The Problem
The objective of this paper is to present an approach for the estimation
of Pu concentration in soil at the Area 13 "safety-shot" site. We do
not attempt to evaluate the Pu concentration at each "point" in space.
Indeed, the variability of soil sample analyses is so large that it does
Accordingly, we will
not make much sense to aim at very local values.
The size of these
estimate average Pu concentrations over grid cells.
cells may be chosen at will, in order, for example, to match the definiWith the
tion of safety standards given, e.g., in acres or hectares.
present data, it is convenient to consider 100- x 100-foot cells, defined
by a sampling grid covering the region surrounding GZ where Pu activity
is highest.
Due to extreme skewness of the Area 13 data, our analyses are performed
in logarithmic scale.
It would be easiest for the statistician if he
could also report results in that scale, but unfortunately, safety
standards are not stated in log scale.
In this paper, final results are
given as both arithmetic and geometric means since it is not clear which
estimate is to be preferred for comparison with safety standards.
The
serious bias problems encountered in transforming results from logarithmic
to arithmetic scale are discussed.
The Data
In 1957, a device containing plutonium was blown apart in Area 13 by
chemical explosives partly to test for "safety" against fission reactions
in an accident situation involving an atomic weapon.
A consequence of
the test was the contamination of the immediately surrounding desert
soil and vegetation with Pu and Am.
The area was fe..ced off and the
contamination has been monitored since that time.
Since 1971, the
Nevada Applied Ecology Group has studied the area by taking field instrument surveys and by collecting soil, vegetation, and animal tissue
samples.
This present study makes use of the following data accumulated
in Area 13:
1,
Pu concentrations (in uCi/m*) determined by wet chemistry on surface
(top 5 cm) soil samples taken at random locations within Am activity
strata (count:
2.
174).
These data are plotted in Figures 3 and 4.
Am concentrations in surface soil, obtained from FIDLER readings
(in counts per minute (cpm)) at one foot above the surface.
These were taken
(a)
at 145 of the above random locations and at 9 others where Pu
was not measured,
(b)
in a 400-foot systematic grid over the entire area bounded by
an outer fence (count:
352),
367
,
t
f
,
i
t