plutonium-240 and americium-241
have long-term significance
because they can be resuspended
in the atmosphere and are present
edible foods and correlate these
data with appropriate dietary
models, dose models for each of
p.8 for a discussion of the
behavior of plutonium in marine
environments.) However, the doses
predicted for the transuranics are
‘very low compared with those for
locally grown foods at Bikini and
in lagoon water. (See the article on
cesium-137 and strontium-90.
A detailed data base
In 1977, we began an environmental research program on Eneu
Island (Bikini Atoll) to measure
concentrations of radionuclides in
edible foods and thus provide more
precise dose estimates. In our experiments, we were abie to evaluate
subsistence crops planted in 1970
by the Trust Teritory government.
Weare determining the concen-
tration, rate of uptake, and
residence time of strontium-90,
cesium-137, plutonium-239 plus
plutonium-240, and americium241 in locally grown subsistence
crops in the atoll ecosystem. We
are also identifying concentration
ratios (i.e., ratios of the concentration of radionuclides in various
edible fruits to that in the soil) and
correlation ratios {i.e., ratios of the
concentration of radioactivity in
different species of plants) for use
in predictive environmental dose
models. A comprehensive model of
the transport, recycling, and ultimate disposition of long-lived
radionuclides can then be
developed.
To estimate the radiation dose
to a given population, we measure
the radionuclide concentrations in
Se Se
the radionuclides, and proposed
living patterns. There were few
Enewetak Atolls. We had to establish our owntest plots of
coconuts, Pandanusfruit, breadfruit, papaya, banana, squash,
sweet potatoes, and other crops
(on Enjebi Island at Enewetak Atoll
and Eneu Island at Bikini Atoll).
The test plots helped us to understand the relationship between
the concentration of radionuclides
in edible fruit and in the soil. With
accurate concentration ratios (i.e.,
fruit concentration/soil concentra-
tion), we can predict the concentration of radionuclides in foods that
will eventually grow on islands and
atolls currently devoid of foodproducing plants.
The concentration ratio for
cesium-137 in coconut meat (as
measured in ourtest plots) is 6, and
the concentration of cesium-137 in
soil on Enjebi Island is 20 pCi/g.
Thus, we can predict that coconuts
planted on Enjebi island will have a
cesium-137 concentration of
roughly 120 pCi/g. Also, knowing
that people in this area eat approximately 150 g of coconut per
day, we can predict that they will
ingest an average of 18000
picocuries of cesium-137 per day
(approximately 300 000 pCi/dayis
the limit of intake specified by the
International Commission on
Radiological Protection}.
Our field research projects are
designed to evaluate the environmental residence time ofcritical
radionuclides in the atoll
ecosystem. Cesium-137 and
strontium-90 are responsible for
over 95% of the total predicted
' doses, and both have radiological!
half-lives of about 30 years.
However, if environmental
processes are acting to reduce
available cesium-137 and
strontium-90 inventories so that the
residence time is significantly less
than 30 years, then the predicted
doses will be much lower, and longrange land-use plans in the atolls
could be altered.
Wealso study the mechanisms, ~
processes, and controlling features
of radionuclide cycling in the
ecosystem. Our goalis to find ways
to minimize the flow of radionuclides into subsistence foods and
to reduce the available inventory
in the soil.
Results
The results of our assessmentat
Enewetak Atoll are listed in
Table 1; the ingestion dose in-
cludes that due to food and water,
and the inhalation dose is nealigible. Today, about 60% of the food
eaten by the people of the Marshall
Islands is imported. However, when
such food is unavailable (because
of shipping delays or weather
conditions), they must subsist on
locally grown foods. Hence, two
types of diet are specified.
The dose rates predicted for the
southem half of the atoll are much
lower than the 100-mrem/yr
background dose (at sea level) in
the U.S. However, the maximum
annual doses predicted for Enjebi
Island (in the northern part of the
atoll) do approach the current
Federal limit for an individual (500
mrem per year). Table 2 lists the