Water pollution has been defined as the presence in water of harmful and objectionable material – obtained from sewers, industrial wastes and rainwater run-off – in sufficient concentrations to make it unfit for use.

We have long used air, land and water resources as ‘sinks’ into which we dispose of the wastes we generate. These disposal practices leave most wastes inadequately treated, thereby causing pollution. This in turn affects precipitation, surface waters, and groundwater, as well as degrading ecosystems.

Pollution from agriculture, industry and domestic wastewater is making water resources, both surface water and groundwater, increasingly scarce and decreasingly poor in quality.
The sources of pollution that impact water resources can develop at different scales (local, regional and global) but can generally be categorized according to nine types: organic matter; pathogens and microbial contaminants; nutrients; salinization; acidification (precipitation or runoff); heavy metals; toxic organic compounds and micro-organic pollutants; thermal and silt and suspended particles.

Atmospheric contamination from industrial plants and vehicle emissions leads to dry and wet deposition. This causes acidic conditions to develop in surface water and groundwater sources and at the same time leads to the destruction of ecosystems. Acid deposition impairs the water quality of lakes and streams by lowering pH levels (i.e. increasing acidity), decreasing acid-neutralizing capacity, and increasing aluminium concentrations. High concentrations of aluminium and increased acidity reduce species diversity and the abundance of aquatic life in many lakes and streams.

Industrial discharge returned without treatment has high organic content, leading to rapid growth of algae, bacteria and slime, oxygen-depleted water, and thermal pollution. Discharge can affect a relatively large volume of water and have numerous impacts on human health. Polluted water may affect fishing grounds, irrigated lands, municipalities located downstream, bathing water and can have significant transboundary effects.

An estimated 80% of the pollutants entering coastal waters, mostly from land-based sources, are transported via rivers, and there are clear links between upstream river basins and associated coastal zones.

In the United States alone, it is estimated that industry generates about 36.3 billion kg of hazardous organopollutants each year, with only about 10% disposed of in an environmentally responsible manner.

Information from:
2nd United Nations World Water Development Report, ‘Water, a shared responsibility’
the United Nations Statistics Division (UN Stat) Environmental Glossary

Source: UNESCO Water Portal, May 2006