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Guide for Characterization of Sites Contaminated with Energetic Materials
29
Soil Sample Mixing and Subsampling
Because only a small portion (subsample) of the 500-g or larger sample is
taken for analysis, the bulk sample has to be thoroughly mixed to allow for
representative subsampling. This can be achieved by air-drying, sieving, grind-
ing, and mixing the bulk sample, after subjectively removing vegetation (organic
debris) and pebbles. The degree to which a sample is ground (the most time-
consuming step) should meet the data quality objectives of the sampling plan.
Past experience has shown that soil samples collected in areas contaminated by
explosives carried in liquid waste stream may not have to be ground to as fine a
particle size as soils collected from an impact range (Jenkins et al. 1996a, 1997b;
Walsh and Ramsey, in press).
To air dry at room temperature, the bulk sample should be spread out on a
clean flat surface and left in the dark. Typically this step takes about 24 hours,
after which the sample should be passed through a sieve. The initial sieving often
is based on the definition of soil, which encompasses particles of 2 mm and less,
in size. Method 8330 specifies a 2-g subsample, which requires that the bulk
sample be ground using a mortar and pestle until it passes through a 30-mesh
sieve (0.60-mm). Before grinding it is advisable to screen the bulk sample for
high levels of EM. Sample grinding is not recommended when screening results
have indicated EM concentrations greater than 1,000 mg/kg. Even when a 20-g
or larger subsample is taken, grinding is still recommended to obtain a repre-
sentative subsample. Before collecting a subsample the ground sample should be
thoroughly mixed. Furthermore, it is recommended that the ground sample be
spread out in a thin layer and multiple units (30 or more) be taken from random
locations to build a subsample (place sample container on scale, add units until
desired weight is obtained) of the appropriate weight for the intended method of
analysis. Care should be taken when collecting each unit so that particles of all
sizes are represented in the same proportions, that is, a visual inspection should
establish similar particle size distributions for both the subsample and the bulk
sample. An alternative method of obtaining subsamples following grinding is to
use a rotatory sample divider.
Analysts are cautioned that subsamples removed from bulk soil samples
collected from impact ranges that were air-dried, ground with mortar and pestle,
passed through a 30-mesh sieve, and thoroughly mixed, but were not ground to a
fine powder, were found to be highly variable in analyte concentrations (Walsh
and Ramsey, in press). Walsh and Ramsey (in press) determined that the percent
relative standard deviations (%RSD) were typically greater than 50% for sets of
twelve subsamples ranging in size from 2 to 50 g. The range in absolute analyte
concentration for these subsamples often was on the order of a magnitude, and in
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