Page 309 - Soil and water contamination, 2nd edition

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exchange capacity and base saturation. In the USA and Europe, the emission and
atmospheric deposition rates of acidifying components peaked in the 1970s and 1980s. Since
then, the rates of atmospheric deposition of these and other compounds have declined as
a result of environmental policies. For example, total annual emissions of SO in the USA
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dropped from 35.0 million tonnes in the 1970s to 15.1 million tonnes in 2005 and in
Western Europe they decreased from 29.3 million tonnes in the 1970s to 6.2 million tonnes
in 2005 (Smith et al., 2011). A recent study in a number of spruce forests in the northeastern
USA (Lawrence et al., 2013) showed only a modest recovery of the soil in response to the
declining acid deposition, but not at all sampling sites. The study showed that, in general,
exchangeable aluminium in the topsoil began to disappear when the mobilisation of
aluminium decreased and the topsoil was replenished by decaying plant litter, which has
low levels of aluminium. However, calcium levels in the soil remained low, because the soil
material is not rich in this element and weathers very slowly. This study demonstrates that
soil recovery is slow process. It is also important to note that globally, the total emissions
and deposition of acidifying components have not decreased as dramatically as they have
in North America and Europe. For example, in China, the fastest growing economy in the
2000s, the total annual emission of SO rose from 7.3 million tonnes in the 1970s to 32.7
2
million tonnes in 2005 (Smith et al., 2011). The average annual bulk nitrogen deposition
-1
-1
-1
-1
also rose from 13.2 kg N ha y in the 1980s to 21.1 kg N ha y in the 2000s. In the
industrialised and agriculturally intensified regions of China, current nitrogen deposition
rates match the peak levels of deposition in Western Europe in the 1980s (Liu et al., 2013).
16.4.4 Effects of soil erosion and deposition
Erosion and deposition of soil particles is an important vector for contaminant redistribution
in catchments (Stone, 2000; Walling and Owens, 2003). Obviously, this effect only applies
to sediment -associated contaminants. The various processes involved in the erosion and
deposition depend on landscape position and characteristics such as slope gradient, slope
length, soil infiltration capacity, soil erodibility, and vegetation (see Chapter 12). If the topsoil
has been enriched with contaminants as a consequence of agricultural inputs or atmospheric
deposition , the contaminants may be translocated from hill slopes to sedimentation areas
downstream, such as valley bottoms or floodplains . This leads to contaminant losses in erosion
areas and gains in sedimentation areas and, consequently, to patterns of contaminants and
nutrients that are closely related to landscape topography (e.g. ; Van der Perk et al., 2004;
2007; Xiaojun, 2010). In addition to soil erosion induced by rainfall events, soil particles are
also moved laterally by tillage. The net translocation is downslope and its rate depends on
the slope gradient. As in the case of rainfall-induced soil erosion, this results in a net loss of
soil material and associated contaminants on convex parts of the landscape and a net gain on
concave parts (Lobb et al., 1995; Quine et al., 1999; Heckrath et al., 2005).
Van der Perk et al. (2002) investigated the effect of soil erosion and deposition on the
spatial distribution of Chernobyl-derived 137 Cs deposition values in the arable part of the
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small Mochovce catchment (1.4 km ) situated in a hilly part of the Danube Lowlands in
western Slovakia. This study combined a straightforward long-term sediment redistribution
model presented by Govers et al. (1993) (see Section 12.6) and geostatistical interpolation
of point samples of 137 Cs activity in soil to distinguish the effects of sediment erosion and
deposition from other sources of variation in 137 Cs. Figure 16.8 shows the interpolated
137
137
pattern of Cs deposition density. Enhanced Cs activity in soil was found in the bottoms
of the side-valleys and in a small floodplain area in the central part of the study area,
137
whereas diminished Cs activities were found on the steep slopes in the eastern part of the
catchment. At the scale of the entire catchment, soil erosion and deposition accounted for
137
only 25 percent of the total variation in Cs activity in soil. This rather low value could be

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