Water

Water is an odorless, tasteless, and transparent compound that is a critical factor in all chemical, physical, and biological processes. As far as is known, water exists freely and in great abundance on only one planet in our solar system, Earth.

Background

Although water could exist on Earth without life, life could not exist without water. It is the most abundant liquid on Earth. In its solid and liquid forms, water covers about 70 percent of Earth’s surface. It exists in gaseous form as water vapor in the lower atmosphere, varying from close to 0 percent to about 4 percent by volume from region to region. Water constitutes most of the living tissue in humans: about 92 percent of blood plasma, 80 percent of muscle tissue, 60 percent of red blood cells, and more than 50 percent of most other tissues.

Water Properties

Water is a compound of two atoms of hydrogen and one of oxygen, giving it the well-known chemical formula H₂O. It has some unique properties. It can exist naturally in three states on Earth: solid, liquid, and gaseous. Furthermore, under normal pressure, when heated from 0 degrees Celsius, the melting point of water, to 4 degrees Celsius, it contracts and reaches its highest density. This unusual thermal condition contrasts sharply with most other substances, which expand and experience decreasing density when they are heated. Therefore, ice is less dense than water and will float. This property has substantial implications, as it enables water to freeze from the surface downward, thereby allowing circulation to continue under the frozen surface so that fish can survive. Submarines that travel under the Arctic ice pack could not do so were it not for water’s unusual thermal property.

Water is an excellent solvent, so much so that “pure water” is hard to find in nature. Water has the highest specific heat of all common substances. Specific heat is the amount of heat that a fluid needs to raise the temperature of a unit volume by 1 degree. This is an important property, as the enormous heat capacity of water has an equalizing effect on Earth’s climate. Maritime locations have a milder than those that are located in continental interiors. Thus, the average annual temperature ranges between the warmest and coldest months for Winnipeg, Canada, and the Isles of Scilly, England, are 39 degrees and 8.3 degrees Celsius, respectively. Even though both places are at 50 degrees north latitude, the temperatures in the Isles of Scilly are moderated by their oceanic location, whereas Winnipeg is in the middle of a large continent.

The high heat capacity of water is closely associated with some other unusual properties of water, namely the latent heat of fusion and vaporization. The latent heat of fusion is the amount of heat per unit mass (80 calories per gram) that is necessary to change a substance completely at its melting point to a liquid at the same temperature. This means that if heat is applied to ice at 0 degrees Celsius, the temperature of the ice remains constant until all of the ice has melted. Note that the term “latent” indicates a change in state without a change in temperature. In a similar manner, the latent heat of vaporization is the amount of heat per unit mass (539 calories per gram) required to change a liquid completely at its boiling point to a gas at the same temperature. This means that if heat is applied to water at 100 degrees Celsius, the water begins to boil and the temperature remains the same until all the water has boiled away. The old saying “a watched pot never boils” reflects the fact that water needs an enormous amount of heat before it can reach its boiling point and undergo a phase change from liquid to vapor. The processes of fusion and vaporization are reversible and thereby represent two of the most important energy transformations in the environment, as they strongly influence Earth’s climate.

Water boils at 100 degrees Celsius at sea-level pressure, which is one of the highest boiling points of any fluid on Earth. This property differs from the general rule that the boiling point of a fluid goes up as its molecular weight increases. This rule does not apply to water, which has a relatively low molecular weight. Viscosity increases with increasing pressure for nearly all fluids. This is not the case with water, for which viscosity decreases as pressure increases. This property explains why water, which is under high pressure in a water-distribution system, is able to flow, rather than dribble, out of a kitchen tap.

The hydrogen bonding of water allows its surface tension to be two to three times greater than that of most common liquids. This property explains why certain insects can “walk on water” and why steel needles can float. Surface tension (cohesion) and the tendency of water to wet solid surfaces (adhesion) cause capillarity, which allows water to “climb” a wall or tube. If water had a much weaker or smaller surface tension (and therefore weaker capillary forces), soil water, which is necessary for plant life, would be unable to overcome gravity.

Distribution of Water

Earth is a well-watered planet. Thus, hypothetically, if the entire surface of Earth could be leveled off and the ocean depths filled with the continents, the planet would be covered with water to a depth of more than 3 kilometers. By far, most of the world’s water (97 percent) is contained in the oceans. Another 2 percent is locked in ice caps and glaciers. This means that almost all the water in the world (99 percent) is either salty or frozen. The remaining water is accounted for by groundwater to a depth of four kilometers, freshwater lakes, saline lakes and inland seas, soil moisture and water in the unsaturated zone, and the atmosphere. Finally, if one measured the average volume of all the rivers on Earth, the estimated amount would only be 0.0001 percent of the total water on the planet.

