Water pollution can be defined as "any biological, chemical, or physical change in water quality that has a harmful effect on living organisms or makes water unsuitable for desired uses." In the United States, approximately 44% of lakes, 37% of rives, and 32% of estuaries are unsafe for recreation due to toxic water pollutants. Such pollutants fall into three main categories: 1) biological, such as bacteria or viruses; 2) chemical, including heavy metals, nutrients, pesticides, and wastes; 3) physical, such as sediment, radioactive material, and heat.
Each year, about 1.5 million Americans become ill as a result of bacterial contamination in drinking water. Other examples of biological pollutants include viruses, protozoa, and parasitic worms. These infectious agents enter the environment from human and animal wastes, and they cause a variety of serious diseases. The United States Environmental Protection Agency uses the number of coliform bacteria per 100 milliliters of a water sample in order to determine the severity of biological pollution in water. The EPA recommends that drinking water contain zero colonies per 100 milliliters, and that swimming water contain no more than 200 colonies per 100 milliliters.
Heavy metals represent a common type of chemical pollution in water. They can be found naturally in bedrock and sediment or they may be introduced into water from industrial sources and household chemicals. Heavy metals harm humans through direct ingestion of contaminated water or through accumulation in the tissues of other organisms that are eaten by humans. The following are some common heavy metals found in water:
- Mercury (Hg): Enters the environment through the leaching of soil due to acid rain, coal burning, or industrial, household, and mining wastes. Causes damage to nervous system, kidneys, and vision.
- Lead (Pb): Sources include paint, mining wastes, incinerator ash, water from lead pipes and solder, and automobile exhaust. Causes damage to kidneys, nervous system, learning ability, ability to synthesize protein, and nerve and red blood cells.
- Cadmium (Cd): Sources include electroplating, mining, and plastic industries, as well as sewage. Causes kidney disease.
- Arsenic (As): Enters the environment through herbicides, wood preservatives, and mining industry. Causes damage to skin, eyes, gastrointestinal tract, and liver. May also cause cancer.
- Aluminum (Al): Enters the environment through leaching due to acid deposition. Causes anemia and loss of bone strength, and may also contribute to dementia and Alzheimer’s disease.
Nutrients constitute a second category of chemical water pollutants. Sources of nitrogen and phosphorus, two common plant nutrients, include animal wastes, agricultural runoff, and sewage. When these nutrients enter a body of water in large quantities, they cause eutrophication. Because eutrophication significantly lowers the levels of dissolved oxygen in water, many species of fish can no longer survive. In addition, consuming water that contains excess levels of nitrates may reduce the bloodstream’s oxygen-carrying capacity, leading to a number of undesirable health effects for humans.
Oil, another chemical pollutant, is introduced into aquatic environments through leaks from oil tankers or dumping down storm drains. Each year, humans discharge approximately three to six million metric tons of oil into the ocean. In 1989, the oil tanker Exxon Valdez spilled eleven million gallons of oil into Alaskan waters. Over 300,000 birds and 2,500 otters were killed, and the total environmental damage amounted to a cost of over fifteen billion dollars.
Another type of chemical water pollutant is radioactive waste. Examples include radioactive isotopes of iodine, radon, uranium, cesium, and thorium. These chemicals enter aquatic ecosystems through discharge from nuclear power plants, processing of uranium and other ores, nuclear weapons production, and natural sources. The harmful effects of radioactive waste when ingested through drinking water include genetic mutations, miscarriages, birth defects, and certain cancers.
Sediment, solid fragments of inorganic or organic material that do not dissolve in water, represents the most significant source of water pollution, physical or otherwise. Sources of sediment include erosion, deforestation, and agricultural and hydroelectric projects. Sediment chokes and fills lakes, reservoirs, harbors, and other aquatic environments, reducing photosynthesis and disrupting aquatic food webs. Sediment may also carry pesticides, bacteria, and other harmful substances, and it can destroy the feeding and spawning grounds of fish.
Heat is another physical water pollutant. Excessive heat in water results when large quantities of water are used for cooling of electric power plants. Each year, almost half of the water withdrawn in the United States is used for such cooling. Thermal pollution in water lowers dissolved oxygen levels and makes aquatic species more susceptible to disease, parasites, and toxic chemicals. Thermal shock occurs when an organism adapted to a certain temperature range is suddenly exposed to a temperature outside of that range. Thermal pollution results in death for many aquatic species.
Thinkquest Team "Fish," March 2005, Disclaimer and copyright information