Water pollution is the degradation or contamination of water by substances, organisms or forms of energy that adversely alter its physical, chemical or biological condition or make it unsuitable for particular uses. Polluted water can occur in rivers, lakes, reservoirs, wetlands, groundwater, estuaries, coastal waters and other aquatic environments. Pollution is not restricted to visibly dirty water. Clear water can contain dissolved toxic chemicals, pathogens, excess nutrients or other contaminants that are not apparent to the eye. Conversely, naturally occurring substances can make water unsuitable for some purposes without necessarily constituting pollution in the regulatory sense. Water quality is therefore assessed according to measurable characteristics and intended uses such as drinking, recreation, agriculture, aquatic life or industrial supply.
Water pollution can involve physical, chemical and biological changes. Physical pollutants include excessive sediment, suspended solids, litter, heat and changes in turbidity. Chemical pollutants include nutrients, pesticides, petroleum compounds, industrial chemicals, salts, metals and other dissolved substances. Biological contamination includes bacteria, viruses, protozoans and other organisms capable of causing disease or ecological disruption. Thermal pollution can alter water temperature and therefore affect dissolved oxygen, metabolism and habitat suitability. Pollution can also involve changes to acidity, salinity or oxygen concentration rather than the introduction of a single recognisable toxic substance. Because aquatic ecosystems respond to combinations of stressors, pollution assessment often considers multiple variables simultaneously.
Water pollution can arise from point sources and nonpoint sources. A point source is a discernible and confined conveyance through which pollutants can be discharged, such as a pipe, channel or other discrete outlet. Nonpoint pollution comes from diffuse areas and is commonly transported by rainfall, snowmelt, drainage, seepage or atmospheric deposition. Urban runoff, agricultural runoff and erosion can therefore introduce pollutants over broad areas rather than from one easily identifiable discharge point. The distinction matters for monitoring and management because point sources can often be sampled at a particular outlet, whereas diffuse pollution requires understanding land use, precipitation, soil processes and watershed transport.
Water pollution can affect human health directly and indirectly. Pathogens in contaminated water can cause infectious disease, while chemical contaminants can create acute or chronic health risks depending on the substance, concentration, exposure pathway and duration. Contaminated drinking water is particularly significant because ingestion provides a direct route of exposure, but people can also encounter pollutants through recreation, food harvested from contaminated waters and contact with polluted sediments. Some contaminants accumulate in organisms or become more concentrated through food webs. The health significance of a pollutant therefore cannot always be judged from its concentration in water alone because persistence, bioavailability, transformation and biological accumulation also influence risk.
Aquatic ecosystems can be damaged by pollution through several interacting mechanisms. Excess nutrients can stimulate algal blooms and subsequent oxygen depletion. Sediment can cloud water, smother habitats and interfere with feeding or reproduction. Toxic substances can damage organisms directly or disrupt physiological processes. Heated water can alter metabolic rates and reduce habitat suitability for species adapted to particular temperature ranges. Salinity changes can stress freshwater organisms, while persistent contaminants can remain in sediments and groundwater long after the original release. Pollution can therefore affect organisms at individual, population, community and ecosystem levels, sometimes producing effects far downstream from the location where contaminants entered the water.
Water pollution is also a water cycle problem because contaminants can move between environmental compartments. Rainfall can wash pollutants from land into streams, while infiltration can carry dissolved contaminants into groundwater. Groundwater can subsequently discharge into rivers, wetlands or coastal waters. Atmospheric pollutants can be deposited directly onto water surfaces or onto land, from which they are later transported by runoff. Some substances are transformed chemically or biologically as they travel, while others persist. Consequently, water pollution cannot always be understood by examining a single river or lake in isolation. The surrounding watershed, groundwater system, atmosphere, land use and upstream activities may all contribute to the observed water quality.
Pollution prevention generally involves reducing contaminants at their sources, treating wastewater, controlling runoff, protecting wetlands and riparian areas, managing agricultural inputs, preventing spills and monitoring water quality. Wastewater treatment can remove or transform many physical, biological and chemical pollutants before discharge. Urban stormwater controls can capture sediment and pollutants before they reach streams. Agricultural measures can reduce nutrient and pesticide losses by improving application practices and retaining vegetation. Groundwater protection is particularly important because contamination can be difficult and expensive to reverse once pollutants enter an aquifer. Effective management therefore combines source control, treatment, land management, monitoring and enforcement rather than relying on a single intervention.
A common misconception is that water pollution always produces visible colour, odour or floating waste. Many important contaminants are invisible and may require laboratory analysis to detect. Another misconception is that pollution is synonymous with any substance present in water. Natural waters normally contain dissolved minerals, organic matter, microorganisms and sediments, and some naturally occurring constituents can be harmless or beneficial at normal concentrations. Pollution generally concerns harmful alteration, excessive concentrations or conditions that impair ecological or human uses. Water pollution is therefore best understood as a problem of water quality and environmental function rather than simply the visual appearance of water.
