Fresh water is water with less than 0.5 parts per thousand dissolved salts. Fresh water may be found in lakes, rivers, and bodies of groundwater. The ultimate source of fresh water is rain (or, to be more precise, the precipitation of atmospheric water vapor, see also hydrologic cycle).

Water salinity based on dissolved salts in parts per thousand (‰)
Fresh water	Brackish water	Saline water	Brine
< 0.5 ‰	0.5 - 30 ‰	30 - 50 ‰	> 50 ‰

Access to unpolluted fresh water is a critical issue for the survival of many species, including humans, who must drink fresh water in order to survive. Only 3% of water on the Earth is fresh water, and over two thirds of this is frozen in glaciers and polar ice caps (see water resources).

Aquatic organisms

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Many species can live in fresh water, including freshwater fish species. However, fresh water creates a hypotonic environment for aquatic organisms. This is problematic for some organisms, whose cell walls will burst if excess water is not excreted. Some protists accomplish this using contractile vacuoles, while freshwater fishes excrete excess water via the kidney. Although most aquatic organisms have a limited ability to regulate their osmotic balance and therefore can only live within a narrow range of salinity, some fishes have the ability to migrate between fresh water and seawater.

See also

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• Desalination
• Limnology
• Seawater

References

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Liquid water | Hydrology

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