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169
6. POTENTIAL FOR HUMAN EXPOSURE
Carroll et al. (1999) reported distribution coefficients (K
d
) ranging from 3x10
4
to 8x10
4
for plutonium in
bottom sediments from the Kara Sea. The International Atomic Energy Agency (IAEA)-recommended
K
d
value for sediments is 1x10
5
for plutonium (Carroll et al. 1999; IAEA 2004).
Plutonium can be taken up from various environmental media into plants and animals. The primary
factor that governs whether plutonium in soil can be taken up by plants roots is the presence of soluble
forms of plutonium in adjacent subsurface soils. The highest concentrations of plutonium in plants are
generally found in the roots where plutonium is present as a surface-absorbed plutonium complex, a
stabilized complex, or a soluble plutonium complex (Garland et al. 1981). Concentration ratios
(concentration of plutonium per gram of dry plant tissue divided by concentration in the soil) of 1x10
-6
–
2.5x10
-4
were calculated based on radioisotope experiments in soybean plants grown in controlled
environments. The stems and leaves were found to possess lower overall concentrations of plutonium
than the roots, but higher concentrations of soluble plutonium (Cataldo et al. 1987).
In studies on orange trees, Pinder et al. (1987) found that
238
Pu was deposited on the leaf or soil surface,
remained there, and that no measurable quantities were transferred to the fruits. Grain crops grown near
the Savannah River Plant, Aiken, South Carolina, were found to contain higher concentrations of
plutonium the closer to the facility they were grown. During harvesting, plutonium from soils or straw
was resuspended and mixed with the crop. Plutonium in vegetable crops grown at Oak Ridge National
Laboratory, Oak Ridge, Tennessee, contained higher plutonium concentrations in the foliage biomass
than in the fruit. Peeling of potatoes and beets removed 99% of the residual plutonium (DOE 1980d).
The Mayak Production Association (PA), which processed weapons-grade plutonium from 1949 to 1952,
discharged radioactive wastes into the Techa River, which belongs to the Kara Sea basin of the Arctic
Ocean. Akleyev et al. (2000) reported plutonium accumulation coefficients ranged widely from 2x10
-4
to
0.8 for vegetation samples the area of the Techa River. The authors noted that plutonium accumulation is
highly species-dependent.
Plutonium uptake by grazing herbivores was predominantly located within the animal's pelt and
gastrointestinal tract (DOE 1980i). Rodents studied near the Los Alamos and Trinity sites in New
Mexico support this claim. DOE (1980i) found no evidence of bioconcentration through the food chain
from soil to plants to rodents. They concluded that soil was the source of plutonium in rodents. In
contrast, a study by Sullivan et al. (1980) showed that rodents absorbed more
238
Pu when it was
incorporated into alfalfa (by growing it in soil containing plutonium) than when it was administered in the
PLUTONIUM
170
6. POTENTIAL FOR HUMAN EXPOSURE
inorganic form (Sullivan et al. 1980). This study suggests that plutonium bound to organic compounds
may have increased availability.
Plutonium was found to bioaccumulate in aquatic organisms, primarily at the lower end of the food chain.
The bioconcentration factors (i.e., the amount of the chemical found in the organism divided by the
concentration in the surrounding water over the same time period) were 1,000 for mollusks and algae,
100 for crustacea, and 10 for fish (WHO 1983). Plutonium is concentrated in the bones of fish rather than
in muscle tissues, as seen by whole fish to muscle tissue ratios of 40:1 (NCRP 1984). Swift (1992)
reported that whole-body concentration factors for juvenile lobsters did not exceed 250 over an exposure
period of 49 days in seawater containing
237
Pu.
237
Pu was found to accumulate mostly in the gills and
exoskeleton.
6.3.2
Transformation and Degradation
Plutonium isotopes are transformed by radioactive decay to either uranium or americium based on the
isotope, but as an element, plutonium cannot degrade. The most common isotopes of plutonium are
238
Pu,
239
Pu,
240
Pu, and
241
Pu with respective half-lives of 87.7, 2.410x10
4
, 6.56x10
3
, and 14.4 years (Lide 2005).
The half-lives for
239
Pu and
240
Pu are very long, and only a small amount of transformation would occur
over a human lifetime.
241
Pu has a much shorter half-life and would undergo transformation over a
human lifetime.
241
Pu decays into
241
Am, which has a half-life of 432.7 years and is also an alpha particle
emitter (Baum et al. 2002). Information on the radioactive transformation of various plutonium isotopes
can be found in Table 4-3.
6.3.2.1 Air
Plutonium does not undergo transformation processes in the air beyond those related to radioactive decay.
Radioactive decay in air is not significant for those plutonium isotopes with long half-lives compared
with residence times in the atmosphere. For plutonium injected into the atmosphere from a weapon
detonation, the residence half-time of particulate debris in the troposphere of approximately 30 days
(Bennett 1979; Nero 1979).
6.3.2.2 Water
The solution chemistry of plutonium is complex due to the ease with which it can undergo oxidation-
reduction reactions and its extreme oxophilicity (tendency to bind oxygen) of plutonium cations (Clark et
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