Sources of ion supply to natural inland waters include not only rocks and soils but also the atmosphere, whose significance has been under-estimated. Atmospheric materials are transferred to surface waters by rain or snow, as dry fallout, or in gaseous form; the sources are the sea, land surfaces, volcanoes, products of air pollution, or organic debris. Ion supply by soil and rock weathering, which is usually more important than atmospheric supply, involves solution, oxidation-reduction reactions, activity of hydrogen ions, and complex formation. Transfer from soils to waters is influenced by ion exchange and by modes of water percolation.
Five principal environmental factors—climate, geology, topography, biota, and time—interact to determine ionic concentration and composition of atmospheric precipitation, soil solutions, and lake and river waters, although the extent to which each applies is not well understood. Investigation of situations in which only a single factor varies effectively should do much to clarify the role of each in determining the ultimate composition of natural waters.