of vegetation to be cropped to natural island vegetation that can grow in one
year during the rainy season.
Considering one annual crop of vegetation, the
observed C.R. of about 3 or 4, and the maximum density of vegetation growth of
about 1.5 to 2 kg/m@-y, repeated annual cropping would only reduce the time
required to reach current Federal guidelines for !37Cs in the soil column from
90 y to 75 y.
This is not an efficient method for reducing the dose in view
of the extraordinary effort and commitment required to debrush the island and
dispose of the material on an annual basis for the next several decades.
Matrix
immobilization
(clinoptilolite).
matrices.
has
been*#evaluated using mica and a zeolite
These materials sequester or bind !37Cs in their structural
We have observed a reduced uptake of !37Cs in plants grown with
these materials added to the soil.
these materials in the soil
breadfruit, Pandanus trees,
However, the application and mixing
of
with the massive surface roots from coconut,
and scrub vegetation
is
not
practical.
In
addition, these materials bind the !37cs and keep it in the soil system and
interfere with the natural flow mechanism for !37Cs out of the soil column
into the ground water.
The clays and zeolites would also be totally foreign
soils to the natural carbonate matrix of the Atoll ecosystem.
The
two most
competition.
reasonable alternatives
are
excavation
and
chemical
Excavation of the top 40 cm of the soil column is an effective
way of removing !37cs, 90sr, 239+240py, and 241Am from the island.
The major
drawback to excavation is the tremendous environmental insult that results.
The 30,000 mature coconut trees would have to be knocked down, along with the
existing breadfruit, Pandanus, papaya trees, and all natural scrub vegetation, —
and disposed of along with the soil.
Also, the top 40 cm of soil contains
almost all of the organic and humic material in the soil column.
The organic
material supplies the majority of the water-retention capacity of the soil and
most of its nutrient capacity.
As a result of excavation, the island surface
would be void of, or very limited in, organic matter and water-retention
capacity, and would resemble the carbonate sands of the beach.
if this method were chosen,
a long-term commitment would be required to
rebuild the soil and revegetate the island (BARC, 1988).
to rebuild the
develop,
soil
organic
is uncertain.
Consequently,
matter,
which
has
taken
The length of time
decades
or more
to
Plants can be grown on the sandy soil devoid of
organic material, but a long-term commitment of supptying required nutrients
and water would be required.
7\