Page 297 - Soil and water contamination, 2nd edition

284 Soil and Water Contamination

in contaminant immissions at the soil surface, or vertical variation over the soil profile due
to, for example, temporal variation of contaminant immissions. The level of contamination is
mostly expressed in concentrations per unit mass, but in some studies of the lateral variation,
the contaminant concentrations are integrated over the mass of soil present in a soil profile
to obtain amounts per unit area, often referred to as deposition density. One application of
this unit of measurement is to express radioactive contamination of soil due to atmospheric
-2
deposition (Bq m ). To convert concentration to deposition density, the following expression
can be used:
C d C b (16.1)
d
-2
-2
-1
-3
where C = deposition density [M L ] or [T L ], C = concentration [M L ] or activity
d
-3
-1
concentration [T L ], ρ = dry bulk density of the soil, d = depth of soil profile . If the dry
b
bulk density is constant over the soil profile, the solution to Equation (16.1) is:
C C d (16.2)
d b
-3
-3
-1
where C = depth-averaged concentration [M L ] or activity concentration [T L ].
In the following sections the causes and effects of various kinds of variation of soil
contamination will be illustrated and further discussed on the basis of a number of selected
case studies.
16.2 NATURAL VARIATION IN BACKGROUND CONCENTRATIONS
Elements and substances that occur naturally in the environment exhibit concentration
patterns in soil that are closely linked to bedrock lithology, geological mineralisation (i.e.

the hydrothermal deposition of metals in ore bodies), and soil type (e.g. Bini et al., 1988;
Salminen and Gregorauskien, 2000; Rawlins et al., 2003; Myers and Thorbjornsen, 2004).
Concentrations of many substances, including metals and nutrients, are generally higher in
fine-textured soils than in coarse-textured soils. Metal concentrations also tend to be higher
in geologically mineralised areas. In recent years, many geological surveys have mapped the
spatial patterns of elements in various environmental media, including the topsoil and the
subsoil, The maps have been published in geochemical atlases, which provide data at national
scale (e.g. Gustavsson et al., 2001; Caritat and Cooper, 2011; Rawlins et al., 2012; Mol et al.,
2012) or at the international, continental scale (e.g. Salminen et al., 2005; Reimann et al.,
2012).
Because contaminants mostly enter the soil from above through atmospheric deposition,
the application of fertilisers, manure, and pesticides on agricultural land, or the deposition of
contaminated sediments on floodplains, it is in the topsoil that contaminant concentrations
are generally highest. There are obvious exceptions, for example when contaminants are
buried underground or supplied from below through upward seepage of contaminated
groundwater. In areas where these exceptions are not present and if the contaminants have
not migrated into deeper soil layers, the concentration of an element or substance in the
uncontaminated subsoil is generally considered to be an appropriate proxy for the natural
background concentration. However, it is important to recognise that natural background
concentrations may vary within a soil profile as a result of natural soil-forming processes,
including deposition of rainwater components, aerosols, sediments, and organic matter and
the translocation of substances within the soil.
Figure 16.1 shows two example maps from the FOREGS geochemical atlas of Europe
(Salminen et al., 2005), which depict the spatial distribution of lead and mercury in the

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