been accomplished, decisions concerning the kind of data to collect, the
precision in estimates needed to meet objectives, and the actual sampling
locations can be made.
No attempt is made here to provide a complete review of the literature regarding
the design of transuranic field studies. However, the references included
should illustrate various designs that have been used in past studies. Our
discussion of possible approaches to deciding whether cleanup is needed should
be considered only as a guide to what might be done in actual practice.
To
our knowledge, no cleanup operation has yet been conducted using the probability
approach described here.
Practical problems in the field will require modifi-~
cation of our suggestions.
PRINCIPAL STEPS IN DESIGNING TRANSURANIC FIELD STUDTES
Certain factors should be considered in designing transuranic field studies.
The description of these which follows has been adapted from Cochran (1963,
pp. 5-8).
Objectives
Identification of objectives is necessary in order to choose the most appropriate field sampling design (see Eberhardt, 1976, for discussion).
Since most
studies have multiple objectives, a compromise design may be necessary.
Some
common objectives are:
1.
Geographical distribution (spatial pattern) of the contamination over the
study site
(horizontal
and with depth).
2.
Inventory (total amount) of contamination present in soil (worldwide
fallout or local contamination).
3.
Amount of radionuclides present in ecosystem components (soil, vegetation,
air, invertebrates, vertebrates).
4,
Changes in concentration in ecosystem components over time.
5.
Rate of movement of radionuclides from one component to another (soil to
vegetation,
sediments to water,
etc.).
6.
Resuspension by wind or from the activities of man and animals.
7.
Determination of whether a cleanup operation is necessary or to verify
that cleanup has been successful.
8.
Evaluation of the hazard to man from a given contamination (this may
include all of the above objectives).
576
Population to Be Sampled
Cochran (1963) defines "population" to be the aggregate from which the sample
is chosen.
It is important to distinguish between the "target population" and
the "sampled population." In transuranic field studies, the target population
is that part of the environment about which information is wanted. The sampled
population refers to the population from which samples are actually collected.
Inferences always pertain to the sampled population. This is an important
point since the sampled population may not correspond to the target population.
As an example, consider sampling the sediments of a pond known to be contaminated by plutonium. The objective might be to estimate the average Pu concentration in these sediments. The target population could be the sediment layer
in all areas of the pond, whereas the sampled population might be restricted
to certain portions of the pond due to practical reasons. Since samples are
drawn from only the "sampled population," conclusions from these samples may
not apply to the sediments in those areas of the pond that could not be sampled.
As another example, consider sampling an area contaminated locally by a nuclear
detonation or safety-shot test. The "target population" for soil at these
sites is usually surface soil (to some specified depth) over the entire site
regardless of terrain, rock content of the soil, vegetation cover, etc.
Efforts should be made to insure that the "sampled population" is identical to
the “target population” so that no portion of the study area is systematically
excluded from sampling.
For some kinds of field studies, this goal may be
impossible to achieve. For example, suppose that we wish to estimate the
total deposition of transuranics from worldwide fallout since the start of
atmospheric testing in 1945. The target population could be considered to be
the surface area of the entire world, whereas the sampled population is restricted to land sites that have acted as fallout collectors since 1945.
Those parts of the target population not included in the sampled population
are bodies of water, inaccessible land areas such as mountains, locations
where transuranics might have accumulated or washed away, or undisturbed sites
that could not be sampled due to practical problems, or deliberate exclusion.
Data to Be Collected
Careful consideration must be given to insure that all data required to meet
the study objectives will be collected. This step requires a clear understanding of objectives so that all essential information is obtained.
It may also
be necessary to collect certain data in order to properly interpret other
data. For example, if im sttu (field) measurement instruments are used, it
may be necessary to collect additional soil samples to calibrate the in situ
readings with wet chemistry soil concentrations. These special studies may
need to be done before the main study begins.
Precision Desired
Any estimated quantity is subject to error since only a small proportion of
potential environmental samples can be collected and analyzed. This error can
be reduced in a number of ways such as by taking more samples, using more
efficient designs, and statistical analyses, or using more accurate instrumentation and wet chemistry techniques, Since these techniques usually require