Water Pollution: Causes and Effects

Water pollution occurs when sewage, industrial effluent, agricultural runoff or other waste enters

a river, lake, pond or groundwater body faster than natural processes -- dilution, sedimentation,

and breakdown by naturally present micro-organisms -- can remove it, so that the water's quality

degrades to the point where it harms aquatic life or becomes unsafe for human use.

Sources of water pollution are usefully divided into two categories.

Point-source pollution enters water from a single, identifiable outlet -- for example, a pipe

discharging untreated sewage or industrial effluent directly into a river, or a factory's waste

outfall into a lake. Because the source is a single fixed location, point-source pollution is

comparatively easy to monitor, regulate and, when necessary, treat before discharge.

Non-point-source (diffuse) pollution has no single identifiable outlet; instead, it accumulates

from many scattered, small contributions across a wide area -- for example, fertiliser and

pesticide residues washed off farmland by rain, or oil and debris washed off city streets into

storm drains. Because it arrives from many directions at once, non-point-source pollution is much

harder to trace, regulate and control than point-source pollution.

Major categories of water pollutant and their effects on aquatic life include:

• Disease-causing agents (pathogens) from untreated sewage -- bacteria, viruses and parasites -- which make water unsafe to drink and can spread waterborne disease.
• Oxygen-demanding wastes -- biodegradable organic matter from sewage, food-processing waste and similar sources -- which is broken down by aerobic bacteria that consume dissolved oxygen from the water as they do so, potentially suffocating fish and other oxygen-dependent organisms if the load is large (see Section 13.8 on BOD).
• Plant nutrients, chiefly nitrogen and phosphorus compounds from sewage and fertiliser runoff, …