Romney et al. (1970) were the first te investigate the vertical dis-
tribution of
Pu (739°?"tpu) in the Safety Shot areas.
Some 7?%pu
was found to have migrated te 9~ to 12-cm depths only 1.3 years after the
event. Two profiles taken 10.8 years after the event showed similar
distributions and indicated that deeper sampling would probably reveal
additionat ??%pu penetration.
Between 1972 and 1973 a concerted effort was launched by NAEG to describe the inventory and distribution of uranium, plutonium, and
americium at the Safety Shot Sites.. This provided a more detailed
evaluation of the vertical distribution of the nuclides.
et al.
Essington
(1976) evaluated some 70 soil profiles from 7 test areas to de-
termine the maximum depth of 239,249py and 7"! am penetration and their
distribution within the soil profile. As with the observations at
Enewetak and Bikini, the 73%+?""py qistributions were extremely
variable but could be categorized inte three groups:
Oo
°
+
ad
oO
oo
(Ww9) Hidad 10S
54
lOOF
20
Fig.
5.
0.01
0.1
0.38 pCi/g
(Adapted from Nevissi et al.,
1976).
Distribution of 239-240py in Bikini Atoll Soils
0.1
FRACTION OF TOTAL 25%240py
and abnormal
normal,
disturbed,
(altered by the physical and chemical conditions of the
soil). Figure 6, trace A shows a normal or unaltered 299 +240py profile
where the concentrations decreased with depth in a regular fashion described, perhaps, by a miltiple exponential distribution. Figure 6,
trace B shows a disturbed 777+?"°py profile and differs from the
Frofile A in the large residual activity in the lower portions of the
profile.
This profile tepresents the 239,240p) distribution in a
streambed area, where it is presumed that intense storm runoff has
mixed the streambed materials and effectively distributed the
739, 24Ony
throughout the depth sampled.
Similar profiles were reported by Lynch
and Gudiksen (1973) and Nevissi (1976) for highly sorted beach materials
at Enewetak and Bikini Atclls. Other evidence of disturbance included:
that of small mammal digs, post-event decontamination efforts, and the
growth and reduction of blow sand mounds prevalent in the desert environment.
Figure 6, trace C represents a rather widespread observation
reflecting the effects of soil formation on 239,240b) distribution.
The small increase in 73%+?"°pu was correlated with the top of the "B"
horizon, the zone of clay accumulation.
A very important aspect of plutonium migration into soils is the effect
of time. Unfortunately, no experiments were designed nor were any
measurements taken which would allow a detailed comparison of short
and long term downward movement of plutonium.
However,
one of the pro-
files collected by Romney (1976) in 1958, 1.3 years after the safety
shot event, was compared to a profile collected in 1973 by NAEG from
a point 50 m away {Essington, 1976). The two profiles are shown in
Fig.
7,
normalized to fraction of
total
24r,24uny in the profile.
The
two profiles are similar, however the 1973 profile indicates a slightly
deeper penetration of plutonium. The 1958 profile indicates that most
of the plutonium found in the soil migrated shortly after deposition,
and only a small amount may have moved to greater depths thereafter.
In comparing these two profiles one must consider the sampling and
analytical variability associated with each sample,
Analytical variabilities (specifically radioactive counting statistics) for the 1973
samples are in the order of the symbol size; however, the variability
for the 1958 samples was not given.
55