Page 319 - Soil and water contamination, 2nd edition

306 Soil and Water Contamination

of sediment during flooding is greater than in the areas further away from the river, where
a minor dike protects the washland from low floods.
• The maximum concentrations in the soil profile are also greatest near the main river
channel. The reasons for this are twofold. First, the part of the washland section near
the river receives more input of contaminated sediment . After deposition, the fresh
sediment is mixed with the topsoil through the dispersive action of bioturbation
and, possibly, tillage (although most of the washlands are used as grasslands with
little tillage). This dispersion effect causes the maximum concentration to decrease
(compare Figure 11.9). Second, heavy metal contamination of suspended sediments
tends to decrease with increasing discharge: for example, due to dilution with less
contaminated sediments from slopes (see also Chapter 18). As a consequence low-
lying washlands that are not protected by a minor dike are already inundated during
relatively low discharges and thus receive sediments that are more contaminated than
parts of the washlands that are only inundated during high discharges (Middelkoop
et al., 2003).
• The concentration in the top 10 cm of the soil is lower near the main river channel than
further away from the river. Again, this can be attributed to the larger sediment inputs on
the washland near the river. The decreased contamination levels of the Rhine sediments
since the early 1970s have caused the heavy metal contamination in the topsoil to
diminish gradually. This effect is less pronounced in the areas that receive less sediment,
because the thinner layer of less contaminated sediment has been mixed with the more
contaminated sediments underneath.
• The decreased concentrations during the 1940s are not visible in either of the profiles.
This is probably because the decrease in concentrations occurred only during a relatively
short period; the mixing processes in the soil profile have blurred the two distinct
concentration peaks of the 1930s and the 1960s into one peak.

It should also be mentioned that sediment deposition on floodplains is a discontinuous
process that only occurs during high discharge events. If such events do not happen for a
prolonged period of years, a significant part of the contamination history will not be
reflected in the soil profile . Therefore, the peak concentration in the profile of a floodplain
soil does not necessarily represent the year when the concentrations in suspended sediment
were at their maximum (Delsman, 2002).

16.6 FURTHER READING ON CONTAMINANTS IN SOIL

• Mirsal, I.A., 2008, Soil Pollution: Origin, Monitoring & Remediation, 2nd edition, (Berlin:
Springer).
• Ginn, J.S., and Russel Boulding, J., 2003, Practical Handbook of Soil, Vadose Zone, and
Ground-Water Contamination: Assessment, Prevention, and Remediation, 2nd edition,
(Boca Raton FL: Lewis/CRC).
• Merrington, G., Winder, L., Parkinson, R., and Redman, M., 2002, Agricultural
Pollution: Environmental Problems and Practical Solutions, (New York: Spon Press).

EXERCISES

1. Explain briefly:
a. which factors control the natural background concentrations of substances in soils;
b. why heavy metals are usually more mobile in sandy soils than in loamy soils;

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