and Petroleum Products
pollution is commonly perceived as being a problem associated mainly
with the marine environment, but in fact oil and related substances
account for about one-quarter of reported pollution incidents of
fresh waters in Britain; and probably the pattern is not much different
in other parts of the world. Also, about one-quarter of all the
oil released to the seas by human activity is estimated to enter
via rivers. The sources of oil pollution are the usual ones - illegal,
negligent or accidental discharges, plant failures and so on - but
to these must be added inputs from roads and railways, particularly
in the case of traffic accidents, and discharges associated with
oil extraction from inland oilfields and processing plants.
effects of oil discharges vary enormously, because the characteristics
of the receiving waters and the various types of oil are themselves
very variable. Fortunately many small discharges have relatively
trivial effects, but substantial discharges can have severe impacts.
Generally, light oils are relatively volatile and disperse quickly,
though they are extremely toxic and can cause severe local damage.
Fast-flowing waters often recover rapidly, over a few weeks or months,
but lentic waters are much more susceptible to long-term damage.
Heavy oils are less toxic, but can have marked physical effects
in the substratum and the banksides, which of course will be reflected
in changes to the biological community. Heavy oils may remain in
situ for long periods; they do, over time, decompose biologically
and chemically, but in so doing impose a high BOD. Frequently, the
effects of oil pollution are not dissimilar from those of heavy
organic pollution, with varying levels of toxic effects adding to
the overall impact. Oils, unlike most forms of water pollution,
can also have damaging effects on terrestrial organisms, such as
plants, birds and mammals living at the water's edge, since heavy
oil contamination usually lead,, to deposition of oil on the banks
of likes or rivers.
the use of oil dispersants, which are commonly used in dealing with
marine spills, is avoided in freshwater because of their high toxicity.
Also, the physical conditions of freshwater habitats are often amenable
to containment and recovery of oil by booms, skimmers and/or tile
use of absorbent materials.