Chapter Summary

The hydrologic cycle constantly purifies and redistributes fresh water, providing an endlessly renewable resource. More than 97 percent of all water in the world is salty ocean water. Lakes, rivers, and other surface freshwater bodies make up less than 0.01 percent of all the water in the world, but they provide more than half of all water for human use and for habitat and nourishment for aquatic ecosystems that play a vital role in the chain of life.

In the United States, about one-tenth of the water we withdraw from our resources is for direct personal use. Our two largest water uses are agricultural irrigation and industrial cooling. Only about half the water we withdraw is consumed or degraded so that it is unsuitable for other purposes; much could be reused or recycled. Water conservation and recycling would have both economic and environmental benefits.

Water shortages in many parts of the world result from rising demand, unequal distribution, and increased contamination. Water storage and transfer projects are a response to flooding and water shortages. Giant dams and diversion projects can have environmental and social costs far above the benefits they provide. Among the problems they pose are evaporation and infiltration losses, siltation of reservoirs, and loss of recreation and wildlife habitat. Many conservationists prefer watershed management and small dams as means of flood control and water storage.

Any physical, biological, or chemical change in water quality that adversely affects living organisms or makes water unsuitable for desired uses can be considered pollution. Worldwide, the most serious water pollutants for human health are pathogenic organisms from human and animal wastes. We have traditionally taken advantage of the capacity of ecosystems to destroy these organisms, but as population density has grown, these systems have become overloaded and ineffective.

In industrialized nations, toxic chemical wastes have become an increasing problem. Agricultural and industrial chemicals have been released or spilled into surface waters and are seeping into groundwater supplies. The extent of this problem is probably not yet fully appreciated.

Pollution levels in the ocean are increasing. Major causes of ocean pollution are oil spills from tanker bilge pumping or accidents, and oil well blowouts. Surface runoff and sewage outfalls discharge fertilizers, pesticides, organic nutrients, and toxic chemicals that have a variety of deleterious effects on marine ecosystems.

Silt and sediment make up the greatest quantity of water pollutants. Biomass production by aquatic organisms, land erosion, and refuse discharge all contribute to this problem. Salts and metals from highway and farm runoff and industrial activities also damage water quality. In some areas, drainage from mines and tailings piles delivers sediment and toxic materials to rivers and lakes.

Appropriate land-use practices and careful disposal of industrial, domestic, and agricultural wastes are essential for control of water pollution. Natural processes and living organisms have a high capacity to remove or destroy water pollutants, but these systems become overloaded and ineffective when pollution levels are too high. Reducing pollution sources is often the best solution to our pollution problems.