Water can exist in various forms within the environment, including: (1) liquid water of oceans, lakes and ponds, rivers and streams, soil interstices, and underground aquifers; (2) solid water of glacial ice and more-ephemeral snow, rime, and frost; and (3) vapor water of cloud, fog, and the general atmosphere. More than 97% of the total quantity of water in the hydrosphere occurs in the oceans, while about 2% is glacial ice, and less than 1% is groundwater. Only about 0.01% occurs in freshwater lakes, and the quantities in other compartments are even smaller.

Each compartment of water in the hydrosphere has its own characteristic chemistry. Seawater has a relatively large concentration of inorganic solutes (about 3.5%), dominated by the ions chloride (1.94%), sodium (1.08%), sulfate (0.27%), magnesium (0.13%), calcium (0.041%), potassium (0.049%), and bicarbonate (0.014%).

Surface waters such as lakes, ponds, rivers, and streams are highly variable in their chemical composition. Saline and soda lakes of arid regions have total salt...