Water Pollution

H

Thermal Pollution

Water is often drawn from rivers, lakes, or the ocean for use as a coolant in factories and power plants. The water is usually returned to the source warmer than when it was taken. Even small temperature changes in a body of water can drive away the fish and other species that were originally present, and attract other species in their place. Thermal pollution can accelerate biological processes in plants and animals or deplete oxygen levels in water. The result may be fish and other wildlife deaths near the discharge source. Thermal pollution can also be caused by the removal of trees and vegetation that shade and cool streams.

III

Sources of Water Pollutants

Water pollutants result from many human activities. Pollutants from industrial sources may pour out from the outfall pipes of factories or may leak from pipelines and underground storage tanks. Polluted water may flow from mines where the water has leached through mineral-rich rocks or has been contaminated by the chemicals used in processing the ores. Cities and other residential communities contribute mostly sewage, with traces of household chemicals mixed in. Sometimes industries discharge pollutants into city sewers, increasing the variety of pollutants in municipal areas. Pollutants from such agricultural sources as farms, pastures, feedlots, and ranches contribute animal wastes, agricultural chemicals, and sediment from erosion.

The oceans have become increasingly polluted. Pollutants reach the sea from adjacent shorelines, from ships, and from offshore oil platforms. Sewage and food waste discarded from ships on the open sea do little harm, but plastics thrown overboard can kill birds or marine animals by entangling them, choking them, or blocking their digestive tracts if swallowed.

Oil spills often occur through accidents, such as the wrecks of the tanker Amoco Cadiz off the French coast in 1978 and the Exxon Valdez in Alaska in 1992. Routine and deliberate discharges, when tanks are flushed out with seawater, also add oil to the oceans. Offshore oil platforms also produce spills: The second largest oil spill on record was in the Gulf of Mexico in 1979 when the Ixtoc 1 well spilled 530 million liters (140 million gallons). The largest oil spill ever was the result of an act of war. During the Persian Gulf War, Iraqi forces destroyed eight tankers and onshore terminals in Kuwait, releasing a record 910 million liters (240 million gallons). An oil spill has its worst effects when the oil slick encounters a shoreline. Oil in coastal waters kills tidepool life and harms birds and marine mammals by causing feathers and fur to lose their natural waterproof quality, which causes the animals to drown or die of cold. Additionally, these animals can become sick or poisoned when they swallow the oil while preening (grooming their feathers or fur).

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Water pollution can also be caused by other types of pollution. For example, sulfur dioxide from a power plant’s chimney begins as air pollution. The polluted air mixes with atmospheric moisture to produce airborne sulfuric acid, which falls to the earth as acid rain. In turn, the acid rain can be carried into a stream or lake, becoming a form of water pollution that can harm or even eliminate wildlife. Similarly, the garbage in a landfill can create water pollution if rainwater percolating through the garbage absorbs toxins before it sinks into the soil and contaminates the underlying groundwater (water that is naturally stored underground in beds of gravel and sand, called aquifers).

Pollution may reach natural waters at spots we can easily identify, known as point sources, such as waste pipes or mine shafts. Nonpoint sources are more difficult to recognize. Pollutants from these sources may appear a little at a time from large areas, carried along by rainfall or snowmelt. For instance, the small oil leaks from automobiles that produce discolored spots on the asphalt of parking lots become nonpoint sources of water pollution when rain carries the oil into local waters. Most agricultural pollution is nonpoint since it typically originates from many fields.

IV

Controlling Water Pollution

In the United States, the serious campaign against water pollution began when Congress passed the Federal Water Pollution Control Act in 1972 and a set of amendments, known as the Clean Water Act (CWA), in 1977. This law enunciated a national goal to end all pollution discharges into surface waters, such as lakes, rivers, streams, wetlands, and coastal waters. The law required those who discharge pollutants into waterways to apply for federal permits and to be responsible for reducing the amount of pollution over time. The law also authorized generous federal grants to help states build water treatment plants that remove pollutants, principally sewage, from wastewater before it is discharged.

Since the passage of this legislation, most of the obvious point sources of pollution in the United States have been substantially cleaned up. Municipal sewage plants in many areas are now yielding water so clean that it can be used again. Industries are treating their waste and also changing their manufacturing processes so that less waste is produced. As a result, surface waters are far cleaner than they were in 1972. In 1990 a survey of rivers and streams found that three-quarters of these waters were clean enough for swimming and fishing. Cleaning up the remainder of these rivers and streams will require tackling the more difficult problems of diffuse, nonpoint source pollution.

Congress first took up the nonpoint source problem in 1987, requiring the states to develop programs to combat this kind of pollution. Since interception and treatment of nonpoint pollution is very difficult, the prime strategy is to prevent it.

In urban areas, one obvious sign of the campaign against nonpoint pollution is the presence of stenciled notices often seen beside storm drains: Drains To Bay, Drains To Creek, or Drains To Lake. These signs discourage people from dumping contaminants, such as used engine oil, down grates because the material will likely pollute nearby waterways. Householders are urged to be sparing in their use of garden pesticides and fertilizers in order to reduce contaminated runoff and eutrophication. At construction sites, builders are required to fight soil erosion by laying down tarps, building sediment traps, and seeding grasses.

In rural areas, efforts are under way to reduce pollution from agricultural wastes, fertilizers, and pesticides, and from erosion caused by logging and farming. Farmers and foresters are encouraged to protect streams by leaving streamside trees and vegetation undisturbed; this practice stabilizes banks and traps sediment coming down the slope, preventing sediment buildup in water. Hillside fields are commonly plowed on the contour of the land, rather than up and down the incline, to reduce erosion and to discourage the formation of gullies. Cows are kept away from streamsides and housed in barns where their waste can be gathered and treated. Increasingly, governments are protecting wetlands, which are valuable pollution traps because their plants absorb excess nutrients and their fine sediments absorb other pollutants. In some places, lost wetlands are being restored. Despite these steps, a great deal remains to be done.

In the United States, the Environmental Protection Agency (EPA) is in overall charge of antipollution efforts. The EPA sets standards, approves state control plans, and steps in (if necessary) to enforce its own rules. Under the Safe Drinking Water Act (SDWA), passed in 1974 and amended in 1986 and 1996, the EPA sets standards for drinking water. Among other provisions, the SDWA requires that states map the watersheds from which drinking water comes and identify sources of pollution within those watersheds. While America’s drinking water is among the safest in the world, and has been improving since passage of the SDWA, many water utilities that serve millions of Americans provide tap water that fails to meet the EPA standards.

The EPA has equivalents in many countries, although details of responsibilities vary. For instance, the federal governments may have a larger role in pollution control, as in France, or more of this responsibility may be shifted to the state and provincial governments, as in Canada. Because many rivers, lakes, and ocean shorelines are shared by several nations, many international treaties also address water pollution. For example, the governments of Canada and the United States have negotiated at least nine treaties or agreements, starting with the Canada-U.S. Boundary Waters Treaty of 1909, governing water pollution of the many rivers and lakes that flow along or across their common border.

Several major treaties deal with oceanic pollution, including the 1972 Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter and the 1973 International Convention for the Prevention of Pollution from Ships (known as MARPOL). International controls and enforcement, however, are generally weak.

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