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Water Degradation

Water is a “renewable” resource that provides essential services, is constantly restored by the hydrologic cycle, and can be degraded when used or altered faster than it can be replenished. Freshwater degradation occurs when the physical, chemical, or biological characteristics of water become harmful to the environment or organisms, including humans, by which the usefulness of the water resource is in some way reduced. The quality of water as a resource, and therefore the acceptable level of degradation, depends on the intended use of the water and consequently also depends on the desires of multiple, often conflicting stakeholders. The globally variable distribution of water resources, the burgeoning human population with increasing consumptive uses, and increasing waste production and pollution make water degradation an important societal and ecological concern. This entry discusses the sources, distribution, and use of freshwater; water stresses, causes, and drivers; indicators and assessment of water degradation; and water degradation prevention measures.

Sources, Distribution, and Uses of Freshwater

Water covers about 70% of Earth's surface area; 97.5% of this is saltwater, contained mostly in oceans and seas, and 2.5% is freshwater. Most freshwater, about 68.7%, is frozen in ice caps and glaciers, 30.1% is stored as groundwater, and the remaining small fraction (<1%) is found as surface water in lakes and rivers, as soil moisture, and in the atmosphere and biota. At any time, usable freshwater amounts to about 0.01% of the world's total water. The world's continents receive an estimated annual precipitation of 110,000 km3 (cubic kilometers), which is variably distributed by ecological region.

Plastic and trash pollution in Bicaz Lake, Romania

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Source: Stéphane Bidouze/iStockphoto.

Although we have enough water at a global scale, the distribution of freshwater has been a problem. P. Gleick indicated in 1993 that two thirds of the world's population lives in areas receiving only one quarter of the world's annual rainfall. The global freshwater demand per capita is rising substantially as a result of economic development and population growth. Agriculture represents 70% of the global annual freshwater use (FAO-AQUASTAT). Meeting the rising water demands and the increasing reliance on irrigation to produce food will be a challenge for the growing population and for economic activity in the face of the decline in the finite freshwater resources and increases in pollution.

Water use can be classified as consumptive when water is physically removed for a specific purpose, such as public potable water supply, irrigation, livestock, mining, and others. Non-consumptive uses include hydroelectric power generation, fisheries, navigation, environmental flow, and others that require a certain amount of water at a given point and time but that do not necessarily remove water from its source.

Water Stresses, Causes, and Drivers

The degrading factors that influence water quality can be chemical, biological, or physical, often with compounding interactions among these three causes.

Chemical Water Pollution

Chemical water pollution is the reduction of the usefulness of water by contamination with one or more degrading substances, such as sediments, organic wastes, nutrients, metals, salts, and many other natural and synthetic chemicals that can be caused by both anthropogenic (human caused) and natural processes. Humans have a great impact on water quality through their consumptive uses and also in the generation of pollution. Several components of the hydrologic cycle can also affect the quality of water; for example, evaporation from surface pools can concentrate salts and other chemicals by reducing the overall water volume, and runoff and infiltration can leach harmful levels of metals into groundwater. Degradation of water quality due to processes in the hydrologic cycle is referred to as background pollution or natural contamination to distinguish it from degradation from anthropogenic causes.

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