FIDLER data obtained at random soil sample locations
immediately prior to
taking the soil sample,
They found the method (termed “double sampling")
to
be applicable for higher levels of contamination where
the correlation between
FIDLER readings and Pu wet-chemistry concentrations
was sufficiently high and
the cost ratio of FIDLER to wet-chemistry analyses
sufficiently low,
Once surveys have estimated the spatial distribution
Pattern, additional
survey readings and/or soil samples are sometimes
taken along a transect
running from GZ down the "hot line” or center of the
fallout pattern.
Closer
spacing between sampling may be used near GZ in order
to estimate more accurately the rapid changes in concentration levels likely
to occur in that area.
This was the general approach used at the NAEG safety~sho
t sites, the Plutonium
Inventory and Distribution Program at NTS (Church et
al., 1974; Brady and
Church, 1975), and at the Trinity site in New Mexico
(Hakonson and Johnson,
1973). The purpose of such transect samples is usually
to gather information
to facilitate the design of more intensive studies.
Soil samples collected
along a single transect are usually too few and widely
spaced to be useful for
estimating inventory.
If the objectives of the study require additional informatio
n on soil concentration, there are several design options open to
the investigator depending
on objectives.
If the in situ or aertal surveys have satisfied the requireme
nts
for information on the spatial distribution of contaminat
ion, then soil samples
may be collected primarily to estimate inventory,
In this case, the survey
information may be useful for stratifying the study
site within which soil
samples are collected. The idea of stratification is
to enclose areas of
different concentration levels into Separate strata
(subareas) so that the
vartability between samples within strata is as small
as possible relative to
the variation between stratum means.
If the sample locations within strata
are chosen at random, this design is termed stratified
random sampling (Cochran,
1963).
Stratified random sampling was used at the NAEG safety-sho
t sites
(Gilbert and Eberhardt, 1974; Gilbert et al., 1976a)
and tended to result in
estimates of inventory with smaller standard errors
than if stratification had
not been used (Gilbert et al., 1975).
If in situ or aerial measurements are not sufficiently
well correlated with
soil concentrations, or if such surveys have not or cannot
be done, the design
of the intensive soil sampling plan may need to fulfill
the two objectives of
spatial distribution and inventory. One approach is to
arrange a rather fine
grid over maps of the study area and to choose at random
a number of squares
defined by the grid. This approach was used for sampling
soil on most islands
in the 1972 Enewetak Radiological Survey (Lynch and
Gudikson, 1973).
The
selection process can be performed using a random number
table such as produced
by the Rand Corporation (1955), or by using a good pseudo-ra
ndom number generator on a computer. The locations actually sampled would
be the centers of the
randomly chosen squares.
An alternative design would be to first stratify the
area based on 24Jag
Ge(Li) scans on soil samples collected on a grid.
Once the strata are determined, a grid within each stratum (grid size need not
be the same for all
strata) could be established. Then two or more locations
could be chosen at
random within each grid square in each stratum.
580
The above discussion has concerned contamination from local sources. In this
situation, it is usually best to avoid sampling at only "undisturbed" locations,
since these locations may not be representative of the area (Eberhardt, 1976).
However, when estimating total deposition from worldwide fallout, it is common
to sample only undisturbed sites (Hardy and Krey, 1971).
The distinction is
whether interest centers on estimating the existing spatial distribution and
inventory or on estimating the total amount deposited over time. Undisturbed
sampling locations have also been used to estimate the spread of contamination
from the Rocky Flats plant (Krey and Hardy, 1970). The issue involved here is
important for other study objectives, also. Consider a study for evaluating
the need to clean up an area,
In terms of potential health risk, it may be
more important to determine where most of the contaminant is presently located
than to know only the total amount initially deposited.
Sampling to Assess Relationships
After information has been obtained about the spatial distribution of transuranics, there may be interest in estimating the amounts of contamination in
various ecosystem components and the movement rate of transuranics between
components. The design of these studies depends to a large extent on the
particular ecosystem being studied. The design of such studies should be
based on whatever information is available or can be obtained on where the
bulk of the contamination is located, and how this contamination is likely to
move through the environment.
Three ecosystem-type studies may be briefly mentioned. These include NAEG
studies in Nevada at 10 safety-shot sites referenced above.
Soil and associated
vegetation samples were collected at ail 10 sites according to a stratified
random sampling plan. Small mammals were also collected at some sites, and in
Area 13 (Project 57), cattle were grazed in the Pu-contaminated areas to
obtain information on uptake and tissue distribution of Pu due to grazing a
contaminated area (Smith ef al,, 1976).
An important objective of these NAEG
studies is to evaluate the potential hazard to man from this contamination. A
provisional Pu transport and dose estimation model has been developed for this
purpose by Martin and Bloom (1976). Gilbert et al. (1976) gave an initial
synthesis of the Area 13 Pu data for soil, vegetation, small mammals, cattle,
and a "hypothetical man." Other transuranic ecosystem studies include those
at the Savannah River Plant and in. some of the canyons at Los Alamos. At
Savannah River (McLendon et al., 1976), samples of soil, a resuspendible
fraction of soil, vegetation, grasshoppers, and cotton rats were collected and
their Pu (238py, 239°248py) concentrations compared in order to study the
transport of Pu in a humid climate.
The primary objective of the Los Alamos
studies (Hakonson et al., 1976) was to estimate the ?38pu,
>
Pu, and
137¢g levels in canyon ecosystem components (stream sediments, vegetation,
small mammals) as a function of distance below waste discharge areas.
The design of such studies is complicated due to incomplete knowledge of how
transuranics move through the environment, and of the spatial patterns and
correlations over time and space.
Under the assumption that samples collected
near each other are more alike than those from further apart, the collection
of adjacent soil and vegetation samples is often practiced. However, Gilbert
et al. (1975) illustrate that in desert environments, the correlation between
581