Water

V

Natural Water Cycle

Hydrology is the science concerned with the distribution of water on Earth, its physical and chemical reactions with other naturally occurring substances, and its relation to life on Earth; the continuous movement of water between the earth and the atmosphere is known as the hydrological cycle (see Water Cycle). Under several influences, of which heat is predominant, water is evaporated from both water and land surfaces and is transpired from living cells. This vapor circulates through the atmosphere and is precipitated in the form of rain or snow. See Meteorology.

On striking the surface of Earth, the water follows two paths. In amounts determined by the intensity of the rain and the porosity, permeability, thickness, and previous moisture content of the soil, one part of the water, termed surface runoff, flows directly into rills and streams and thence into oceans or landlocked bodies of water; the remainder infiltrates into the soil. A part of the infiltrated water becomes soil moisture, which may be evaporated directly or may move upward through the roots of vegetation to be transpired from leaves. The portion of the water that overcomes the forces of cohesion and adhesion in the soil profile percolates downward, accumulating in the so-called zone of saturation to form the groundwater reservoir, the surface of which is known as the water table. Under natural conditions, the water table rises intermittently in response to replenishment, or recharge, and then declines as a result of continuous drainage into natural outlets such as springs. See Spring.

VI

Composition

Because of its capacity to dissolve numerous substances in large amounts, pure water rarely occurs in nature.

During condensation and precipitation, rain or snow absorbs from the atmosphere varying amounts of carbon dioxide and other gases, as well as traces of organic and inorganic material. In addition, precipitation carries radioactive fallout to Earth’s surface.

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In its movement on and through Earth’s crust, water reacts with minerals in the soil and rocks. The principal dissolved constituents of surface and groundwater are sulfates, chlorides, and bicarbonates of sodium and potassium and the oxides of calcium and magnesium. Surface waters may also contain domestic sewage and industrial wastes. Groundwaters from shallow wells may contain large quantities of nitrogen compounds and chlorides derived from human and animal wastes. Waters from deep wells generally contain only minerals in solution. Almost all supplies of natural drinking water contain fluorides in varying amounts. The proper proportion of fluorides in drinking water has been found to reduce tooth decay. See Fluorine.

Seawater contains, in addition to concentrated amounts of sodium chloride, or salt, many other soluble compounds, as the impure waters of rivers and streams are constantly feeding the oceans. At the same time, pure water is continually lost by the process of evaporation, and as a result the proportion of the impurities that give the oceans their saline character is increased. See Ocean and Oceanography.

VII

Water Purification

Suspended and dissolved impurities present in naturally occurring water make it unsuitable for many purposes. Objectionable organic and inorganic materials are removed by such methods as screening and sedimentation to eliminate suspended materials; treatment with such compounds as activated carbon to remove tastes and odors; filtration; and chlorination or irradiation to kill infective microorganisms. See also Sewage Disposal.

In aeration, or the saturation of water with air, water is brought into contact with air in such a manner as to produce maximum diffusion, usually by spraying water into the air in fountains. Aeration removes odors and taste caused by decomposing organic matter, and also industrial wastes such as phenols and volatile gases such as chlorine. It also converts dissolved iron and manganese compounds into insoluble hydrated oxides of the metals which may then be readily settled out.

Hardness of natural waters is caused largely by calcium and magnesium salts and to a small extent by iron, aluminum, and other metals. Hardness resulting from the bicarbonates and carbonates of calcium and magnesium is called temporary hardness and can be removed by boiling, which also sterilizes the water. The residual hardness is known as noncarbonate, or permanent, hardness. The methods of softening noncarbonate hardness include the addition of sodium carbonate and lime and filtration through natural or artificial zeolites which absorb the hardness-producing metallic ions and release sodium ions to the water (see Ion Exchange; Zeolite). Sequestering agents in detergents serve to inactivate the substances that make water hard.

Iron, which causes an unpleasant taste in drinking water, may be removed by aeration and sedimentation or by passing the water through iron-removing zeolite filters, or the iron may be stabilized by addition of such salts as polyphosphates. For use in laboratory applications, water is either distilled or demineralized by passing it through ion-absorbing compounds.

VIII

Water Desalination

To meet the ever-increasing demands for fresh water, especially in arid and semiarid areas, much research has gone into finding efficient methods of removing salt from seawater and brackish waters. Several processes are being developed to produce fresh water cheaply. Distilled water obtained by desalination is slightly acidic and usually has minerals added to it after processing to restore chemical neutrality and to provide certain minerals needed by the human body, as well as to improve taste.

Three of the processes involving evaporation followed by condensation of the resultant steam are known as multiple-effect evaporation, vapor-compression distillation, and flash evaporation. The last-named method involves heating seawater and pumping it into lower-pressure tanks, where the water abruptly vaporizes (flashes) into steam. The steam then condenses and is drawn off as pure water. In 1967, Key West, Florida, opened a flash-evaporation plant and thus became the first city in the United States to draw its fresh water from the sea.

Freezing is an alternate method, based on the different freezing points of fresh and salt water. The ice crystals are separated from the brine, washed free of salt, and melted into fresh water. In another process, called reverse osmosis, pressure is used to force fresh water through a thin membrane that does not allow the minerals to pass. Sunlight can be used to heat water in a solar still for desalination, but this process is considered inefficient for large-scale needs.

Electrodialysis has been used to desalt brackish waters. When salt dissolves in water, it splits into positive and negative ions, which are then removed by electric current through anion and cation membranes, thus depleting the salt in the product water. Although developmental work on electrodialysis is continuing, a number of commercial plants are in operation. In 1962 Buckeye, Arizona, became the first town to have all its water supplied by its own electrodialysis-desalting plant, which could provide about 2,460,000 liters (about 650,000 gallons) of water daily.

One major problem in desalination projects has been the cost of producing fresh water. However, water shortages and the growing demand for fresh water in many parts of the world such as the Middle East, Asia, and Australia, and in California and Florida in the United States, have made desalination a viable option. Reverse osmosis, combined with different kinds of filtering, has become a widely used method, in part because the process typically demands less energy than some other types of desalination. The so-called multistage flash distillation process is also commonly used. Sometimes desalination can be combined with energy production, a practice known as cogeneration. Desalination facilities have been built alongside nuclear power plants and solar power plants, as well as near conventional fossil-fuel power plants. See Solar Energy; Waterpower; Water Supply and Waterworks.

For other functions of water, see Erosion; Geology; Metabolism.

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