Water pollution occurs when undesirable foreign substances are introduced into natural water. The substances may be chemical or biological in nature. Common pollutants include human or animal waste; disease-producing organisms; radioactive materials; toxic metals such as lead or mercury; agricultural chemicals such as pesticides, herbicides, or fertilizers; acid rain ; and high-temperature water discharged from power plants, often called "thermal pollution." Pollutants in water are dangerous for human or animal consumption and harm crops. High temperatures may cause algae to grow rapidly, rendering water unfit for consumption.
Point sources of pollution, such as an oil leak from a pipeline or chemical waste from a factory, can often be controlled. Nonpoint sources, such as runoff sediment and nitrate-rich water from feedlots represent larger amounts of pollution and are difficult to identify and remedy. Pollution from nonpoint sources may pass into streams or aquifers, covering a wide area.
Although water has been identified on several planets, none has as much water as Earth, of which 70 percent is covered with water. Approximately 97.4 percent of the water on Earth is found in oceans and is too salty for human consumption. An additional 2.6 percent is freshwater found in underground bodies of water called aquifers or frozen in glaciers or polar ice caps. Less than 0.02 percent of Earth's water is present in lakes, rivers, or the atmosphere.
In a few places, water is pure enough to drink directly from wells or springs, but increasingly water must be treated to remove dangerous contaminants, and substances such as chlorine, chloramines, or ozone must be added to kill harmful bacteria.
Pollutants in water are commonly measured and reported as parts per million (ppm) or parts per billion (ppb). A solution that contains 2 grams(0.071 ounces) of lead in 1 million grams (2,205 pounds) of water (1,000 liters, or 264.2 gallons) is a 2 ppm solution. A 1 ppb solution of calcium contains 1 gram (0.036 ounces) of calcium in 1 billion grams (2,205,000 pounds) of water. A concentration of 1 ppm is the same as 1 milligram(3.6 × 10 −5 ounces) per liter.
While it is impractical to remove all impurities from water, the Safe Drinking Water Act, passed by the U.S. Congress in 1974, gives the Environmental Protection Agency (EPA) the authority to set limits for harmful contaminants in water. For each substance, the EPA establishes Maximum Contaminant Level Goals (MCLGs), levels at which the substance can be consumed over a long period of time with no known adverse effects. This level is defined as the amount of impurity that could be present in two liters of water drunk by a person weighing 70 kilograms (154 pounds), each day for seventy years, without ill effects. In addition, the EPA sets Maximum Contaminant Levels (MCLs) of substances for exposure at any single time. A single exposure to concentrations of pollutants below the MCL is considered to be harmless. The MCLG of lead is 0; continuous exposure to lead in any concentration is considered hazardous. The MCL of lead is 0.015 ppm. Both the MCLG and MCL of mercury are set at 0.002 ppm.
Specialized analytical equipment allows technicians to monitor pollutants. In the field, pH meters are used to measure acidity and turbidometers measure the presence of suspended solids. Samples taken to laboratories are analyzed by gas chromatography to determine the presence of organic
compounds such as vinyl chloride, by emission spectroscopy to detect heavy metals , and by high performance liquid chromatography (HPLC) to detect pesticide residues. Such instruments are capable of detecting as little as one part per trillion of pollutants in water.
For much of history, humans used waterways and bodies of water as waste dumps. When the human population was low, fewer people were exposed to the effects of pollution, and the sources were fewer and produced less pollution. During the Industrial Revolution of the nineteenth century, water pollution was recognized as a danger to public health.
Even early settlers were concerned with water quality. Two hundred years before laws were written to protect consumers from lead poisoning, Benjamin Franklin wrote of a family that suffered gastrointestinal pains after drinking water collected from their lead roof. During the trek west, members of wagon trains avoided drinking from stagnant pools, some of which contained large amounts of alkali.
As populations and production grew, industrial and household refuse accumulated, and it became clear that many discarded materials did not simply disappear, but were spread through the water table, absorbed by lower forms of life and passed up the food chain, causing deaths, birth defects, and mental problems. Now, many beaches are closed occasionally or permanently due to pollution, and at a time when populations of fish have decreased, many areas are unsafe for fishing. Water pollution represents an especially dangerous problem in developing nations, which have high populations and manufacturing facilities that do not meet safety standards.
The most dangerous forms of water pollutants include sewage, which frequently contains dangerous pathogenic organisms; oil and hydrocarbons; heavy metals; radioactive substances; pesticides and herbicides; and corrosive substances such as acids and bases.
In developed countries, few direct sources of water pollution should exist, but homeowners still discharge motor oil, antifreeze, pet waste, and paint into storm sewers, and small manufacturers sometimes ignore proper disposal procedures. In developing countries, businesses and households often discharge wastes directly into streams or ponds that are also used for water supplies. Many sources contaminate water supplies indirectly. Indirect sources of pollution include runoff of waste from feedlots or runoff of agricultural chemicals from farmlands; leaking oil from pipelines, wells, or platforms; and large amounts of sediment from streets and parking lots.
Most industrial operations are required to treat wastewater before discharging it into rivers. Wastes from feedlots are collected in lagoons, settled, collected, and used for fertilizer. Heavy metals and organic compounds from industry are often reclaimed from wastewater and recycled, decreasing manufacturing costs. Sewage from homes undergoes at least two stages of treatment. Primary treatment consists of sedimentation and dyeing of solids, which may be used as fertilizer. Secondary treatment consists of aeration of the remaining liquid, through a process of stirring, trickling over filters, and spraying; aerobic bacteria oxidize much of the remaining organic matter. Tertiary treatment, using antibacterial agents such as chlorine or ozone, may be used to produce effluent water that is safe for further use.
MacKenzie, Susan Hill (1996). Integrated Resource Planning and Management: The Ecosystem Approach in the Great Lakes Basin. Washington, DC: Island Press.
Stanitski, Conrad L.; Eubanks, Lucy P.; Middlecamp, Catherine H.; and Pienta, Norman J. (2003). Chemistry in Context: Applying Chemistry to Society, 4th edition. Boston: McGraw-Hill.