Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 124

transect(s). In this process it is essential to make sure that two types of controls are also
taken. The first is to make sure that the first and last samples are taken from outside the
area that is assumed to be contaminated. Also, samples from various depths must include
a sample obtained from below the contaminated material.
In Figure 5.2 transects are taken in an area containing three soil types. Note that the
transect lines are chosen in order to obtain samples from each soil type; the figure also
illustrates taking both surface and subsurface samples at regular intervals. Also, some
transect samples are taken outside the suspected contaminated areas. These samples are
essential to establish the boundaries of the contaminated areas.
Normally it is important to take into consideration what soil type is being sampled, and
this is true for the transect samples. In this case, however, it is only important to label
samples so that the soil type from which the sample is taken can be determined later. This
information is useful for the development of the detailed sampling plan. At this point, the
most important information to gather is the level and extent of the contamination
throughout the contaminated area. An advantage of transect sampling without regard to
soil type is that if analytical results are consistent for the different soil types the detailed
sampling plan and the final analytical work is simplified, which in turn saves resources,
including money.
During transect sampling, however, unusual soil characteristics should be noted; that
is, any indication that the soil types are different from those mapped or that the soil
characteristics have not been noted elsewhere. If the soil is mapped as a silt loam and an
area with sandy texture is encountered in the transect sampling, this is important. A sandy
soil will have absorptive, retentive, and extraction properties that are very different from
silt loam. Layers that are unusually hard or compacted must also be noted, because they
will affect contaminant movement, analytical results, and applicable remediation
procedures.
The transect sampling results can also be used to identify zones of contamination (i.e.,
some areas contaminated at uniform but relatively lower levels of contaminants than
surrounding areas). When determining the number of samples needed, these areas can be
treated separately from other areas (see Chapter 6), since they may, for example, require
less sampling. This leads to increased efficiency and less cost to the project.
The analytical method chosen for analyzing the transect samples should be simple,
rapid, and inexpensive. Accuracy and precision should be of secondary concern. Use of
an entirely different analytical procedure for the detailed sampling and analysis is
appropriate. Also, note that if the two different analytical methods give similar results,
confidence in the validity of the whole sampling and analysis process is greatly increased.
Preliminary Nitrate Sampling
I worked with some researchers concerned with nitrate contamination in well
water. They had ten wells they were sampling and wished to know which
wells were in need of monitoring. At the time I was using a nitrate ion
selective electrode (ISE) for some other work and was asked if I could
analyze their samples. Some of our colleagues scoffed at the idea, saying that