Page 316 - Soil and water contamination, 2nd edition

Patterns in the soil and in the vadose zone 303

Figure 16.14 Modelled leaching concentrations of two pesticide substances A and B at 1 m soil depth in Euope.
(Tiktak et al., 2006).

concentrations are relatively high in many areas of southern Europe. This can be explained by
the often extremely low organic matter contents (< 1%) in these areas (Tiktak et al., 2006).

16.5.3 Effects of contamination history

Changes in contaminant input over time may become manifested in the vertical distribution
of contaminant concentrations in the soil profile. In Section 16.4.3, the evolution of the
concentration profile following a momentary input of a conservative tracer was discussed (Figure
16.12). It is to be expected that periodic input of soluble chemicals will similarly be reflected
in the soil profile, provided that sufficient water percolates downward through the soil and
the solutes undergo little sorption to solid surfaces. In the case that the chemicals are strongly
sorbed to the soil solids, translocation of soil material by bioturbation or tillage operations is
the predominant manner of vertical transport. However, these mixing processes homogenise
the topsoil and blur the vertical variations in concentrations. The contamination history will
then only be reflected in an increased deposition density over time, while the evolution of
contamination inputs over time will not be expressed in vertical concentration gradients in
the soil profile. In the case of contaminants with high affinity for soil solids, the contaminant
input into soil is associated with sediment input. Historical variations in inputs of contaminants
with high affinity for soil solids may in this way result in vertical concentration profiles in the
sediment deposit: for example, in floodplain deposits or bed sediments (e.g. Ayres and Rod,
1986; Winkels et al., 1998; Middelkoop, 1997, 2000; Tebbens et al., 2000). In this case, the
vertical concentration profile largely reflects the evolution of sediment contamination over time.
Middelkoop (1997) reconstructed the history of heavy metal contamination of Rhine
sediments using undisturbed sediment cores from three dike breach ponds located in the
floodplains along the river Waal. These ponds had undergone more or less continuous
deposition of sediment when the washlands had flooded sufficiently rapidly, so he was
able to achieve a temporal resolution of less than five years. The sediments were dated by

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