On Bikini Island, however, the radionuclide concentrations for '37Cs and
90sr in ground water do exceed EPA recommended guidelines for drinking water.
Although
the
radionuclide concentrations
exceed the
EPA quidelines for
drinking water, the ground water does not provide a source of usable water in
any case because it 13 too saline to be potable.
We have taken groundwater samples at each of the next two largest
islands, Nam and Enidrik, and, in both cases, the ground water is very saline
and unpotable.
In our study of Bikini Istand, werhave observed that since 1975 there has
been a decrease in the 137c¢. concentrations in the ground water and an
increase in the salinity,
wae attribute this to the continued growth and
development of the root system of the thousands of coconut trees planted on a
30'-grid across the island in 1970 and 1971. The isiand had been cleared
before the start of the agriculture program and from 1971 through 1975 the
trees were very small with rather small root systems.
The trees grew to
significant height from 1975 to 1987, with a corresponding increase in the
root zone; now, the top 40 cm of soil has a high density of coconut roots. At
the same time that the coconuts started reaching maturity, a dense undergrowth
of native vegetation was reestablished under the coconut canopy, also with a
corresponding increase in root mass.
.
As a result of the maturation of the coconut trees and native vegetation,
more and more of the annua] rainfall is required for evapotranspiration by the
plants, consequently, less and less of the annual rainfall percolates downward
through the soil column to reach the ground water.
In fact, we estimate the
annual water requirement of the current stand of vegetation on the island to
be 40 to 50 inches, or essentially equal to the average annual rainfall.
The !37cs and 90s; reach the ground water by being dissolved in the soil
water and carried into the lens when there is adequate rainfall to recharge
the lens, 1.€., to provide a flow of water down the entire length of the soil
column
into
the groundwater
lens.
As
a result of
the
increased
water
requirement of the growing vegetation and the resulting reduction of water
Flow to the’ lens, the amount of 137cs transported to the lens has been less
and the '3/Cs concentration has been reduced.
At the same time, the lack of, or reduced, flow of fresh water from the
annual
rainfall
Inventory to the lens has created a situation, over the past
several years, where the Salinity of the lens has increased.
oa
wool
te
69
Only on Eneu