1042 RADIATION ECOLOGY
Strontium-90 accumulation normally is greater in children
than in adults, because growing children are building bone at
a greater rate and use a larger amount of calcium. A study of
species in the deer family (cervidae) graphically demonstrated
the effects of^90 Sr fallout. The levels of^90 Sr in the antlers of
deer rose continually from 1947 through 1955, then remained
constant for 2 yrs, and rose again in 1958. The concentration
of^90 Sr was more than 8 as high in 1958 as it was in 1947.
With the cessation of major weapons tests in the 1960’s, the
levels began to drop off.
Cesium-137 is another major fission product that is
found in fallout and untreated radioactive waste effluents.
Cesium behaves chemically very much like potassium and
follows the same metabolic route in plants and animals as
potassium does. It enters plants directly through the leaves
after being deposited by rain, and so it appears in plant tis-
sues more quickly than does^90 Sr.
From there on the route of^137 Cs is much the same as^90 Sr;
it appears in the milk and muscles of cattle that eat contami-
nated plants, and it enters the human body in food. Once in
the human body, it becomes part of muscle tissue and so has
an almost uniform distribution throughout the body. It stays
there for only about 4 months. Since its half-life, like that of(^90) Sr, is about 28 yrs, little of the (^137) Cs undergoes radioactive
decay while in the body.
The transfer of^137 Cs and^90 Sr from plants to animals
also has been observed in species that are not important as
food sources for man. The coconut crab, a land animal that
lives on a diet of fruit and nuts on islands in the Pacific, was
found to have accumulated radioactive materials as a result
of the Pacific test explosions. Strontium-90 was found in the
skeleton, and^137 Cs was found in the soft body parts—direct
results of eating contaminated vegetation.
The third radionuclide or fission product of importance
in fallout is radio-iodine (^131 I). The chemistry of radioactive
(^131) I is exactly like that of natural (^127) I, which is not radioac-
tive. Therefore, its concentration in the body depends only
on the concentration in the source material. Iodine becomes
concentrated in the thyroid glands of vertebrate animals,
where it can cause cancer of the thyroid and damage to other
tissues. Like^90 Sr and^137 Cs, it enters plants as a result of
radioactive fallout and then enters humans either by way of
the plants themselves, or by way of contaminated milk from
cows that eat the plants. The radioactivity of thyroid glands
removed from certain animals can serve as a sensitive indi-
cator of^131 I in the environment, because the concentration of
(^131) I in the thyroid can be as much as 10,000 higher than
the concentration in nature. The black-tailed jack-rabbit is a
useful animal for such measurements. It has a large thyroid
gland that is easy to remove and weigh. The level of radioac-
tivity in each gram of its thyroid tissues varies directly with
the fallout on vegetation.
Finally there are man-made radionuclides (activation
products) which are important because they are the isotopes
of elements which may be essential to plants and animals.
Some of these also may enter the environment as activation
products resulting from reactor operations or nuclear explo-
sives. Examples of activation radionuclides are cobalt-60
and zinc-65. In aquatic or marine environments these radio-
nuclides have been found to accumulate in food organisms
especially shellfish and mollusks. Generally^60 Co can be
anticipated to be accumulated by organisms or to be retained
in organically enriched materials such as forest floor humus
and organic sediment. Zinc-65 is of particular concern in
marine environments where it is likely to be accumulated in
clams and oysters. However, being an activation rather than
a fission product its presence depends more on appropriate
stable elements present which in turn are exposed to fast
neutrons than on fissionable material.
RADIOSENSITIVITY OF ECOLOGICAL SYSTEMS
Although there is much current concern about the possible
effect of low level, chronic radiation on ecological systems,
there is relatively little comprehensive scientific data on
TABLE 1
Radionuclides of ecological importance
Category Major radionuclides Ecological importance
Naturally occurring
radionuclides
Uranium Thorium Actinium series
elements Potassium-40 Carbon-14
Major contributors to background radiation (long
half-lives)
Fission products Strontium-89, 90, 91 Yttrium-90, 91
Zirconium-95 Niobium-95
Ruthenium-103, 106 Rhodium-106
Iodine-131 Cesium-137 Barium-137,
140 Lanthanum-140 Cerium-141,
144 Praeseodymium-143, 144
Neodymium-147 Promethium-147
Enter ecological systems through fallout or waste disposal
(half-lives ranging from a few hours to 30 yrs)
Radioisotopes of
elements essential
to organisms
Hydrogen-3 Cobalt-60 Carbon-14
Sodium-22, 24 Phosphorus-32
Sulfur-35 Potassium-42
Calcium-45 Manganese-54 Iron-59
Copper-64 Zinc-65
Used as tracers in both radionuclide cycling and radiation
effects studies on organisms and ecological systems
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