In order to get an approximation of the amount of soil that would have
to be removed to bring about a given dose reduction, one needs to determine
the three dimensional distribution of the radioactive contamination.
Figure 3
shows the average 90, activities (pCi/gm) in soil samples collected to a
depth of 15 cm on JANET.
Similar figures for 13766, 6000, and 23954 may
be found in Appendix II of NVO-140.
In addition to the 15 cm deep samples,
radioactivity distribution as a function of depth ("profile samples") was
measured in fourteen locations on JANET.
Data from these profiles are presented
in Figs. B.8.2.a-n of Appendix II of NVO-140.
Inspection of these profiles
indicates that, on the average, about 40 cm of soil would have to be removed
to reduce the activity in the top 2 cm layer by a factor of 10.
In addition,
as the depth increases the slope of the activity-vs-depth curve tends to
decrease, i.e., the activity levels do not go to zero, even at depths greater
than 100 cm.
Table 5 shows pertinent data for 90g,
In an attempt to quantify this distribution and obtain an approximation of
the "average profile" for calculational purposes, 905, and 13765 data for each
of the fourteen profile samples have been reproduced in Tables 6 and 7.
average values for 905, for each sampling depth are plotted in Fig. 4.
The
It is
apparent that from the surface to about 30 cm the 906, specific activity is
decreasing with a "soil half thickness" of 8.4 cm, while in the 30 to 85 cm
depth range the half thickness increases to 22 em.
The levels do not get as
low as those found on the southern islands (10.5 pCi/gm) at any depth down to
180 cm.
Those profile samples which lie in or closest to the subsistence
agriculture areas of Figure 2 have been averaged and plotted in Fig. 5.
In
this set, the half thickness is only 4 cm from the surface to 10 cm, but
increases to 25.5 cm in the 10 to 85 cm depth range.
1376, data is plotted in Figs. 6 and 7.
-9-
Similar treatment of the
In Fig. 6, where all samples are
wet M,