Page 298 - Soil and water contamination, 2nd edition

Patterns in the soil and in the vadose zone 285

subsoil (samples were taken from the C horizon – a 25 cm thick section within a depth range
of 50 to 200 cm). This figure shows that high Pb content is found in subsoil in northern
Portugal and Galicia, southern Portugal and the Spanish Sierra Morena, the Massif Central
and the Massif Armoricain in France, the Alps, the Black Forest in southwest Germany, the
Ore mountains in the border area between Germany and the Czech Republic, the Tyrrhenian
fringe of Italy, the karstic coastal areas of Croatia and Slovenia, Slovakia, the Attica region in
Greece, and the Dalarna-Jämtland region (Sweden) in the Central Scandinavia Baltic Shield.
In the majority of these areas the mercury values are also elevated, but the mercury content
in the subsoil is also high in southern Spain, western Austria, southern and western Germany,
and eastern Slovakia. The lead and mercury values are notably low in the metamorphic
basement rocks of Fennoscandia and, consequently, in the Pleistocene glacial drift derived
from these rocks in the northern Germany, Poland, Denmark and the Baltic states. Low
values are also found throughout parts of central and eastern Spain and northeastern
Greece where there are granite, gneiss, schist, and sedimentary rocks. These maps clearly
demonstrate that lithology and geological mineralisation are important factors influencing
the occurrence of high concentrations of lead and mercury – and other metals – in the soil
(De Vos et al., 2006).

16.3 VARIATION DUE TO CLAY AND ORGANIC MATTER CONTENT

The contaminant concentration in a soil or sediment sample depends not only on the
natural background concentration and the amount of contaminant immitted, but also on
the adsorption properties of the soil material. For organic contaminants, the adsorption
properties are largely determined by the organic matter content , whereas for metals they
depend on both the organic matter content and clay content . Because the clay and organic
matter content in soils can vary over several orders of magnitude, their spatial variation
laterally and vertically may be the dominant cause of spatial variation in environmental
concentrations of contaminants in soils and sediments (see Van der Perk and Van Gaans,
1997). For example, Figure 16.2 shows the resemblance of spatial patterns of clay content
and the concentrations of As and Zn within the topsoil of a field near Pozzuolo del Friuli,
Italy (De Zorzi et al., 2002).
The relation between organic matter and clay content on the one hand and contaminant
concentrations on the other represents the natural preferential adsorption to the surfaces of
clay minerals and organic matter. It holds for naturally occurring background concentration s
as well as for elevated concentrations in polluted soils. However, in polluted soils, the
quantitative relationships may be very different from those in unpolluted soils (Salomons
and Förstner, 1980; Middelkoop, 1997). Middelkoop (1997) demonstrated that the effect
of clay and organic matter content becomes stronger with increasing degree of pollution of
river sediments from the Waal and Meuse rivers in the Netherlands. Using multiple linear
regression he evaluated the relationships between clay (particle size < 2 μm) and organic
matter content and heavy metal concentrations (Cu , Cd , Pb , and Zn ) for three subsets
of sediments. The first two subsets comprised recent overbank deposits from washlands
(embanked floodplain sections) along the river Meuse and river Waal (a distributary of the
river Rhine ), respectively, with the Meuse sediments being more polluted than the Waal
sediments. A third subset comprised more or less unpolluted pre-industrial sediment that had
been deposited between about 1500 AD and 1750 AD. Although these sediment samples
were not contaminated by historical industrial activities, due to pre-industrial human
activities they might have slightly higher metal concentrations than natural background
values. For instance, in Meuse sediments dating from Roman times (Subatlanticum) Tebbens
et al. (2000) found Pb concentrations that were higher than natural background values; these

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