Water as a Resource

There are several characteristics that pertain to water as a resource. First, water is a renewable resource. As governed by the hydrologic cycle, it is continuously going through the processes of evaporation, convection, and advection in the atmosphere; precipitation; interception and transpiration by vegetation; overland flow; infiltration and percolation through the soil and unsaturated zone to the groundwater in shallow, intermediate, and deep aquifers; and base flow from groundwater to streams for eventual transport to the ultimate sink on Earth, the oceans. In the oceans, it evaporates again to continue the cycle. The quantity of water on Earth is relatively fixed, although the quality is affected by numerous activities.

Second, water is ubiquitous on Earth. It can be found almost anywhere, although it may be too salty or frozen to use directly. Available and abundant freshwater resources, like resources, are unevenly distributed. Thus, water must be transported long distances to supply the needs of major metropolitan areas. For example, New York City gets most of its water from Delaware River basin reservoirs, some two hundred kilometers away. Los Angeles depends on water that is transported hundreds of kilometers from Northern California, the Owens Valley east of the Sierras, and the Colorado River.

Third, water can be considered a common property that has poorly defined property rights. Even during droughts, when potential consumers may be excluded, water is sometimes treated as a free commodity. Society recognizes the expenses associated with the diversion, treatment, and distribution of water but does not recognize the cost of the water itself. The western United States stands out as a major exception to the common property concept, because ownership of water does occur, usually on a first-come, first-served basis.

Fourth, water is relatively inexpensive (although in certain drought-prone regions, in the face of population growth, its scarcity is a growing concern). The combination of the common property aspect of water, water-supply technology, and economies of scale make water an unusually cheap commodity even though it is essential for life and has no substitute. For example, treated public water in the United States delivered to a domestic user costs about thirteen cents per liter (five cents per gallon).

Water Use

The various ways that water is used can be dichotomized into offstream and instream use. Offstream use pertains to water that is diverted (withdrawn) from surface water or groundwater sources and transported to the place of use. This includes water that is used for domestic, commercial, irrigation, livestock, industrial, mining, and thermoelectric power purposes. Each of these seven categories of offstream water use has a different effect on the potential for reuse of the return flows. For example, the return flow for irrigation is often contaminated by pesticides, herbicides, salts, and fertilizers to such an extent that it has minimal reuse potential. An unfortunate illustration of this situation occurs on the lower Colorado River near Yuma, Arizona, where the United States built a large desalinization plant in order to reduce the salinity of the water for the irrigated areas in nearby Mexico. The plant opened in 1992 and has experienced numerous operating problems. In contrast, the reuse potential of most of the water discharged from thermoelectric plants is high, because the major change in the water is an increase of temperature.

Instream water use occurs without the water being diverted from surface or groundwater sources. These uses include navigation, low flow maintenance to benefit aquatic ecosystems, hydroelectric power generation, and wastewater assimilation. Although instream uses have an impact on the quality and quantity of water resources for all uses, numerical estimates of the amount of instream use are difficult to obtain with the exception of hydroelectric power generation.

Diversion of freshwater resources varies considerably from country to country. One useful measure of existing or potential water shortages is to examine total annual diversions as a percentage of the annual renewable water supplies for that country. Some countries, such as Canada and the United States, are well within the limits of their overall renewable water supplies, although the drier areas of the Southwest are reaching the limits of local resources. Other countries in arid regions such as Libya and Saudi Arabia are in excess of their renewable supplies and are therefore mining their groundwater reserves.

Water-use data for the United States have been compiled at five-year intervals by the US Geological Survey on a statewide basis since 1950. These five-year water-census reports provide an invaluable summary of water-use trends and patterns. As expected, total water withdrawals increased from 1950 to 1980 as the population increased. However, beginning in 1985 and contrary to expectations that water use would simply continue to increase as population increased, water use actually declined and then remained stable through 2000. Water use continued to decline between 2000 and 2015. It is hypothesized that technological changes, such as irrigation practices, the introduction of low-flow toilets, and a growing awareness of water conservation, has led to a more efficient use of water.

