Figure 172 also shows that the concentration of
om
187Cs in the dict closely parallels the course of fallout rate, but is delayed essentially one year in reaching its maximum level. This delay is due to the lag
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—|96|—+--1962—--1963+-1964-+— 1965+ 966+1 967+1968 +1969
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Fic. 172—""Cs in the Chicago area 1961-1969; deposition
rate, accumulated deposition, concentration in diet, internal
food items. Throughout the time interval considered,
the accumulated deposition of #27Cs is either increasing or remaining at a nearly constant maximum level.
The primary dependence of the '7Cs in the diet upon
fallout rate rather than accumulated radioactivity is
clearly evident, and reflects the fact that foliar and
stem uptake far outweighs root uptake in the case
of 7Cs under most circumstances. It should be
pointed out, however, that recent work in Florida‘?
has shown fairly constant dietary levels of '’Cs, presumably due to root uptake caused by local soil char-
acteristics.
The effective half-time for 7Cs in the Chicago diet
shown in Figure 172 is about 18 months, and the halftime in manis closely the same, as illustrated by the
internal dose curve. The slight increases in deposition
rate, diet 137Cs, and resultant dose in the last two
years is attributed to the input of new nuclear debris
from Chinese and Frenchtesting.
Most of the foodstuffs in the diet considered are
terrestrially produced; hence we may presumethat the
relatively rapid decrease in *"Cs concentration ob-
dose to man.
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Properly modified, such models may also be useful in
assessing the dosimetric aspects of other modes of
environmental release of "Cs and Sr, and by extension may be applied to other contaminants.
Fairly extensive measurements of 1°7Cs in air, soil,
food, and man have been made routinely at Argonne for a number of years. These data allow the
observation of temporal trends and indicate something
of the transport through, the environment of this
particular substance. Indeed, it is the transport and
environmental persistence of 17Cs which justifies
activity coinciding with high precipitation. From
1963 through 1967 the deposition rate decreased with
a half-time of about 12 months, which is in accord
with the observed half-residence time for fallout
137s in the stratosphere.
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a maximum in 1963 and has a half-width of ~4 years.
The fine structure of annual deposition is characterized by maximal deposition rates in the spring and
early summer due to the higher levels of airborne
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Figure 172, where the deposition rate is shown to reach
x
TTT
nuclear weapons testing in 1961-1962 serves as an illustration. The duration of this pulse is indicated in
%
1000
TF
continuation of these studies.
The pulse of 1*Cs generated by the large scale
Accumulated Deposition
CONCENTRATION IN FISH CpCi/kg wet wt J
&
ee
| 1964196519661967-119681969—
YEAR
Fic. 173—'"Cs in the Red Lakes, Minnesota area 1964-1969;
deposition rate, accumulated deposition, concentration in perch,
northern pike, and walleye.