|
joon
ean
on of Bikini Lagoon
id sources of radiogoon.
1) in the annual
ial to the concen-
nvironment,
On
il detected radioave decreased in
‘orders of magni-
radionuclide contime is not con-
> was most rapid
some years (e.g.
d 1965), the connuclides changed
but real changes
Ne tried to correrith the peak in
Northern Hemid amount deposperiod wasinsigobserved change.
lerived from only
suggest that unie atoll may lead
concentrations of
the lagoon envi1965 the concenidionuclides have
ter correction for
it recycling from
ological activity,
ee ee ee eee
c {North equatorial current)
Transuranics in Bikini Lagoon
and surface runoff, or some combination of
these or other biogeochemical processes,
are responsible for replenishing activity
levels of some radionuclides in the lagoon
at rates that compensate for the rates of removal.
No model can adequately account for
these unanticipated changes in concentration unless the specific mechanisms responsible for the changes are understood. However, even though a degree of uncertainty
exists, we can use a simplified model of the
lagoon environment based on the diagram
in Fig. 6 and the coral data to describe the
rate at which the radionuclides are exchanged between the lagoon and open
ocean and the rate at which specific radionuclides are recycled from theatoll.
The statement of the mass balance in
terms of the change in the amountof a radionuclide, no, in the lagoon water with
time is
dn = kyno— Ane + kong + Kym,
dt
(1)
where k, is a universal rate constant in yr!
and is independent of the particular radioactive species considered. k, is the mean
residence time of the lagoon water. A is
the radiological decay constant in yrand
k, is the rate constant in yr? defining the
supply of a particular dissolved species
from all diagenetic processes. If m, the
quantity of species ns supplied to the lagoon from the ocean reservoir, is small
compared to the amounts contributed by
the atoll, as it is for all radionuclides de-
tected in the coral except ®Sr and 1°"Cs,
Eq. 1 reduces to
SM = na ky + A) + Kans.
(2)
The change in ns, the quantity of species
m2 supplied to the lagoon from diagenetic
processes, with timeis
d
7 =—g(A + ke).
(3)
Solving Eq. 3 and substituting the solu-
tion in 2, the solution for m2 as a function
of timeis
737
Table 6. ks and ky values computed from Eq.
3 for specific radionuclides.
Radionuclide
k,
9 - mk 30
239py
0.07
0.12
240py
0.07
0.091
241 py
0.06
3.90
238py
0,13
0,014
135eu
0.06
1.25
20754
0.13
0.39
80cq
0.12
1.52
Ne —_
= Noe -At “'e-™ p-kit +
ke
k, -k,
noe! (e-Fat — eit)
x
(4)
We propose that the radionuclide concentrations in each annual growth section
are proportional to the amount of species
nm. in the surrounding water environment
during the respective year of growth. The
last nuclear test series at Bikini was held
in 1958. Taking 1958 as t, there was an
amount M39 of species nz in the atoll reser-
voirs, We assume that the rate at which
the lagoon is flushed with uncontaminated
ocean water is rapid enough so that after
5 years e** can be taken to be zero. Equation 4 then reads
ny (after 1962-1963) = kye-™"#, (5)
where ko = (Kontago) /( ki — ke).
Using the data retained in the coral sec-
tions from 1962 to 1972 we can compute a
best-fit unique value of ky and ke from Eq.
5. These values for each radionuclide detected are listed in Table 6. Substituting
the values of ky and ky into Eq. 4, and now
using the 1958 and post-test year coral
data, we get an average value for k, of
1.98 + 0.14 yr.
The lagoon volume along with any dissolved species is exchanged 1.98 times a
year with the open ocean. The residence
time of the lagoon is 127 to 198 days. From
calculated flows into and out of Bikini Lagoon, Von Arx (1954) estimated that during the tradewind season one lagoon vol-