vegetation was
not
collected,
concentration in soil
the
relative difference
at an island will
in
radionuclide ~
reflect the relative difference
expected in the radionuclide concentrations in vegetation from that island.
Concentrations
of
radionuclides
in
vegetation may depend on
radionuclides’ distribution with depth in the soil column.
trying to evaluate this dependence.
the
~
We are currently
The low uptake of 90sr, the very low
uptake of trgnsuranic radionuclides by plants, and the general binding of
these radionuclides, especially 99Sr, by the soil matrix, makes them less
mobile than !37cs.
Thus, once Sr, Pu, and Am are mixed to a depth of 40 cm,
as has happened at both Bikini
and Eneu Islands to varying degrees,
the
redistribution to the surface and also down the soi? column is a much slower
process than is the case for '3/cs.
The 13/Cs is distributed in a more
exponential] manner on Eneu Island because of the biological availability of
'37Cs in coral soil and, thus, its uptake by plants.
Because of its mobility
in the Atoll ecosystem, '37cs is brought to the surface by the process of
uptake
into
redistributed
plants
down
and
subsequent
the soil
litter
fall
column by rainfall
and
is
subsequently
and other environmental
processes.
CONCENTRATION RATIO
Almost all plants at the Atoll take up !37cs from the soil. The Atoll
soils are, in general, low in available K; thus, !37Cs is readily accumulated
in the plants as an analog for needed K. Most of the plants concentrate !3/Cs
at a level proportional to the concentration of !3’Cs in the soil.
This
accumulation of !37Cs has been represented in terms of a concentration ratio,
C.R. This ratio is almost always greater than 1 for !3’cCs, as is shown in
Table 32. Most of the edible foods have C.R.s greater than 2 for !37Cs.
However, the C.R. for 90Sr is almost always less than 0.01, and for 239+240py
and 24lam less than 0.0001, indicating a discrimiration against these
radionuclides in the edible portion of food crops.
The source of 90s¢,
239+240py, and 241am in food crops is definitely root absorption.
All of the
edible portion of the food crops that we evaluate at the Atoll are covered by
at least one, if mot several, layers of thick, protective skin or husks; when
these layers are peeled away, there is very little chance that resuspended
material is included in the sample.
a
In addition, our resuspension studiesat
66