Decisions based on a set of composite samples will,
quently observed during the use of on-site ana-
for practical purposes, always provide greater sta-
lytical methods in which duplicate subsamples are
tistical confidence than for a comparable set of
analyzed and differ by more than an order of mag-
individual samples. In the study discussed above
nitude. Grant et al. (1993) conducted a holding
by Jenkins et al. (1996a, b; 1997), the composite
time study using field-contaminated soils that
samples were much more representative of each
were air-dried, ground with a mortar and pestle,
plot than the individual samples that made up the
sieved, subsampled in triplicate, and analyzed
composites. Using a composite sampling strategy
using Method 8330. Even with such sample prepa-
usually allows the total number of samples ana-
ration, the results failed to yield satisfactory pre-
lyzed to be reduced, thereby reducing costs while
cision. (The relative standard deviations [RSDs]
improving characterization. Compositing should
often exceeded 25%, compared with RSDs below
be used only when analytical costs are significant.
3% at two other sites.) Subsampling in the field is
An American Society for Testing and Materials
much more challenging because complete sample
(ASTM) guide was developed on composite sam-
processing is not feasible. However, most screen-
pling and field subsampling (Gagner and Crockett
ing procedures specify relatively small samples,
1996, ASTM 1997).
typically a few grams.
To reduce within-sample heterogeneity, two
Stratified sampling designs
methods can be employed: either homogenization
Stratified sampling may also be effective in re-
and extraction or analysis of a larger sample. Un-
ducing field and subsampling errors. Using his-
less directed otherwise, an analyst should assume
torical data and site knowledge or results from
that information representative of the entire con-
preliminary on-site methods, it may be possible
tents of the sample container is desired. Therefore,
to identify areas in which contaminant concentra-
the subsample extracted or directly analyzed
tions are expected to be moderately heterogeneous
should be representative of the container. The
(pond bottom) or extremely heterogeneous (open
smaller the volume of that subsample removed for
detonation sites). Different compositing and sam-
analysis and extraction, the more homogeneous
pling strategies may be used to characterize dif-
the entire sample should be before subsampling
ferent areas, thereby increasing the likelihood of
(e.g., a representative 0.5-g subsample is more dif-
a more efficient characterization.
ficult to obtain than a 20-g subsample from a 250-
Another means of stratification is based on par-
g sample). Collecting representative 2-g
ticle size. Because explosives residues often exist
subsamples from 300 g of soil is difficult and can
in a wide range of particle sizes (crystals to
require considerable sample processing, such as
chunks), it is possible to sieve samples into vari-
drying, grinding, and riffle splitting. Even in the
ous size fractions that may reduce heterogeneity.
laboratory, as discussed above, obtaining repre-
If large chunks of explosives are present, it may
sentative subsamples is difficult. An ASTM guide
be practical to coarse-sieve a relatively large
has been developed to help in this regard (Gagner
sample (many kilograms), medium-sieve a por-
and Crockett 1996). Although sample-mixing pro-
tion of those fines, and subsample the fines from
cedures such as sieving to disaggregate particles,
medium screening as well. This would yield three
mixing in plastic bags, etc., can and should be used
samples of different particle size and presumes
to prepare a sample, extracting a larger sample is
that heterogeneity increases with coarseness. Each
perhaps the easiest method of improving repre-
fraction would be analyzed separately but not
sentativeness. For this reason, 20 g of soil is ex-
necessarily by the same method (visual screening
tracted for the Cold Regions Research and Engi-
of the coarser fractions for chunks of explosives
neering Laboratory (CRREL) method, and the
may be possible) and then could be summed to
same approach may easily be used to improve re-
yield the concentration on a weight or area basis.
sults with most of the on-site methods shown in
In addition, aqueous disposal of explosives waste-
Table 3. The major disadvantage of extracting the
waters, such as those found in washout lagoons
larger sample is the larger volume of waste sol-
or at spill sites, often results in preferential sorp-
vent and solvent-contaminated soil that needs dis-
tion to fine-grained materials, such as fines or
posal.
clays, particularly for nitroaromatics.
The effectiveness of proper mixing in the field
is illustrated in the recent reports by Jenkins et al.
Reducing within-sample heterogeneity
(1996a, b; 1997). Duplicate laboratory analyses of
The heterogeneity of explosives in soils is fre-
the same samples, including drying, grinding,
6
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