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Fresh water

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Natural color of fresh water in the lake in broad daylight
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   Natural colour of fresh water in the lake in broad daylight

   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 ‰

Water distribution

   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. However, only 3% of water on the Earth is fresh water, and
   about two-thirds of this is frozen in glaciers and polar ice caps. Most
   of the rest is groundwater, but 0.3% is surface water. Fresh water
   lakes contain seven-eighths of fresh surface water. Swamps have most of
   the rest, but a little is in rivers. The atmosphere only contains 0.04%
   of fresh water (see water resources).

Aquatic organisms

   Two people reflected in the water of a fish pond
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   Two people reflected in the water of a fish pond

   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 fish 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 fish have the ability to migrate between
   fresh water and seawater.

   Retrieved from " http://en.wikipedia.org/wiki/Fresh_water"
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   with only minor checks and changes (see www.wikipedia.org for details
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