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UNIT 1: WHERE DO WE STAND? THE STATE OF THE GLOBAL ENVIRONMENT
By the year 2025,
as much as twothirds
of the
world population
may be subject to
moderate to high
water stress.
Water Pollution
A change in natural water quality implies water pollution. While natural events
such as storms, cyclones and mudflows can cause temporary deterioration, the
term ‘pollution’ implies the more serious and longer-term water quality problems
that result from domestic, industrial and agricultural activity.
Water pollution is critical, especially given the increasing demand for high-quality
water for drinking, recreation and industry, coupled with the increased use of
water for sanitation and waste disposal – all the water being taken from the same
source. Groundwater or aquifer pollution is a particular problem, as this water has
a longer residence time in the natural hydrological cycle than surface water.
Some 1,500 substances have been listed as aquatic pollutants. The main sources
of marine water pollution are:
• Organic waste from sewerage and other industrial and agricultural
sources (the primary source);
• Oil spills or discharges from transport, offshore installations, terminal
operations, coastal refineries and municipal and industrial activity;
• Heavy metals discharged through mining, smelting and refining;
• Organochlorines and PCBs discharged by industrial processes;
• Radioactivity (which can also occur naturally, mainly from
potassium-40 and other decay products);
• Heat or thermal pollution.
The Impacts of Water Pollution
The impacts of water pollution depend on the quantity and type of pollutants
and the ecological conditions in the environment that receives them. Some water
pollution impacts are briefly discussed below.
• Organic pollutants such as sewage and agricultural waste break
down in the presence of oxygen, through bacterial activity that
reduces the oxygen concentration in the water. This can be naturally
supplemented through the digestion of oxygen from the surrounding
air. The digestion of oxygen is, however, a slow process and anaerobic
bacteria begin to thrive as oxygen levels drop. The end products –
hydrogen sulphide, methane and ammonia – are toxic.
• Water pollution causes cultural euthrophication. Cultural
euthrophication is the human-induced enrichment of water by
inorganic nutrients such as phosphates and nitrates, which are
discharged into water bodies through urban sewage and agricultural
effluent, especially animal waste. Cultural euthrophication first
stimulates plant growth and in extreme cases can result in algae
blooms. These plants are not simply anaesthetic in terms of smell,
slime and flies; they also severely restrict the transparency of water
to light. This results in a series of chain reactions leading to the
widespread death of fish and aquatic invertebrates. The loss of these
species affects other animals further up the food chain, including
plants and fish-eating birds.
• Heavy metals in water bodies can concentrate in filter-feeding
molluscs such as mussels. This may have adverse effects on other
aquatic flora and fauna, and on human health when people eat
contaminated shellfish.
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