Excluding water withdrawn for thermoelectric power, irrigation represents the largest use of in the United States, accounting for approximately 65 percent of total water withdrawals. In 2015, the three states with the largest irrigation withdrawals are California (16 percent), Idaho (13 percent), and Arkansas (10 percent). In terms of source, surface water and groundwater account for 76 percent and 24 percent, respectively, of the total amount of freshwater withdrawals.

Public water supply pertains to the diversions made by public and private (investor-owned) systems that are delivered to many users for domestic (residential), commercial, industrial, and thermoelectric power purposes. Surface water accounts for 63 percent of the total fresh water diverted by public water systems. It does not include industrial self-supplied water or the thousands of individual homes and farmsteads in the United States that have their own wells. About 15 percent of the US population have their own wells. Even in the most densely populated state in the nation (New Jersey), the estimated portion of the population that has its own wells has remained at about 10 percent for several decades.

Water Disputes

Because water is essential for life, disputes over its use not only are numerous but also have been going on for several thousand years. In arid areas, such as the Middle East, water is crucial for irrigated agriculture. Thus, Turkey’s decision to build reservoirs for irrigation in the headwaters of the Tigris and Euphrates Rivers, which are in its territory, may deprive the downstream states Iraq and Syria of water on which they have come to depend. The allocation of the waters of the Jordan River among the neighboring states of Israel, Jordan, Lebanon, and Syria in another politically sensitive and drought-prone area is related to the viability of peace in the region. With the small exception of some limited of groundwater that accrued from ancient pluvial periods, Egypt is totally dependent on the Nile River, which originates in Ethiopia and Lakes Albert and Victoria in east-central Africa. Any large diversion of the Nile by the upstream states would have a major impact on Egypt.

The Colorado River and its tributaries begin in the Rocky Mountains in Wyoming, Colorado, and New Mexico and flow for 2,333 kilometers through Utah, Arizona, Nevada, and California before emptying into the Gulf of California in Mexico. Although agreements exist among the seven states and Mexico regarding water allocation, problems have developed and are likely to worsen in the future, because the initial allocation was predicated on an average flow that was based on an above-normal precipitation cycle. In the face of drier or more normal precipitation cycles, the allocations have to be reduced, with obvious harm to the large users in the basin, particularly those who use the water for irrigation.

The Chicago diversion scheme provides a good example of an international agreement on water allocation that was settled amicably. As Chicago grew during the late nineteenth century, drinking water was obtained from a nearby and abundant source, Lake Michigan. Serious health problems developed when Chicago’s sewage was sent back to the same lake. In order to maintain the quality of the drinking water, the Chicago Sanitary and Ship Canal was connected with the Illinois River, which flows into the Mississippi River. Because excessive out-of-basin diversions from Lake Michigan would affect navigation farther downstream at Montreal and Quebec on the Saint Lawrence River, an international agreement between Canada and the United States was reached early in the twentieth century that allowed a diversion of eighty-five cubic meters per second.

Water Quality

Until relatively recently, societies were more concerned with water quantity than with water quality. However, this began to change as growing concentrations of industry and increased population density led to larger amounts of impurities being released into local water sources. By the end of the nineteenth century, the Thames River near London and other rivers near large European cities were so polluted that the rivers became anaerobic (containing no dissolved oxygen) and emitted offensive odors. Fish could not survive in these waters. It became obvious that wastewater from residential and commercial sources had to be treated prior to release into a receiving watercourse.

One solution to the problem in urban areas has been to construct public sewers that connect to wastewater treatment plants, which have helped to improve water quality. In more rural areas, septic systems and well-constructed latrines are generally used to handle wastewater. However, there are countries where unimproved sanitation facilities, such as public and open-pit latrines, are used by large segments of the population. Thus, access to improved sanitation for the total population (urban and rural) varies from an estimated low of 9 percent for Chad and Eritrea in Africa to 100 percent for such countries as Canada, Israel, Japan, and the United States.

