transuranics in environmental soils is lacking.
on transuranic chemistry
Although the literature
in soils was not discussed, many of the papers
reviewed either hypothesized, concluded, or in isolated cases measured
the fact that plutonium had migrated into the soils and probably was
in particulate form, either as plutonium oxide particles, hydrated plutonium oxide colloids, or plutonium particles aggregated to soil parti-
cles.
particles.
However, the migration of plutonium as soluble complexes,
or in conjunction with soil particles also occurs to a sufficient degree, and some plutonium does migrate rapidly and to great depths, as
evidenced by the ground water study at Enewetak.
Close examination of many of the published profiles indicates that the
true front of plutonium migration was not observed due either to inThe
sufficient depth of sampling or inadequate detection capability.
Even
though the plutonium distributions
in soil
are quite variable,
there has been an attempt at a rough categorization.
It appears that
irrespective of the source, plutonium profiles in soil can be categorized
into three groups:
normal,
disturbed,
and abnormal.
Normal profiles exhibit no obvious profile development and the plutonium
distribution appears
to reflect a complex exponential
function.
This
type of profile is more likely to be associated with an undisturbed soil.
Only a limited number of observed plutonium profiles can be categorized
as normal.
The disturbed plutonium profiles result from activities associated with
construction, burrowing animals, water and wind erosion, and redeposition after the initial deposition of plutonium,
The plutonium distributions
in these disturbed soils are quite varied.
a uniform distribution of plutonium with depth,
samples were collected,
as was
They might show
that is,
as deep as
found on beaches and open areas of
implication is that a small amount of plutonium may have migrated to
greater depths than those measured.
The few inferences that can be drawn from observations of the duration
of migration suggest that for a short period of time after deposition
onto soil there
is a rapid penetration of plutonium into the soil.
In-
dications are that a smal) amount of plutonium moves rapidly, and to
some depth, into the soil as though the plutonium were complexed and
relatively uneffected by the attractive forces of soil particles. For
large concentrations of freshly deposited plutonium this effect may
ultimately present a significant environmental hazard.
Studies
should
be conducted to assess the magnitude of the possible mechanisms involved.
The turnover rate of fallout plutonium has been calculated to be 5-6
years for each 5 cm of depth giving rise to the exponential depth proinfiles in undisturbed soils. This observation appears to be somewhat
dependent of the total rainfall of the area.
Enewetak and Bikini Atolls, or in a desert wash area where runoff from
intermittent but severe rainstorms actively mix the top layers of
material.
Many of the early scil profiles were collected by coring techniques
Abnormal plutonium distributions are generally unexplainable and a
problem was recognized and much of the later work used techniques (such
variable.
as the "trench method") which provided some control over that
series of hypotheses have been used to suggest the possible mechanisms
responsible for individual observed distributions.
These possibilities
range from faulty analyses of some fraction (or fractions) to unidentified mechanical disturbances, to the physical and chemical interactions of plutonium with the soil constituents.
Understanding of the
abnormal distributions may be a key
chemical
predicting
which may have introduced a cross-contamination variable,
However, this
higher-than-real concentrations of plutonium at depth,
the coring
One cannot argue at this point with definitive data that
remains
method is faulty, only that the variable does exist and
unanswered.
to describing the physical and/or
interactions effective in the redistribution of plutonium or
transuranic nuclides within soils.
For
instance,
the accumulation of
ACKNOWLEDGEMENT
Plutonium in specific soil zones which are related to soil forming
factors,
nized.
such as A and B horizons or carbonate
The filtering action of
specific soil
lenses,
horizons
has been recogfor small
colloids containing plutonium or plutonium particles, and the potentially
different chemistry among the
factors
zones,
for plutonium migration rates
may be the long term governing
in soils.
The authors are aware
of no study currently in progress to address these problem areas.
The authors wish to thank Mr.
T.
Ensminger,
Early in the history of plutonium distribution studies, it was thought
that plutonium was so tightly bound to soil, that plutonium could not
penetrate into
the soil
in the short time that has elapsed since
first introduction into the soil.
of soluble
ionic plutonium since
its
This may be true for the migration
it may be
70
tightly bound by
Oak Ridge National Labora-
tory (ORNL) for his assistance in querying the Nevada Applied Ecology
on of
Information Data Base, for references dealing with the distributi
‘
transuranic nuclides in soils.
soil
71