Page 177 - Soil and water contamination, 2nd edition

164 Soil and Water Contamination

of organic compounds include fuel, pesticides , plasticisers, surfactants, solvents, pigments, or
coolants. They may cause environmental pollution as they enter the environment as a result
of industrial processes, product use and applications, spills and leaks, combustion of fuel,
dumping or incineration of organic wastes or wastes contaminated with organic compounds,
and application of pesticides.
The dispersal and persistence of organic compounds in the environment are principally
controlled by their physico-chemical properties. Organic pollutants may occur in the
solid, liquid, and gas phases. Some organic compounds evaporate readily and completely
and remain in the gas phase when exposed to the atmosphere at normal environmental

temperatures and pressures. These – mostly liquid – compounds are often referred to as
volatile organic compounds (VOCs ). VOCs include pure hydrocarbons (e.g. benzene ,
toluene ), partially oxidised hydrocarbons (e.g. acetone), and organic compounds containing
chlorine, sulphur or nitrogen (e.g. methylene chloride , methyl chloroform). Semi-volatile
organic compounds (SVOCs ) comprise another set of organic compounds, which evaporate
slowly and partially when exposed to the atmosphere.
Some groups of organic compounds, for example carbohydrates, alcohols, and organic
acids, contain polar functional groups that allow the formation of hydrogen bond s (e.g.
hydroxyls). Consequently, they are hydrophilic and soluble in water. More polar organic
compounds, particularly those containing oxygen and nitrogen , tend to be more water-
soluble than less polar organic compounds. However, most organic pollutants are non-polar
and are hydrophobic. The inability to form hydrogen bonds prevents water from solubilising
these compounds and mixing with them, and so they are excluded from the aqueous phase .
Liquid hydrophobic organic compounds, such as petroleum and benzene , are therefore often
referred to as non-aqueous phase liquids (NAPLs). Given similar structures of the organic
molecules, compounds with higher molar masses are generally more hydrophobic and less
volatile. Hydrophobic organic compounds tend to occur both attached to solid surfaces and
as liquid droplets. They interact strongly with the organic matter associated with the solid
phase. To describe the affinity of an organic compound for water and organic matter, the
octanol–water partition coefficient K is commonly used (see Section 2.5.3).
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The behaviour of immiscible NAPLs in the subsurface depends on their density.
Compounds with densities less than the aqueous phase will float on the capillary fringe if
a sufficient amount of the compound is present. The NAPLs are called light non-aqueous
phase liquids (LNAPLs). Dense non-aqueous phase liquids (DNAPLs ) have densities
greater than the aqueous phase and will sink until an impermeable barrier is reached. This
flow behaviour of NAPLs will be discussed further in Section 11.4.
The persistence of organic compounds in the environment is largely controlled by
their degradation rate, which is promoted by microorganisms (biodegradation ) or light
(photochemical degradation). Organic chemicals are distributed in a continuum, from very
reactive to extremely persistent ones. The latter, referred to as persistent organic pollutants
(POPs), degrade very slowly, with long environmental half-lives ranging from two months
to several decades in water, soil, and sediments. POPs include anthropogenic chemicals such
as pesticides (for example DDT , dieldrin , aldrin , and hexachlorobenzene ) and industrial
chemicals (for example PCBs , dioxins , and furans). In general, POPs are hydrophobic, and,
because they persist in the environment for long periods, are prone to bioaccumulation as
they are transferred through the food chain. Furthermore, most POPs are semi-volatile, and
therefore tend to be transported long distances in the atmosphere and deposited long range.
This has resulted in ubiquitous contamination by POPs of the global environment; they are
found even in remote regions where they have never been used or produced (e.g. the Arctic
and Antarctic).
Many organic compounds occur only in very small concentrations in soil and water, but
even at these low levels can cause severe ecotoxicological effects. Therefore, these compounds

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