days for each of the radionuclides used. The difference between their results and ours is probably due to the effects of rain in
removing soluble materials, but rain should have similar effects on
particulate materials.
Considering the large number of variables involved (e.g., plant
species, local weather conditions, the particle-size composition of
material deposited on plants, the developmental stage of foliage, etc.),
one should expect to find a wide range of apparent effective half-lives