Introduction to Contaminated Site Management - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 17

this much simpler terminology, the water-unsaturated layer above the groundwater
table is often simply referred to as 'soil', while the water-saturated layer under the
groundwater table is often called 'groundwater'. Note that in this topic 'soil' is
sometimes also used in the wider definition, for example, in the context of 'soil
policy', or 'contaminated soils'. Clearly, both terms refer to both soil (in the more
narrow definition of water-unsaturated upper layer) and groundwater.
An important difference between the upper soil and the aquifer is that ground-
water is an important consumer product. This implies that clean groundwater has
intrinsic value. From this perspective, the aquifer could be considered as a protec-
tion target. Soil, on the other hand, is not commonly used as a consumer product, but
principally serves as an indispensable source for many useful products and activities.
It is important to realize that the separation between the upper soil layer and the
aquifer, although from a scientific viewpoint convenient, is transient and partly arti-
ficial. With regard to the presence of contaminants, and the risks related to them, this
separation is rather confusing. The reason for this is that contaminants migrate and
do not necessarily belong to one of the two entities. Contaminants move through-
out the soil-groundwater system, predominantly downwards, but sometimes also
upwards and laterally, without acknowledging any borders between the soil and
groundwater zones.
However, there are important differences between the water-unsaturated upper
soil and the water-saturated aquifer. The upper soil, for example, enables the rapid
transport of volatile contaminants via the gas phase. And the presence of a sub-
stantial amount of organic matter in the upper layer has a strong influence on the
behaviour of contaminants and on the problems associated with these contami-
nants. Generally speaking, transport of water and contaminants is much faster in
the groundwater than in the upper soil.
Another important difference between the upper soil and the deeper layers is the
biological activity. Although organisms are found at every depth in the soil profile
(see Chapter 13 by Swartjes et al., this topic), the number of organisms is higher
in the water-unsaturated upper soil, due to the presence of a gas phase. Within this
upper soil layer, the number of organisms is even higher in the top of the soil, that
is, the organic matter-rich layer that varies from a few centimetres up to several
decimetres.
In many inhabited areas in the world the natural soil profiles are often disturbed.
Many human activities, from the past and the present, are responsible for this fea-
ture, for example, (mechanical) digging activities in cities, tillage in agricultural
areas or the addition to soils of foreign materials such as debris, stones, tar, and
waste materials. Since most contaminated sites are within urban areas and disturbed
soils are complex due to their heterogeneity, urban soil science is seen as a chal-
lenging, current frontier of soil science today (Norra 2006 ). In more extreme cases,
whole new layers of soil material, mostly sand, sometimes clay, have been added
onto the (natural) soil in many urban areas for infrastructural or filling purposes.
Man-made soils, often with a high contribution of extraneous materials, are called
Technosols . Generally speaking, soil structure is lacking in these artificial soil lay-
ers. In the lower parts of the Netherlands, for example, from the 16th century until