The types of water pollution can be categorized on the basis of their effect on human health and the environment. Organic wastes are decomposed by chemical and biological processes that can use up the dissolved oxygen in water that is essential for fish and other aquatic organisms. Excessive amounts of nitrates and phosphates entering surface waters can lead to accelerated aquatic plant growth and organic debris buildup, a process known as eutrophication. Sediments from agricultural and urban land uses can cover benthic (bottom) organisms, clog steam channels, and destroy certain aquatic organisms. Bacteria and viruses that come from animal and human wastes can enter drinking water supplies and cause such diseases as dysentery, hepatitis, and cholera. Heavy metals such as lead and mercury, fibers such as asbestos, and industrial acids are harmful to humans and aquatic ecosystems. Synthetic organic compounds that include water-soluble materials (cleaning compounds and insecticides) and insoluble materials (plastics and residues) can cause a variety of ailments in humans and animals, such as kidney disorders, birth defects, and possibly cancer. Radioactive wastes from commercial and military sources release toxic radiation that causes cancer. Thermal pollution results from heated water being discharged into receiving watercourses, usually from power plants. The additional heat can lead to species change and increased growth rates in many types of aquatic organisms.

An additional problem has developed with the discovery that endocrine-disrupting compounds (pharmaceuticals) and personal care products, collectively known as PPCPs, can be excreted from humans and livestock (animals that are given food additives such as antibiotics, growth promoters, and pharmaceuticals). The array of PPCPs that have been detected in drinking water sources include antibiotics, painkillers, beta blockers, and sex steroids. The majority of the PPCPs wind up in wastewater treatment plants, where they are only partially removed by existing technology. The remaining PPCPs end up in surface streams from overland runoff or get directly into groundwater from septic systems. Currently, there is minimal change in drinking water legislation regarding these products by government regulatory bodies, although there is growing recognition that an increasing amount of PPCPs are entering drinking water supplies without humans’ full knowledge of the dangers to health.

Water pollution sources are often dichotomized as point and nonpoint. Point sources of pollution refer to a known discharge point or outfall from a facility such as a wastewater treatment plant. Although these are individually important, most of the stream pollution comes from nonpoint sources, which are diffuse and scattered throughout the landscape. Nonpoint sources include storm-water runoff from urbanized areas and agricultural runoff from rural areas. Many contaminants from agricultural operations (herbicides and pesticides) are adsorbed onto soil particles, which are washed into the stream during storm events and transported downstream.

Bibliography

Brooks, Kenneth N., et al. Hydrology and the Management of Watersheds. 4th ed., Wiley-Blackwell, 2013.

Cech, Thomas V. Principles of Water Resources: History, Development, Management, and Policy. 3rd ed., John Wiley & Sons, 2010.

Chapelle, Francis H. Wellsprings: A Natural History of Bottled Spring Waters. Illustrated by Katy Flynn Brown, Rutgers UP, 2005.

Clarke, Robin, and Jannet King. The Water Atlas. New Press, 2004.

Dieter, C.A., et al. Estimated Use of Water in the United States in 2015. US Geological Survey, 2018, doi:10.3133/cir1441. Accessed 23 Aug. 2024.

De Villiers, Marq. Water: The Fate of Our Most Precious Resource. 2nd ed., McClelland & Stewart, 2003.

Gleick, Peter H., et al. The World’s Water: The Biennial Report on Freshwater Resources. Vol. 8, Island Press, 2014.

Glennon, Robert. Water Follies: Groundwater Pumping and the Fate of America’s Fresh Waters. Island Press, 2002.

Hunt, Constance Elizabeth. Thirsty Planet: Strategies for Sustainable Water Management. Zed Books, 2004.

Manning, John C. Applied Principles of Hydrology. Illustrated by Natalie J. Weiskal, 3rd ed., Prentice Hall, 1997.

Maupin, Molly A., et al. Estimated Use of Water in the United States in 2010. US Geological Survey, 2014, doi:10.3133/cir1405. Accessed 3 Nov. 2016.

Powell, James Lawrence. Dead Pool: Lake Powell, Global Warming, and the Future of Water in the West. U of California P, 2008.

Spellman, Frank R. The Science of Water: Concepts and Applications. 3rd ed., CRC Press, 2015.

United Nations, World Water Assessment Programme. Water: A Shared Responsibility. Berghahn Books, 2006.

The USGS Water Science School. US Geological Survey, 27 Oct. 2016, water.usgs.gov/edu/. Accessed 3 Nov. 2016.

Ward, Andy D., et al. Environmental Hydrology. 3rd ed., CRC Press, 2016.

Water Resources Mission Area. "Irrigation Water Use." USGS, 1 Mar. 2019, www.usgs.gov/mission-areas/water-resources/science/irrigation-water-use. Accessed 23 Aug. 2024.

Whiteley, John M., et al., editors. Water, Place, and Equity. MIT P, 2008.