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Guide for Characterization of Sites Contaminated with Energetic Materials
7
the groundwater samples, a more detailed soil-sampling plan should be
implemented.
Sample Collection
The unusual nature of explosives as contaminants must also be taken into
consideration for all aspects of the sampling, preparation, and analysis of soil or
water matrices. For instance, explosives are solid at ambient temperature, and
contamination often occurs as various size particles; they dissolve slowly and
sparingly in aqueous solution and possess low vapor pressure (Table 1). There-
fore, explosives compounds are only transported through soil once they are
dissolved in water. Hence, the highest levels of explosive contamination are most
likely to occur directly on or near the soil surface, even at sites that have
remained dormant for many years. Nevertheless, the spread of contamination will
vary, depending upon the specific explosive and the nature of the soil matrix. In
many cases, subsurface soil sampling is needed to delineate the transport
pathway or the contamination plume. Moreover, the crystalline nature of
explosives and their potential association with munition casing fragments often
result in a heterogeneous distribution of contaminant particles in the source
region. Several studies have reported on the extreme short-range spatial vari-
ability that often exists for explosives in surface soils (Walsh et al. 1993; Jenkins
et al. 1996a, 1997a, 1997b, 1997c; Thiboutot et al. 1997). For example, groups of
seven discrete surface samples (>500 g/sample) collected within 120 cm of each
other were found to range in ratios of the highest to the lowest EM concentration,
from 3 to greater than 600, with a median value of 50 (Jenkins et al. 1996a).
To address larger spatial scales and to highlight the use of composite sam-
pling, another study assessed an area between two suspected hot spots (Thiboutot
et al. 1997a, b). For this effort a systematic grid was used to divide the region of
concern into fourteen 6-m 6-m grids, then each grid was further subdivided into
four 3-m 3-m subgrids. Large discrete soil samples (top 5 cm of soil from a
circle with a radius of 77 cm) were collected in the middle of the 3-m 3-m
grids. Along with the analysis of the discrete samples, a composite sample was
prepared from all four of the subgrid samples. Statistical analysis revealed that
the concentration estimates for the grid composites (6 m 6 m) were generally
within 25% of the mean of the four subgrid samples. In summary, these studies
have repeatedly shown that if discrete samples were used to estimate an average
concentration, the sampling error typically exceeded the analytical error by an
order of magnitude. The results have also demonstrated that the homogenization
of discrete samples and subsequent combination to form a composite sample lead
to the minimization of the characterization problems caused by spatial hetero-
geneity. This ability to prepare composite samples that accurately represent the
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