A human adult needs to drink approximately 2 liters of water every day. In addition to this quantity, humans use much larger amounts of freshwater (as opposed to the saltwater of oceans) for cooking, cleaning, industry, and agriculture. Thus, the quality of the freshwater supply is important for virtually every aspect of our lives. The supply of freshwater is replenished as water from the oceans and land evaporates, accumulates in the atmosphere (forming clouds), and then falls again as rain or snow.
Sea water, of course, is filled with ions from the dissolving of salt. It may be surprising, then, to find that freshwater does not consist of pure water, but also contains many dissolved ions. Many of these ions have no effect on our use of freshwater, others are essential for agriculture or health, and still others can be quite undesirable for human freshwater use. Aqueous species in the water supply may also participate in reactions that generate a precipitate (e.g., calcium carbonate, which produces mineral deposits on cooking dishes, water pipes, and water boilers). These precipitates can cause damage to appliances and pipes and can result in loss of energy efficiency. Thus, a proper balance of these ions in the freshwater supply must be maintained, for the water to optimally serve both the environment and humans.
The following links explore two important aspects of the quality of the freshwater supply:
The authors thank Dewey Holten (Washington University) for many helpful suggestions in the writing of this tutorial.
The development of this tutorial was supported by a grant from the Howard Hughes Medical Institute, through the Undergraduate Biological Sciences Education program, Grant HHMI# 71192-502004 to Washington University.