Page 180 - Soil and water contamination, 2nd edition

Organic pollutants 167

BTEXs are among the most hazardous constituents of petrol and solvents. Except for
short-term hazards from concentrated spills, BTEX compounds have been more frequently
associated with risk to human health than with risk to plants and animals. This is partly
due to the fact that plants and animals take up BTEX compounds in very small amounts
and BTEXs tend to volatilise relatively rapidly into the atmosphere rather than persisting
in surface waters and soils. However, BTEX compounds may pose a threat to drinking
water quality when they accumulate in groundwater. Short-term hazards of BTEX include
potential acute toxicity to aquatic life in surface water and potential inhalation hazards.
Acute exposures to high levels of BTEX may cause irritation of the skin, eyes, and respiratory
tract, and depression of the central nervous system. Long-term potential hazards of BTEX
compounds include chronic inhalation and contamination of groundwater. Prolonged
exposure to BTEXs may cause changes in the liver and adverse effects on the kidneys, heart,
lungs, and the central nervous system. Benzene is the most dangerous and carcinogenic
to humans. Its chronic effects encompass the destruction of bone marrow, leading to
a reduction of red and white blood cells. Workers exposed to high levels of benzene in
occupational settings have been found to have an increased incidence of leukaemia. Toluene
may also adversely affect reproduction.
The acronym BTEX suggests that benzene , toluene , ethylbenzene, and xylenes are
often found together at contaminated sites. The principal source of BTEX contamination
is the leakage of petrol from faulty underground storage tanks. Other sources of BTEX
contamination are releases from large bulk facilities, surface spills, and pipeline leaks. Once
released to the environment, BTEXs are liable to volatilisation , dissolution , adsorption , and
biodegradation . Although BTEXs can behave as a LNAPL, which means that the bulk of
the BTEX floats on water, it can also dissolve in water. Compared to the other components
in petrol, such as the aliphatic components, BTEXs are very soluble in water. BTEXs can
be adsorbed by organic matter in soil, but are not sorbed to soil particles as strongly as the
aliphatic components. The bacterial flora naturally present in soil is capable of breaking
down BTEXs under aerobic conditions (Brady et al., 1998). Like the decomposition of
the other mineral oil components, the biodegradation rate depends on temperature and
the supply of oxygen and nutrients. Toluene and – to some extent – xylene can also be
biodegraded under anaerobic conditions (Edwards et al., 1992). In anaerobic sediments, the
natural biodegradation is largely controlled by the amount of bioavailable Fe(III), which can
act as an electron acceptor. Typically, bioavailable Fe(III) constitutes between 10% and 30%
of the total amount of iron present in aquifer material (Manshoven et al., 2010).

9.4 POLYCYCLIC AROMATIC HYDROCARBONS

Polycyclic aromatic hydrocarbons (PAHs ) are a class of over 100 different, very stable
organic molecules that are made up of only carbon and hydrogen and contain two or more
connected benzene rings (see examples in Figure 9.Ib). These molecules are formed during
the incomplete burning of coal, oil and gas, garbage, or other organic substances like
tobacco or charbroiled meat. PAHs are found in coal tar, crude oil, creosote, and roofing
tar. Some PAHs are manufactured and are used in medicines or to make dyes, plastics, and
pesticides . Pure PAHs usually exist as colourless, white, or pale yellow-green solids. PAHs are
commonly divided into two groups, depending upon their physical and chemical properties:
low-molecular-weight PAHs, containing three or fewer aromatic rings, and high-molecular-
weight PAHs, containing more than three aromatic rings. Most PAHs are semi-volatile and
the volatility generally increases with decreasing molecular weight. PAHs containing 6 or
more benzene rings are barely volatile. The US Environmental Protection Agency has defined
a subset of 16 PAHs that are most important from an environmental perspective. These 16

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