A Practical Approach for Site Investigation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 160

are taken, one has to realise that only a very small portion of the soil has been inves-
tigated. Consequently, even with the most intensive Main Investigation, one has to
accept the fact that there will appear differences between the conceptual model and
reality. As mentioned earlier, certainly in the Main Investigation, non-intrusive tech-
niques and screening techniques can play in important role. These techniques can,
at least partly, bridge the gap between the per definition scarce information from
sampling and analysis and the contamination that is really there.
3.9 Sampling Patterns
Specifically in the Main Investigation the issue of sampling patterns arises. Whereas
in the Exploratory Investigation the sampling is the primary target, more structured
sampling patterns can be used in the Main Investigation.
Targeted sampling, also known as judgmental sampling, is a type of sampling
where samples are taken at points that are of specific interest. In the Exploratory
Investigation, these are the locations where soil contamination is expected. It is
obvious that the results obtained with these samples are not representative of the
whole site under consideration. From a statistical point of view, these results will
be highly biased if they are used to predict the quality of the whole site. Still, from
the perspective of the Exploratory Investigation these results are highly valuable as
they provide, based on the Preliminary Investigation, a cost effective indication of
the presence or absence of contamination.
As stated in Section 3.8 , the Main Investigation aims to delineate the contamina-
tion while simultaneously obtaining more information on the level of contamination
of the “hot spots”. Obviously, these two goals can be achieved at the same time.
Consequently there is a need to sample both on the (expected) boundaries of the
contamination as well as in the “hot spots” themselves. The use of a predefined
sampling pattern can then be of help.
For easy interpretation, systematic sampling patterns, sampling grids, are easy to
handle. However, in practice a lot of grid points will often prove to be inaccessible
for sampling due to buildings on the site, underground constructions and pipeline, et
cetera. Systematic patterns imply a risk of biased sampling when the sampling pat-
tern coincides with a (historical) pattern in the site. A simple example is a pipeline
in parallel with the grid lines. Leakage from the pipeline will be found when the grid
is close to the pipeline, while it will never be found when the grid is too far away.
In practice, these risks are not too important and can be acceptable, specifically in
light of the limited soil volume that is sampled and the risk of the resulting bias.
More important perhaps is the fact that a systematic pattern tends to identify a lot
of samples on locations that are of less interest, as there is no real expectancy of
contamination. This can be overcome by using a systematic pattern that focuses on
locations of interest.
Apart from systematic sampling, other patterns are random sampling or stratified
random sampling. In random sampling, the locations are chosen randomly over the
site. In practice this technique will not be used very often as there is no guidance