Page 296 - Soil and water contamination, 2nd edition

16

Patterns in the soil and in the vadose zone

16.1 INTRODUCTION

In the previous chapters, various causes that give rise to differences in environmental
concentrations of contaminants and their partitioning over the different phases (e.g. solid,
liquid, gas, adsorbed, and dissolved phases) have been discussed. These causes include spatial
and temporal variation in 1) the natural background concentration of contaminants, 2) the
amounts and rates of contaminant inputs, and 3) the transport and chemical transformation
processes the contaminants are subject to. The natural background concentration of
chemicals in soil and the vadose zone depends on factors such as the natural composition of
the parent material and the nature and intensity of soil-forming processes (see Section 1.3.2).
What most affects contaminant levels apart from the variation due to natural composition
of the soil are the past and present contaminant inputs to the environment. The transport and
fate of contaminants in soil and in the vadose zone are controlled by many factors, including
climatic factors and the physico-chemical properties of soil and of the contaminant itself.
After arriving in the soil, contaminants may be transported over land by runoff water, be
leached into the soil profile , volatilise into the atmosphere, be taken up by plants or other soil
organisms, or be broken down. Soil properties that affect the rate of contaminant transport
and fate have been discussed in detail in the previous chapters and include soil depth, slope
gradient, infiltration characteristics, soil texture, total porosity , pore size distribution,
permeability , microbial population density and diversity, organic matter content , cation
exchange capacity, soil pH, redox potential, and temperature. All these factors and properties
vary in space and time to various extents and the superposition of these spatially and
temporally varying phenomena results in characteristic, but often complex, spatio-temporal
patterns of contaminants in soil and in the vadose zone. Such patterns may be smooth, with
concentrations that vary continuously in time and space: for example, a contamination
pattern that arises from atmospheric deposition . Alternatively, they may be discontinuous,
with concentrations that exhibit crisp boundaries (see Burrough, 1993): for example, a
contamination pattern that arises from different pesticide application rates on different fields
in a patchy agricultural landscape. Analysis of the spatial and temporal patterns in soil and the
vadose zone may yield site-specific information about the most important pollution sources,
but also fundamental scientific knowledge and understanding about the complex interactions
between the various factors and processes affecting the transport and fate of pollutants in soil.
This knowledge is essential to identify, assess, and delineate areas affected by soil pollution,
and for the effective management of contaminated sites and areas.
The concepts of scale and dimension discussed in Section 1.5 also apply to the aspects of
spatio-temporal patterns in soil and in groundwater and surface water. Although the spatial
patterns of contaminants in soil and the vadose zone extend over three dimensions, many
studies have only considered one or two dimensions. In general, studies on spatial variation of
soil contamination focus either on the lateral variation due to, for example, spatial differences

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Soil and Water.indd 295 10/1/2013 6:45:24 PM