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Water Quality

The quality of any body of surface of ground water is a function of either both natural influences and human influences.

Without human influences water quality would be determined by the weathering of bedrock minerals, by the atmospheric processes of evapotranspiration and the deposition of dust and salt by wind, by the natural leaching of organic matter and nutrients from soil, by hydrological factors that lead to runoff, and by biological processes within the aquatic environment that can alter the physical and chemical composition of water.

Typically, water quality is determined by comparing the physical and chemical characteristics of a water sample with water quality guidelines or standards. Drinking water quality guidelines and standards are designed to enable the provision of clean and safe water for human consumption, thereby protecting human health. These are usually based on scientifically assessed acceptable levels of toxicity to either humans or aquatic organisms.

Nitrate Levels
Nitrate levels: concentrations at river mouths
Click on map to enlarge

Globally, the most prevalent water quality problem is eutrophication, a result of high-nutrient loads (mainly phosphorus and nitrogen), which substantially impairs beneficial uses of water. Major nutrient sources include agricultural runoff, domestic sewage (also a source of microbial pollution), industrial effluents and atmospheric inputs from fossil fuel burning and bush fires. Lakes and reservoirs are particularly susceptible to the negative impacts of eutrophication because of their complex dynamics, relatively longer water residence times and their role as an integrating sink for pollutants from their drainage basins. Nitrogen concentrations exceeding 5 milligrams per litre of water often indicate pollution from human and animal waste or fertilizer runoff from agricultural areas.

An emerging water quality concern is the impact of personal care products and pharmaceuticals, such as birth control pills, painkillers and antibiotics, on aquatic ecosystems. Little is known about their long-term human or ecosystem impacts, although some are believed to mimic natural hormones in humans and other species.


  • Water Quality for Ecosystems and Human Health. 2nd edition. UNEP, ERCE, UNESCO. 2008
  • World Water Development Report 3 ‘Water in a Changing World’. WWAP, 2009
  • Vital Water Graphics. UNEP

Water Quality and the MDGs

Water quality management contributes both directly and indirectly to achieving the targets set out in all eight Millennium Development Goals (MDGs), although it is most closely tied to specific targets of the goal 7, to ensure environmental sustainability. Indicators on water quality can be used to demonstrate progress toward the targets, by plotting trends in water quality over time and over space.

Source: Water Quality for Ecosystems and Human Health. 2nd edition. UNEP, ERCE, UNESCO. 2008

What is “pollution”?

Pollution typically refers to chemicals or other substances in concentrations greater than would occur under natural conditions. Major water pollutants include microbes, nutrients, heavy metals, organic chemicals, oil and sediments; heat, which raises the temperature of the receiving water, can also be a pollutant. Pollutants are typically the cause of major water quality degradation around the world.

Source: World Water Development Report 3 ‘Water in a Changing World’

Did you know?

  • Every day, 2 million tons of sewage and other effluents drain into the world’s waters.
  • Every year, more people die from unsafe water than from all forms of violence, including war
  • The most significant sources of water pollution are lack of inadequate treatment of human wastes and inadequately managed and treated industrial and agricultural wastes
  • The quality of water necessary for each human use varies, as do the criteria used to assess water quality. For example, the highest standards of purity are required for drinking water, whereas it is acceptable for water used in some industrial processes to be of less quality.


To know more