Freshwater is chemically defined as containing a concentration of less than two parts per thousand (<0.2%) of dissolved salts.

Freshwater can occur in many parts of the environment. Surface freshwaters occur in lakes, ponds, rivers, and streams. Subsurface freshwater occurs in pores in soil and in subterranean aquifers in deep geological formations. Freshwater also occurs in snow and glacial ice, and in atmospheric vapors, clouds, and precipitation.

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 (NH₄⁺), and hydrogen ion (H⁺). This hydrogen ion is only present if the solution is acidic; otherwise a hydroxy ion (OH⁻) occurs. The most important of the negatively charged ions (or anions) are sulfate (SO₄²⁻), chloride (Cl⁻), and nitrate (NO₃⁻). Other ions are also present, but in relatively small concentrations. Some freshwaters can...