Freshwater is chemically defined as containing a concentration of less than two parts per thousand (<0.2%) of dissolved salts.
Although water is abundant on the surface of Earth, freshwater is a very limited resource. Freshwater, in all forms, makes up less than 2.8% of the world water supply. Freshwater on Earth exists in several forms. These include lakes, which represent 0.009% of the global water supply, rivers (0.0001%), atmospheric water including vapor, clouds, and precipitation (0.001%), shallow groundwater in soil and subterranean aquifers (0.31%), and polar icecaps and glaciers (2.15%). The supply of water available for human and other biological demands excludes those waters that are saline (salty), situated in the atmosphere, or frozen in icecaps and glaciers. The waters that fit into useable criteria constitute less than 0.5% of all of the water on Earth. Pollution, waste, population growth, and competition over available resources further restrict the availability of freshwater and are likely to become more acute in the future.
Most of the dissolved, inorganic chemicals in freshwater occur as ions. The most important of the positively charged ions (or cations) in typical freshwaters are calcium (Ca2+), magnesium (Mg2+), sodium (Na+), ammonium (NH + 4 ), and hydrogen (H+). The most important of the negatively charged ions (or anions) are sulfate (SO42+), chloride (Cl–), and nitrate (NO – 3 ). Other ions are also present, but in relatively small concentrations. Some freshwaters can have large concentrations of dissolved organic compounds, known as humic substances. These can stain the water a deep-brown, in contrast to the transparent color of most freshwaters.
Lakes in watersheds with hard, slowly weathering bedrock and soils, are at the dilute end of the chemical spectrum of surface waters. Such lakes can have a total salt concentration of less than 0.002% (equivalent to 20 mg/L, or parts per million, ppm). For example, Beaver-skin Lake in Nova Scotia has clear, dilute water, with the most important dissolved chemicals being chloride (4.4 mg/L), sodium (2.9 mg/L), sulfate (2.8 mg/L), calcium (0.41 mg/L), magnesium (0.39 mg/L), and potassium (0.30 mg/L). A nearby body of water, Big Red Lake, has similar concentrations of these inorganic ions. However, this lake also receives drainage from a nearby bog, and its chemistry includes a large concentration of dissolved organic compounds (23 mg/L), which stain the water the color of dark tea.
More typical concentrations of major inorganic ions in freshwater are somewhat larger: calcium, 15 mg/L; sulfate, 11 mg/L; chloride, 7 mg/L; silica, 7 mg/L; sodium, 6 mg/L; magnesium, 4 mg/L; and potassium, 3 mg/L.
The freshwater of precipitation is considerably more dilute than that of surface waters. For example, precipitation falling on the Nova Scotia lakes is dominated by sulfate (1.6 mg/L), chloride (1.3 mg/L), sodium (0.8 mg/L), nitrate (0.7 mg/L), calcium (0.13 mg/L), ammonium (0.08 mg/L), magnesium (0.08 mg/L), and potassium (0.08 mg/L). Because the sampling site is within 31 mi (50 km) of the Atlantic Ocean, its precipitation is significantly influenced by sodium and chloride originating with sea sprays. Locations that are more continental have much smaller concentrations of these ions in their precipitation water. For example, precipitation at a remote place in northern Ontario has a sodium concentration of 0.09 mg/L and chloride 0.15 mg/L, compared with 0.75 mg/L and 1.3 mg/L, respectively, at the maritime Nova Scotia site.