Page 313 - Soil and water contamination, 2nd edition

300 Soil and Water Contamination

16.5 VERTICAL VARIATION
16.5.1 Introduction

Vertical variation of contaminants over the soil profile occurs as a result of temporal variation
of contaminant inputs and losses at the soil surface and subsequent vertical transport
processes that redistribute the contaminants over the soil depth. When considering the
development of soil contaminant profiles, it is useful to distinguish between inputs of
contaminants only and inputs of contaminated sediment . Although any contaminant input
implies an input of mass to the soil, the input of contaminants by atmospheric deposition
or application of agrochemicals does not usually increase the mass or volume of a soil body
significantly. However, erosion or deposition of sediment does lead to a change in soil
profile structure. Long-term application of mulch or sewage sludge may alter the soil profile
structure. Obviously, the soil profile structure is altered when land is excavated or when land
is raised with imported soil material or waste materials.
The concentration profile is continuously being altered by transport processes in soil,
particularly by leaching and bioturbation. In permeable soils in temperate climates, leaching
of dissolved substances due to gravity-driven vertical flow is typical in the unsaturated zone.
In semi-arid and arid climates, vertical upward migration of solutes by capillary action
predominates. Bioturbation, i.e. the reworking of soil material by burrowing animals such
as earthworms, arthropods, moles, and voles, causes dispersion of contaminants in the
soil profile . The effect of the dispersive action of bioturbation on the vertical distribution
of contaminants in soil was discussed in Section 11.3.3 (see Figure 11.9). In addition, in
agricultural soils vertical mixing also occurs as a consequence of tillage operations, which – in
the long term – result in a more or less uniform redistribution of substances over the plough
depth (usually 20–30 cm).

16.5.2 Effects of leaching

The leaching of solutes through the soil profile is affected by a set of physical, chemical,
and biological processes, including infiltration , evapotranspiration , root uptake,
advection , dispersion , sorption , decay, and volatilisation . The soil properties that govern
these processes are often heterogeneous in nature, for example due to the presence of soil
horizons, cracks, and root channels. One particular consequence of the heterogeneities in
physical soil parameters is the occurrence of preferential flow . Preferential flow gives rise to
a rapid leaching of contaminants through the vadose zone , because the percolating flow is
concentrated in macropores or fingers (Keesstra et al., 2012; see also Section 3.2.3). Bowman
et al. (1994) found, for example, that as much as 80 percent of the water percolating through
a soil profile actually passes through only about 20 percent of the cross-sectional area of the
profile. Furthermore, the rapid preferential flow and the exchange between the preferential
flow path s and the soil matrix cause solute breakthrough curve s in the vadose zone to exhibit
earlier arrival and longer tailing than predicted by percolation models that assume flow
through a homogeneous, unsaturated medium.
In addition to using laboratory experiments and model simulations, scientists often study
field-scale solute transport in the vadose zone with tracer experiments. For this purpose,
chemical tracers are put in or on the soil and then the spread of the tracer concentrations
in soil and groundwater is monitored. Hendriks et al. (1999) carried out an experiment
using a conservative bromide tracer on the Bouwing experimental farm in the centre of
the Netherlands from November 1994 to May 1995. The soil was alluvial with a light to
medium-heavy clay texture (clay content varied between 31–44 percent in the top 120 cm).
The soil swells and shrinks, which causes shrinkage cracks to form under dry conditions. An

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