World Book Student

Back

 
Water pollution occurs when human activity causes water to become contaminated by such substances as human and other animal wastes, toxic chemicals, metals, and oils. Water pollution remains among the world’s most serious environmental problems. It can occur in rain; in rivers, lakes, and oceans; and in the water held beneath the ground, called ground water.

Polluted water may appear clean or dirty, but it contains bacteria, viruses, chemicals, or other materials that can cause illness or death. These impurities must be removed before water can be used safely for drinking, cooking, washing, or laundering. Some industries must clean polluted water before using it in manufacturing processes.

Industries and governments have spent billions of dollars to reduce pollution and construct water treatment plants. Most efforts have targeted pollution from point sources. A point source is a direct, easily identified source of pollution, such as a sewerage outlet or factory drain. Pollution from nonpoint sources remains largely uncontrolled. Nonpoint sources include water that runs off construction sites and farmland, carrying soil particles, nutrients, and toxic chemicals into nearby waters. They also include storm water runoff from urban areas, which may carry fertilizer and insecticide from lawns, golf courses, and other planted areas, and salt, oil, and grease from roads and parking lots.

Most water pollution comes from one of three chief sources. These sources are: (1) industrial wastes, (2) sewage, and (3) nonpoint sources.

Industrial wastes. Factories and other industrial facilities discharge pollutants that include many toxic chemicals. They release much chemical waste directly into natural bodies of water. In addition, the burning of coal, oil, and other fuels by power plants and factories releases sulfur and nitrogen oxides into the air. These pollutants, also given off by motor vehicles, cause acid rain that can enter streams and lakes. Acid rain is rainfall that contains such acids as sulfuric acid and nitric acid.

The toxic metal mercury ranks as a widespread water pollutant. Scientists have found high levels of mercury in fish far from industrial areas. This mercury appears to originate primarily in airborne emissions from coal-fired boilers, municipal incinerators, and smelters. See Mercury (Mercury in the environment).

Some industries pollute water without releasing toxic chemicals. They discharge large quantities of hot water into rivers and lakes, harming living things in an effect called thermal pollution. See Thermal pollution.

Sewage consists of human wastes, some garbage, and water that has been used for cleaning. Most sewage in developed countries goes into sewerage systems. These systems carry sewage to treatment plants that remove solids and such dissolved substances as the nutrients nitrogen and phosphorus. Many people even in developed countries live in areas without sewers. Some households use underground septic tank systems to store and treat sewage. Some less developed areas do not have treatment systems. They release untreated sewage into rivers, lakes, and coastal waters.

Nonpoint sources. Water from rain or melted snow drains from farmland into streams, carrying chemical fertilizers and pesticides that have been used on the land. Animal wastes can cause water pollution, particularly through the runoff from farm feed lots that house many animals. Water used for irrigation may become polluted by salt, agricultural pesticides, and toxic chemicals before seeping back into the ground. Runoff from urban areas may also carry wastes into streams.

Water pollution poses a serious public health problem. It also interferes with the use of natural resources and damages the environment.

Human illness. Water polluted with human and animal wastes can spread many diseases, including typhoid fever, cholera, and dysentery. Many communities disinfect their water supplies with chlorine or other chemicals to kill disease-causing germs. However, disinfection does not remove harmful chemicals, such as polychlorinated biphenyls (PCB's) and chloroform, or harmful metals, such as arsenic, lead, and mercury (see Polychlorinated biphenyl (PCB)). The careless release of such toxic wastes, primarily into waste dumps, can lead to the contamination of ground water supplies. Scientists have found PCB's, chloroform, and pesticides in some public drinking water. Some experts think that drinking even small amounts of these substances over many years may have harmful effects.

Reduced use of natural resources. Pollution can prevent people and wildlife from enjoying the full use of natural waters. For example, odors and floating debris may make boating and swimming unpleasant, and the risk of disease can make them unsafe. Oil spilled from ships or offshore wells may float to shore, where it can kill water birds, shellfish, and other wildlife. Water pollution can harm or kill fish directly or by reducing the available food or oxygen. It can reduce fishing catches and make some fish unsafe to eat.

Picture

Healthy water system

Disruption of natural processes. Water pollution can disrupt natural processes in water that turn wastes into useful or harmless substances. This disruption harms living things primarily by lowering the amount of oxygen dissolved in the water.

Aerobic (oxygen-using) bacteria break down organic wastes into simpler substances through a natural process called mineralization. Some of the minerals produced, such as phosphates and nitrates, serve as nutrients for plants. Normal quantities of nutrients help support a balance of life in the water. However, contamination by sewage or fertilizers can add nutrients. When there are too many nutrients, a body of water may suffer from a process called eutrophication.

Picture

How water pollution occurs

In eutrophication, excess nutrients cause an unnatural growth in the numbers of plants, plantlike organisms called algae, fish and other animals, and bacteria. As the algae grow, they block the light needed by the plants to produce oxygen, causing the plants to die and decay. Because the decay process requires oxygen, the additional decay uses up more of the oxygen in the water. Less oxygen remains available for living things. Some types of game fish—such as salmon, trout, and whitefish—cannot live in water with reduced oxygen. Fish that need less oxygen, such as carp and catfish, will replace them. If all the oxygen in a body of water is used up, most forms of life in the water will die.

To control water pollution, people must determine what pollutants the water contains and where the pollutants originate. Point sources can be controlled by treating wastewater to remove harmful substances. Pollution from nonpoint sources requires more complex controls.

Picture

Modern sewage treatment plant: primary treatment

Sewage treatment involves several processes. Primary treatment removes the heaviest solid material, and secondary treatment gets rid of much dissolved organic material and nutrients. Many sewage treatment plants use primary and secondary processes, which can remove up to 95 percent of the waste. Some plants use tertiary treatment methods to remove even more impurities. However, most treated sewage still contains some nutrients and toxic chemicals.

Pretreatment and reduction of wastes. Industries can reduce pollution by treating wastes to remove harmful chemicals before dumping the wastes into water. Industries can also decrease waste by adopting manufacturing processes that reuse polluting chemicals.

Nonpoint control. Researchers have developed operating guidelines called Best Management Practices to help reduce nonpoint pollution from farm, construction, and mining wastes. These practices include landscaping areas in ways that prevent wastes from entering rivers. In urban areas, planners can design housing communities with less pavement and more planted areas to reduce storm water runoff. Yards planted with vegetation that occurs naturally in the area or that needs little fertilizer can reduce nutrients and sediments in runoff.

Laws. Most countries have laws limiting the amounts and kinds of wastes that can be dumped into water. International law and organizations help protect bodies of water that span multiple countries. In 1974, laws developed by the Regional Seas Programme, established under the United Nations Environment Programme, began protecting coastal areas in many parts of the world. The 1982 United Nations Convention on the Law of the Sea, which took effect in 1994, regulates marine pollution from such sources as offshore oil rigs and bodies of water that enter the oceans. International treaties protect several rivers that flow through multiple countries, including the Rhine in Europe, the Zambezi in Africa, and the Río de la Plata in South America. Laws designed to prevent acid rain protect some lakes in North America.

In 1974, the U.S. Congress passed the Safe Drinking Water Act, which authorized the Environmental Protection Agency (EPA) to establish uniform quality standards for more than 200,000 public water systems throughout the United States. Similar laws protect drinking water in many other countries.