Page 181 - Soil and water contamination, 2nd edition

168 Soil and Water Contamination

PAHs include (the number between brackets indicates the number of benzene rings, but note
that PAHs may also contain additional non-benzene rings): acenaphtene (2), acenaphthalene
(2), anthracene (6), benz[a]anthracene (4), benzo[b]fluoranthene (4), benzo[k]fluoranthene
(4), benzo[ghi]perylene (6), benzo[a]pyrene (5), chrysene (4), dibenz[a.h]anthracene (5),
fluorantene (3), fluorine (2), indeno[1.2.3-cd]pyrene (5), naphthalene (2), phenanthrene (3),
and pyrene (4).
PAHs enter the atmosphere mostly as releases from volcanoes, forest and peat fires, fossil
fuels combustion, coke and asphalt production, waste incineration, and aluminium smelting.
In air, PAHs occur mostly attached to dust particles smaller than 1–2 μm. Over a period of
days to weeks, PAHs can break down by reacting with sunlight and other airborne chemicals.
They are also removed from the atmosphere by deposition, as a result of which they reach
the Earth’s surface. The lighter molecular weight PAHs can be revolatilised and redistributed
before they are redeposited. Although PAH levels in air, soil, and water are particularly
enhanced in urbanised regions, PAHs are ubiquitous in the environment, even in remote
areas, due to long-range atmospheric transport. In addition to atmospheric deposition , the
sources of PAHs to terrestrial environments include releases from creosote-treated products,
spills of petroleum products, and application of compost.
The PAH contents of terrestrial plants and animals may be substantially greater than the
PAH contents of soil on which they live, albeit that the PAH contents in terrestrial plants are
often independent from those in soil because PAH compounds are poorly soluble and poorly
available in soils (ATSDR, 2013). A major source of PAHs in plants is the accumulation of
airborne PAHs on plant leaves; plant uptake of PAHs via the root system is often negligible.
The process of accumulation is affected by a variety of factors. It increases with decreasing
ambient temperature, and with increasing leaf surface area and lipid concentration in plant
tissues. Volatile PAHs with a lower molecular weight are primarily subject to dry gaseous
deposition, whereas non-volatile PAHs mainly accumulate on plant surfaces in the form of
dry particulates (Bakker, 2000). The volatile PAHs enter plants primarily through gaseous
diffusion via open stomata, although absorption by the waxy leaf surface accounts for a
portion of the total PAHs in tissues. The waxy surface of leaves intercepts both gaseous and
particle-bound PAHs. Because of their ability to intercept PAHs from the atmosphere, a wide
range of plant species, including lichens, mosses, algae, and trees have been used to evaluate
environmental accumulation of PAHs (e.g. Franzaring et al., 1992; Wegener et al., 1992).
Mosses are particularly effective filters due to their large surface area, their capacity to absorb
large organic molecules, and their ability to obtain water and nutrients from the air.
Surface water contamination by PAHs can occur through runoff from contaminated soil
surfaces (particularly urban runoff ), and direct discharges from industrial and wastewater
treatment plants. In general, most PAHs are not very mobile in soil and water, due to their
relatively low solubilities and strong affinity for organic particulate matter. As a consequence,
PAHs tend to partition into sediments and soils. PAH levels are usually much higher in
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sediments than in surface water, i.e. in the range of μg kg (ppb) rather than ng kg (ppt).
PAHs are potentially hazardous to plants due to photo-enhanced toxicity in the presence
of ultraviolet (UV) or other types of solar radiation (Irwin et al., 1998). PAHs are good
photosensitisers, forming biologically harmful excited or singlet-state oxygen radicals .
Their toxic effects include chlorosis , inhibition of photosynthesis , and diminished biomass
accumulation. Soil contamination by PAHs may also inhibit root growth. PAH toxicity in
aquatic environments is mainly associated with the more soluble compounds, particularly
two-ring compounds such as the naphthalenes, and some heavier PAHs that affect aquatic
macrophytes, benthic aquatic invertebrates (i.e. invertebrates that live on the bed of a
water body), and fish. As a consequence, aquatic animals in the water will have a decreased
individual fitness and will not be able to develop through their life stages successfully and
reproduce effectively. Some PAH compounds, however, appear to stimulate reproduction,

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