Page 68 - Soil and water contamination, 2nd edition

Environmental compartments 55

In agricultural areas, the application of pesticides and fertilisers constitutes an important
source of pollutants in soil. Pesticides are often sprayed directly onto the crops, whereas
fertilisers are either sprayed onto the soil surface or injected into the soil at a shallow depth.
Pesticides comprise a variety of organic compounds (e.g. lindane, isoproturon, atrazine),
which, in turn, may contain heavy metals (e.g. copper (Cu), mercury (Hg), arsenic (As)).
Fertilisers promote plant growth by enhancing the supply of essential nutrients, such as
nitrogen (N), phosphorus (P), and potassium (K). These nutrients are included in organic
fertilisers, such as manure , compost, or sewage sludge , or in inorganic artificial fertilisers,
such as for example diammonium phosphate ((NH ) HPO ) or potassium chloride (KCl).
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As well as containing the major plant nutrients, manure and fertilisers often contain heavy
metals , such as cadmium (Cd) in phosphate fertilisers, and copper (Cu) in pig manure. As a
consequence, the application of fertiliser has resulted in slight but widespread contamination
of most agricultural soils in the western world. Sewage sludge and compost, which are used
as fertilisers and to enhance the soil structure, also usually contain heavy metals as well as
other pollutants; spreading sewage sludge or compost on agricultural land causes these
pollutants to be added to the soil. In many western countries, under recent legislation, sludge
may only be spread if it is relatively free of pollutants. Excess application of fertilisers causes
a build-up of nutrients in soil, which in turn may cause enhanced volatilisation of ammonia
(NH ) to the atmosphere, and the leaching of nutrients to deeper horizons beyond the reach
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of crop roots. The chemical properties of both soil and nutrient species control the extent
and rate at which leaching occurs. In the countryside where sewage treatment systems are
lacking, septic tanks comprise another significant source of soil pollution by nutrients. In
contrast to the application of fertilisers, septic tanks can be considered as point source s.
Soils are also affected by pollutant inputs from the atmosphere. The most notorious
example is the atmospheric deposition of acidifying compounds originating from power
plants, chemical plants, and traffic (e.g. sulphur dioxide (SO ) nitrogen oxide (NO ),
2 , x
which are transformed into sulphuric acid (H SO ) and nitric acid (HNO ), respectively, by
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atmospheric reactions), and agriculture (mainly ammonia (NH ), which is transformed into
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nitric acid in soil). Before the introduction of unleaded fuel, traffic used to be an important
source of lead (Pb) contamination. In the vicinity of roads and highways, traffic is still a
source of atmospheric deposition of benzene and polycyclic aromatic hydrocarbons (PAHs),
both of which are constituents of petrol. De-icers and rubber tyre particles (containing
heavy metals ) also contribute to soil pollution in the vicinity of roads. Atmospheric emission
and subsequent deposition onto soil is also induced by incineration of wastes (primarily
heavy metals, NO ) , PAHs, PCBs ), and metal smelting (heavy metals). In the 1950s and
x
1960s nuclear bomb tests in the open atmosphere caused global atmospheric radioactive
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fallout of various radionuclides (e.g. radioiodine ( I) radiocaesium ( Cs and 137 Cs)
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and radiostrontium ( Sr)). Also the Chernobyl accident in 1986 caused fallout of these
radionuclides over vast areas of Europe. Other radiological accidents have caused radioactive
fallout and soil pollution at a more regional scale . Furthermore, polluted soil particles
resuspended in the air can be deposited elsewhere on the soil surface: for example wind-
blown dust from mine tailings, smelters, or roads. A similar process occurs in rivers , where
contaminated sediments can be deposited on the floodplain soils during flooding. Because
sediment deposition also entails a substantial input of soil material, floodplain soils may have
become contaminated over a considerable depth. Finally, upward seepage of contaminated
groundwater may also be a source of contaminants in the soil profile .
3.2.3 Soil water

Almost the entire part of the soil as defined here belongs to the unsaturated zone, also called
the vadose zone . In contrast to the saturated zone, where almost all the interconnected

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