A Practical Approach for Site Investigation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 147

underneath a road on the gasworks site. Considering their clothing it might well
have been oil or tar that they were pumping. However, the essential message from
a picture like this is that it shows an activity on an unexpected location which was,
probably more or less accidentally, caught on photo. It implies that, whatever effort
will be put into the Preliminary Investigation, one has to accept the fact that not
all potentially contaminating activities can be traced. Certainly not for a site that
has been used for several decades or even centuries. This lack of knowledge of past
activities may not be an excuse to omit the Preliminary Investigation. The exam-
ple shows that soil contamination can be present due to different (often unknown)
activities and consequently that sampling only very small locations on a site, per
definition implies a risk of missing the contamination.
Let us assume another example where a drum containing dioxins is buried under-
ground in an area of 50 by 50 m. This is only a small area which has been identified
as the approximate location where the drum was ditched in the past. Getting a more
precise indication of its location is not very likely in practice. The surface of the area
is 2,500 m 2 , while the surface of the drum is assumed to be 1 m 2 . Consequently,
using one boring in the Exploratory Investigation to determine if there is a contam-
ination, would result only in a 1 to 2,500 chance (0.04%) of actually finding the
drum! Or enhancing the probability of finding the drum to 90% would mean that
2,250 borings are necessary. Obviously, where in the Preliminary Investigation only
limited sampling will be performed, the single boring and thus the probability of
0.04% is more in the direction of what is to be considered as an acceptable effort
than the 2,250 borings.
These examples show the importance of the Preliminary Investigation based
on which it will be determined where samples are to be taken. They also
illustrate the limitations of the conventional technique for soil investigations,
i.e. using (hand) augers to make borings and taking samples to the laboratory.
Other survey techniques might be far more successful in finding the drum, even
without actual intrusion into the soil. A number of different non-intrusive tech-
niques can be mentioned, e.g. magnetometry, metal detection, ground penetrating
radar, or seismic/acoustical techniques. However, although in recent years large
enhancements in the performance of the previously mentioned techniques have been
achieved, specifically by combining several techniques, the circumstances in the soil
will determine the success of these techniques to a large degree. When for instance
the soil contains a lot of debris, when the groundwater table is near the surface,
or when the soil is a heavy clay, the performance of these techniques is limited.
Nevertheless, also the “traditional” technique of borings has under specific cir-
cumstances, as shown in the example, only low performance characteristics. Better
performance might be obtained by excavating trial pits with a mini excavator, but
at the same time the potential exposure risks are much larger compared to borings.
However, specifically in soils with more coarse material, the excavation of trial pits
can be far more practical than borings.
Apart from identifying the locations where a contamination might be present, the
Preliminary Investigation should therefore also provide indications on the most opti-
mal survey technique(s) for that specific location in the Exploratory Investigation.