Page 21 - Soil and water contamination, 2nd edition

8 Soil and Water Contamination

processes during formation. By contrast, the variation in the composition of sedimentary
rocks is principally the result of geomorphological processes of differential weathering and
mechanical sorting during transport of the weathering products. At the same time, the
natural variation in the composition of natural waters is largely attributable to differences in
the chemical composition and weatherability of the bedrock. Whereas the soil composition
reflects the more or less stable solid residue of the bedrock, the composition of natural
waters reflects the easily weatherable fraction of the bedrock. Accordingly, the background
concentration s of the different substances in soil and water can display a wide variation in
geographical space. Nevertheless, the background concentrations are usually low compared to
the concentrations due to anthropogenic inputs. Exceptions are locations near ore outcrops
or volcanic springs, where the soil and water may be enriched in, for example, heavy metals
or radioactivity.

1.3.3 Anthropogenic sources
Human activities may enhance the concentration of polluting substances in soil and water
directly or indirectly. Direct releases of pollutants may occur from a single location or point
source , such as an effluent discharge pipe or a container leaking petrol into the soil, or a
natural outcrop of a mineral vein rich in heavy metals . Pollution around a point source is
mostly confined to a plume in the downstream direction from the source (e.g. Figure 1.1a).
The extent to which the front of the plume spreads from the source depends on the age of
the pollution source, the flow velocity, and the retention along the flow path s. The dispersal
perpendicular to the flow direction is generally limited. In general, point sources are relatively
easily monitored and controlled, because they can often be identified as being caused by a
particular individual or organisation.
Chemicals may also enter the environment over a wide area. Examples are
anthropogenically enhanced atmospheric deposition of acid compounds (acid rain ), global
atmospheric fallout of radionuclides following the nuclear bomb testing in the 1950s and
1960s and nuclear accidents, fertiliser leaching from agriculture fields (see Figure 1.1b),
contaminated runoff from city streets , and deposition of sediment -associated pollutants on
floodplains. A pollutant release that occurs over a wide area is said to have a non-point source
or diffuse source . The pollutant concentration gradients in soil and water originating from
diffuse sources are usually gradual. In addition, relatively small point source s may occur at
many different places that cannot be distinguished individually: for example, discharge
of domestic wastewater into urban groundwater (e.g. via septic tanks ) or surface water, or
a large number of leaking containers in an extensive waste disposal site. The impact from
any one location may be minimal, but the cumulative effect of all source locations can
be substantial. The fact that the pollutants mix in groundwater or surface water makes it
impossible to identify the contribution from each individual source, so the total of different
point source locations acts as a diffuse source .
In addition to direct environmental pollution due to point source or diffuse source
emissions, the release of certain substances into the environment may also lead to indirect
pollution. For example, enhanced deposition of acid compounds may effectively increase
the rate of natural weathering of bedrock. Consequently, this increases the rate at which
minerals, including heavy metals , are released from the bedrock into soil and water.
The extent to which polluting substances in soil and water are enriched by human
activities depends on the background concentration , the amount released, the source type,
and the pollution’s tendency to disperse in the environment. The enrichment ranges between
less than one order of magnitude up to several orders of magnitude. Obviously, a substance
with a background concentration close to zero is more sensitive to enrichment than a
substance with a large background concentration. The enrichment is also greater if large

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