SAMPLING THE ENVIRONMENT FOR RADIONUCLIDES
HISTORICAL REVIEW
Historically, soil sampling techniques originated from agriculture and engineer~
ing projects in which a major objective was detailed characterization of
soils; the components of interest were distributed somewhar uniformly both on
a micro- and macroscale.
Many of the procedures used for sampling of transuranic
nuclides in soils were modifications of those procedures.
Sampling efforts for radionuclides, including transuranics, have been identified
as (1) those for worldwide nuclear explosion fallout, and (2) those for accidental or operational releases of radionuclides. The missions for those two
types of sampling may be different and thus require sampling procedures appropriate to the difference in missions.
The most important facet of sampling of soils is proper definitton of mission
and objectives.
The objective describes the overall result or answer desired,
while the mission sets the quality or acceptability of the result. The mission
Global Fallout
should addtess such
important
factors as:
(1)
the
intrinsic
variability of
the medium to be sampled; (2) the required accuracy and the number of replicates
necessary to provide defensible results; (%3* che choice of procedure needed to
minimize cross-contamination; and (4) the cost of sampling, sample preparation,
and analysis.
The importance of those factors was recognized early in the history of soil
sampling. For instance, Vinson gt ai. (1919) discussed the importance of
horizontal subdivision of the sampling area consistent with soli type; this
work is one of the earliest to stress proper sampling design with due consideration for cost and objectives,
Marbut (1921) and Fillinger (1931) cautioned
against contamination and/or cross-contamination of samples.
A form of crosscontamination recognized in later studies was expressed by Cline (1944):
"Many conflicting results that appear in the literature are due to mixing
unlike horizons." Veatch (1925) stressed the importance of vertical subdivisions
within the same soil type and discussed those subdivisions as discrete populations that differ chemically and physically.
Those subdivisions--or horizons,
in the broad sense--are common to a particular soil type. Munch and Bidwell
(1928) emphasized the importance of a proper sampling method to accurately
represent the total population being sampled.
Cline (1944), in reviewing the
work of Munch and Bidwell, noted the need for replicate samples that can be
evaluated separately to provide a statistical basis for sound sampling design.
Jewell (1936) and Kerr and von Stieglierz (1938) investigated the requirements
to be placed on sample replication to obtain reliable estimates of a particular
unknown.
They emphasized that too often the number of sample units is chosen
arbitrarily and generally is insufficlent to detect the wide variability
associated with the soils.
Hossack (1936) also discussed the importance of an
accurately defined mission that accommodates the degree of variability of the
matrix, numbers of samples, and cost.
One of the more complete reviews of soil sampling is that of Cline {1944}, who
treated such topics as:
(1) concept of sampling, (2) statistics, (3) crosscontamination, (4) sampling tools, (5) compositing of samples, and (6) sample
preparation.
Cline's statement relative to composited samples and the establishment of a specific mission is important and bears repeating:
“If any statistic
other than the mean is required, a single composite sample is completely
inadequate,"
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A program of extensive sampling for fallout radionuclides was initiated in
1953 by Alexander et al. (1960).
Within five years, the program was global in
scope (Harley, 1972).
Emphasis through 1967 was on sampling for 7°Sr; however,
the sampling techniques developed for 30sr have been adopted for the more
recent programs of sampling for plutonium and other radionuclides.
The sampling
method described by Hariey (1972} defined each sampling site to be flat,
prassy, and undisturbed. Both surface and core samples are taken; the surface
sampler is normally 8.9 cm in diameter and 5.1 cm deep, often designated as a
“cookie cutter." Subsurface samples are obtained with a barrel auger to the
desired depth. Ten samples are taken on a transect at 30-cm intervals and
samples from respective depths are composited.
Harley emphasized that soil
sampling will provide data for current status but is not applicable to studies
that involve measurement of short-term changes.
This is because the short-term
changes in radionuclide concentration are generally smal] compared to the
variability in local radionuclide distribution.
The sampling efforts have
been directed predominantly toward global radioactive fallout where areal
distribution and particle size were more nearly uniform and concentrations
were low.
Under such conditions, a complex sampling procedure was not required
as indicated by the following experiment conducted by Harley (1972).
Two
sampling sites, separated by distances of 0.1 m tro 100 km, were chosen at each
of five different geographical areas.
Radioactivity in each sample was measured
and means were calculated for each area.
The area means differed by 2-187,
and the average percentage deviation of the pooled means for all five areas
was 10.6%; there was considerable uniformity in the sample pairs.
Mamuro and Matsunami (1964) have reported the collection of fallout particles
in Japan as large as 20 um diam from Chinese nuclear weapons tests and particles
as large as 1? wm diam from earlier Russian tests.
Uranium and plutonium were
identified in some of the fallout particles.
A limited number of highly
radioactive particles may appear in global fallout, which may result in a high
degree of variability in radioactivity associated with soil samples.
However,
no reports have been found that document a high degree of variability attriburable to the presence of highly radioactive fallout particles,
Hardy et al. (1973) discussed the efforts to determine the distribution of
238py deposited on the earth's surface as a result of the atmospheric burnup
of a SNAP power generator.
WUndisturbed areas at 65 sites around the world
were sampled. Each sample consisted of ten 8.9-cm diam cores taken to a
30-cm depth, representing a surface area of 622 cm?.
No further information
on the method of sampling or quality of results relative to the adequacy of
the sampling method were given.
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