15 4 7 Pollution of Aquatic Systems: Pollution Through Eutrophication, Fecal Materials, and Oil Spills
7.1.3 Kinds of Pollutants
In general terms, a pollutant is anything, which when
added to a body of water, directly or indirectly limits
man’s legitimate use or appreciation of that body of
water because sufficient time has not occurred for the
self-purifying powers of the water to remove the added
materials or conditions. As mentioned earlier, the use
of water can be found in the domestic, agricultural,
transportation, and recreational arenas. A material, of
whatever nature, which when added to water does
violence to the observer’s aesthetic appreciation would
also be regarded as a pollutant. When this broader view
of definition is adopted, one may classify pollutants
into the following four categories: pollution by eutro-
phication, pollution by the addition of harmful or
undesirable organisms, pollution by toxic materials
and industrial effluents, and thermal pollution. The
bulk of the discussion will be on pollution by eutro-
phication; the other three will be discussed very
briefly before the comparatively extensive discussion
on eutrophication.
- Pollution by the addition of harmful or undesirable
organisms
This type of pollution typically occurs when sew-
age is the cause of the eutrophication. It can also
occur when effluents rich in easily degradable
organic materials are added to water, such from a
food processing factory. When sewage derives from
a population harboring enteric pathogens, water
pollution caused by such sewage will naturally con-
tain the pathogens of that community. - Toxic materials and industrial effluents
These include industrial heavy metals, radioactive
materials, and pesticides, which can be harmful to
man in one way or the other. In this group also
would be placed effluents from the chemical industry
and pollution by crude oil when oil-tankers run
aground. Pollution by crude oil during spills and
methods of remediating the situation will be discussed
later in this chapter. - Thermal pollution
Industrialization uses power. When such power is
derived from a nuclear plant, water is needed for
cooling. Such cooling water becomes hot in the
process and is usually discharged into rivers or estu-
aries. Cooling is also done in other industries outside
nuclear establishments, for example, in metal smelting.
As a result of the addition of hot water to a natural
body of water the balance of the population in the
ecosystem may alter, giving rise to a preponderance
of thermophilic organisms. Heated water also ren-
ders bodies of water unusable for some of man’s
legitimate undertakings such as fishing.- Pollution by eutrophication
Pollution by eutrophication is a regular occurrence
in rivers and lakes and will be discussed in the fol-
lowing section.
Oil spills important in oil producing countries
and when accidents occur in the oceans with oil
tankers will be examined at the end of this chapter.
7.1.4 Pollution by EutrophicationEutrophication, also known as nutrient pollution, is a
process whereby water bodies, such as lakes, estuaries,
or slow-moving streams, receive excess nutrients that
stimulate excessive growth of microorganisms including
algae and cyanobacteria. This enhanced growth often
results in a bloom (see below).Point and Nonpoint Eutrophication
Pollution by eutrophication may be point or nonpoint.
Point pollution occurs when pollutants enter directly
into a body of water from identifiable sources. The
well-known Exxon Valdez oil spill best illustrates a
point source water pollution. Nonpoint sources deliver
pollutants indirectly through environmental changes,
and cannot always be indentified directly. An example
of this type of water pollution is when ferti lizer from a
field is carried into a stream by rain, in the form of
runoff, which in turn affects aquatic life. Pollution
arising from nonpoint sources accounts for a majority
of the contaminants in streams and lakes.The Role of Oxygen in Eutrophication
Dissolved oxygen (DO) is a most important factor in
aquatic pollution by eutrophication. The excessive
growth of aerobic bacteria (and to a lesser extent
fungi), which are the primary stabilizers of added
material results in greater demand for oxygen. Under
normal conditions, the nutrient load of a stream is low
and hence the bacterial and fungal populations are also
low. With heavy eutrophication and the consequent
increased microbial growth, the depletion of oxygen
may be so severe that the dissolved oxygen is finally
exhausted and anaerobiosis sets in; consequently, fish,
which also depend on dissolved O 2 , may die, followed
by deaths among crustaceans, rotifers, and protozoa.