2 Sources of pollution
To some extent a river is a self-renewing resource. If polluting discharges to a river are intermittent, the river is often able to return to a clean and unpolluted condition as the pollutants are flushed out and carried down to the sea. In addition, because of the organisms present (e.g. bacteria capable of breaking down organic matter), river water has some capacity for self-purification – unless too many of these organisms are killed off too quickly.
Some pollutants are objectionable because they overload the self-purification processes of the river. As rivers are often the raw water sources for potable supplies, this can have dire consequences.
An example of such pollution is the discharge of domestic sewage effluent to rivers. In small quantities it does no serious harm and may indeed be beneficial, providing a source of organic carbon that provides nutrients to the animals in the river. But if inadequately treated or in excessive quantities, sewage effluent can seriously damage the plant and animal life of a river by reducing the oxygen content of the water. In extreme cases, where the oxygen content is reduced to zero (or nearly so), the river will support very little life, and will become foul smelling and grossly offensive. A river in such a state is obviously not desirable as a water source for potable supply.
Some industrial effluents discharged in large quantities can be similarly harmful. For example, effluents from the food industry are not particularly toxic, but because of their organic content and large volume, they can exert a considerable oxygen demand on the environment in the region of the discharges.
The two pollution sources described above are classified as point sources, as the pollutants are generally collected by a network of pipes or channels and conveyed to a single point of discharge. Non-point or diffuse sources are characterised by multiple discharge sources that cannot be pinpointed. An example of a diffuse source is run-off from fields and roads.
Point sources are easily controlled, but diffuse sources are virtually impossible to collect and control. The latter pose great challenges in efforts to upgrade the quality of rivers.
Lakes are much more vulnerable than rivers to pollution. Once a pollutant enters a lake it will stay for a long time. The flushing effect that characterises rivers is much less evident in lakes, and the dilution factor is much less than is available in the sea. Only the self-purifying ability of the water will abate the pollution in the long term. Lakes are thus particularly prone to eutrophication (Figure 3).
Since river pollutants can be controlled more easily at source, it is useful to know where they originate. A list of specific sources would be very long, but the following categories can be identified:
- discharges from sewage works, which often contain some industrial wastes
- discharges from manufacturing and industrial plants, including mines
- discharges from animal rearing, fish farming and agriculture
- seepage from domestic and industrial landfill sites
- urban surface water run-off.
Problems occur when the natural characteristics of a river are altered by pollutant discharges. We now consider the effects of the following categories of pollutants:
- organic materials
- plant nutrients
- toxic pollutants
- physical pollutants
- biological pollutants.
Please be aware that this categorisation is not absolute. For instance, toxic pollutants may well be organic, too.