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Chapter 7 Obtaining and Preparing Samples for Analysis 197

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Sea bottom Figure 7.5
Sedimentary layers Schematic diagram of grab sampler. When
the sampler reaches the sediment, the jaws
of the grab sampler are closed, collecting a
sample of the sediment.

Sample Collection Solids are usually heteroge-
neous, and samples must be collected carefully if
they are to be representative of the target popula-
tion. As noted earlier, solids come in a variety of
forms, each of which is sampled differently.
Sediments from the bottom of streams, rivers,
lakes, estuaries, and oceans are collected with a bot-
tom grab sampler or with a corer. Grab samplers are
equipped with a pair of “jaws” that close when they
contact the sediment, scooping up sediment in the
process (Figure 7.5). Their principal advantages are
ease of use and the ability to collect a large sample.
Disadvantages include the tendency to lose finer-
grained sediment particles as water flows out of the
sampler and the loss of spatial information, both lat-
erally and with depth, due to mixing of the sample.
An alternative method for sampling sediments
uses a cylindrical coring device (Figure 7.6). The
corer is dropped into the sediment, collecting a col-
umn of sediment and the water in contact with the
sediment. With the possible exception of sediment
at the surface, which may experience mixing, sam-
ples collected with a corer maintain their vertical
profile. As a result, changes in the sediment’s com-
position with depth are preserved. The main disad-
vantage to a corer is that only a small surface area is
sampled. For this reason sampling with a corer usu- Figure 7.6
ally requires more samples.
Schematic diagram of a piston corer in
Soil samples collected at depths of up to 30 cm are easily collected with operation. The weight of the corer is
scoops or shovels, although the sampling variance is generally high. A better sufficient to cause its penetration into the
method for obtaining soil samples near the surface is to use a soil punch. This sediment, while the upward motion of the
piston allows water pressure to help force
thin-walled steel tube retains a core sample when it is pushed into the soil and the sediment column into the barrel of the
removed. Soil samples collected at depths greater than 30 cm are obtained by corer.
digging a trench and collecting lateral samples with a soil punch. Alternatively,