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Organic pollutants 165

are often referred to specifically by the generic term organic micro-pollutants. Although
this term is poorly defined, it is widely used in the environmental literature. Nevertheless, it
should be realised that it does not cover all polluting organic compounds: for example, some
organic compounds, such as petroleum constituents or trichloroethene (TCE ), may occur in
large quantities in soil or groundwater near massive and localised releases.
The following sections cover the sources, use, environmental behaviour, and potential
toxic effects of a selection of environmentally relevant classes of organic pollutants. Given the
vast number of polluting organic compounds, it is impossible, and perhaps even undesirable,
to cover them all in this text. For further reading on environmental organic chemistry and
toxicology , see Schwarzenbach et al. (1993), Alloway and Ayres (1997), Fawell and Hunt
(1988), or Hayes and Laws (1991). For more information on individual substances or
groups of substances, see the detailed toxicological profiles issued by the U.S. Agency for
Toxic Substances and Disease Registry (ATSDR, 2013). Much of the information given in
the following section has been excerpted from these toxicological profiles. Other sources of
information, if any, are also referenced at appropriate places.

9.2 PETROLEUM AND DERIVATIVES

Petroleum or crude oil is a thick, dark brown inflammable liquid formed by the anaerobic
decay of organic matter in conditions of increased temperature and pressure in enclosing
sedimentary rocks . The organic matter breaks down into liquid petroleum and natural gas.
Both the liquid and gas tend to migrate from the source rock (usually shale) through porous
rocks and permeable rock (usually sandstone ), until they encounter impermeable layers,
underneath which they tend to collect. Crude oil consists of a complex mixture of many
different chemical compounds ranging from very volatile, light compounds like pentane
and benzene , to heavy compounds such as bitumens and asphaltenes. It contains mainly
hydrocarbons (i.e. organic compounds made up of only carbon and hydrogen) of the alkane
series, which usually make up about 95 percent of the crude oil. These aliphatic components
are barely soluble in water. Crude oil also includes impurities such as monocyclic and
polycyclic aromatic hydrocarbons . The remaining part is comprised of small amounts of
oxygen , nitrogen , and sulphur, and traces of other elements, such as heavy metals .
Petroleum is extracted from reservoirs by drilling and pumping, after which it is refined
by distillation. The products include kerosene, benzene , petrol, paraffin wax, asphalt, etc.
The four lightest hydrocarbons methane (CH ), ethane (C H ), propane (C H ), and butane
4 2 6 3 8
(C H ) are all gases used as fuel. The alkane chains with five to seven carbons are all light
4 10
and volatile and are used as solvents and dry cleaning fluids. The alkane chains with six to
twelve carbons are mixed together and used for petrol. Kerosene is made up of chains in the
range between 10 to 15 carbons, followed by diesel fuel (10 to 20 carbons), and heavier fuel
oils, such as those used in ship engines. All these petroleum compounds are liquid at room
temperature and are less dense than water. Because oil does not mix with water, it belongs
to the group of LNAPLs . Alkane chains with more than 20 carbons (paraffin wax, tar, and
asphaltic bitumen, respectively) are solid.
Environmental pollution by petroleum and oil products may occur during extraction,
refinement, transport, storage, and use. Spills and leaks are the principal causes of oil
pollution of soil, groundwater, and surface water, and pose a threat to soil and water quality,
plant and animal life, and human health. Traces of oil in water can substantially affect
odour and taste and can make the water unfit for use as drinking water. On sandy soils,
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phytotoxicological effects may already occur at concentrations of 0.5 mg kg (Scheffer and
Schachtschabel, 1989). Possible hazards are also related to impurities in petroleum and
derivatives, such as benzene (see Section 9.3) and PAHs (see Section 9.4), or additives to

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