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6. POTENTIAL FOR HUMAN EXPOSURE
aerosols range from 3–12 and 8–24 months in the polar and equatorial regions, respectively. Removal
half-times from the upper atmosphere to the next lower region range from 6 to 9 months and removal
half-times from the high atmosphere were found to be 24 months (UNSCEAR 2000a). The global fallout
rate of
238
Pu, predominantly from the SNAP-9 accident, as determined by Harley (1980), was
0.002 pCi/m
2
/day (7.4x10
-5
Bq/m
2
/day) based on plutonium levels measured in surface soils. The global
deposition rate of
239,240
Pu was equal to 0.03 pCi/m
2
/day (1x10
3
Bq/m
2
/day) (Corey et al. 1982).
Plutonium deposited on soil surfaces may be resuspended in the atmosphere especially in areas that have
low soil moisture levels, such as the Nevada Test Site. In drier areas, the levels of ambient airborne dust
are expected to be higher than in areas with normal rainfall (Harley 1980). The highest concentrations of
plutonium are likely to be found in the fine silt-clay particle size range. Particles of this size tend to be
transported the farthest distance by wind and water (WHO 1983).
The transport and partitioning of plutonium in soils depends on the form of the compound. The solubility
of plutonium depends on the properties of the soil, the presence of organic and inorganic complexing
agents, the form of plutonium that enters the soil environment, and the presence of soil microorganisms
(Bell and Bates 1988; DOE 1980c; Kabata-Pendias and Pendias 1984; WHO 1983). Plutonium fallout
from the atmosphere, for example, tends to be deposited primarily as the insoluble dioxide (DOE 1987b;
Harley 1980). The majority of plutonium remains within the top few centimeters of the soil surface as the
dioxide form (WHO 1983). Microorganisms can change the oxidation state of plutonium, thereby either
increasing or decreasing its solubility.
The types of organic and inorganic materials disposed of in waste streams can also affect the mobility of
plutonium. For example, in some waste streams, such as the Hanford location, the chelating agent
ethylenediaminetetraacetate (EDTA) was used during the production and processing of plutonium and
was widely present in the mixed wastes at the site (Smith and Amonette 2006). EDTA forms complexes
with plutonium, which will increase its mobility in soils and also possesses the ability to adsorb onto soil
itself, thus reducing the number of available surface sites at which plutonium can adsorb to.
Plutonium will migrate in soils as the hydrolyzed ion or as a complex, formed with organic or inorganic
acids. Mewhinney et al. (1987) found that particles subjected to wetting and drying, such as those found
on the soil surface, released more plutonium than soils continually immersed in a solvent, such as that
found in lakes. This phenomenon is attributed to the formation of a soluble dioxide layer on the particle's
surface during the drying phase. Soil organisms have also been found to enhance the solubility of
PLUTONIUM
168
6. POTENTIAL FOR HUMAN EXPOSURE
plutonium (DOE 1987b). Once plutonium enters the soluble phase, it then becomes available for uptake
by plants. The plutonium(IV) oxidation state is found in plants due to the ability of the environment to
hydrolyze it (DOE 1987c; Garland et al. 1981). Cataldo et al. (1987) postulate that reduction of the
higher oxidation states, such as plutonium(VI), occurs prior to absorption/transport across the root
membrane.
In aqueous solution, plutonium typically exists in one of four common oxidation states and the
environmental fate of plutonium in surface waters is dependent upon both the oxidation state and the
nature of the suspended solids and sediments contained in the water column. Under reducing conditions,
plutonium(III) and plutonium(IV) are the most stable oxidation states, with plutonium(III) dominating at
pH values <8.5 and plutonium(IV) dominating at pH values >8.5 (Smith and Amonette 2006). Under
oxidizing conditions, plutonium(IV), plutonium(V), and plutonium (VI) oxidation states tend to form at
pH values >4. The plutonium(V) and plutonium(VI) oxidation states typically form more soluble
complexes and possess greater mobility than the plutonium(III) and plutonium(IV) complexes. Humic
materials (naturally occurring organic acids) were found to reduce plutonium(V) to plutonium(IV) in
seawater. This was followed by adsorption of plutonium(IV) onto iron dioxides and deposition into the
sediments (DOE 1987h).
The partitioning of plutonium from surface water to sediments in freshwater and marine environments
depends on the equilibrium between plutonium(IV) and plutonium(V), and the interaction between
plutonium(IV) in solution and plutonium sorbed onto sediment particle surfaces (NCRP 1984). Sorption
onto marine clays was found to be largely irreversible (Higgo and Rees 1986). Higgo and Rees (1986)
also found that the initial sorption of plutonium onto clays was effective in removing most of the
plutonium species that would be able to sorb onto the clay. When sorption to carbonate marine sediments
was investigated, it was found that some desorption from the surface would also occur. This behavior
was due to the presence of plutonium carbonate complexes on the sediment surfaces which were sorbed
less strongly than plutonium dioxide complexes (Higgo and Rees 1986). In fact, the formation of
plutonium complexes with organic carbon causes plutonium to remain in solution as a complex (NCRP
1984). Plutonium can become adsorbed onto colloids, small (micrometer) particles that are often found in
groundwater. Adsorption to colloidal particles can enhance the mobility of plutonium in groundwater
(DOE 1999a).
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