Water quality refers to how suitable water is for its intended use, often for drinking. A wide range of water characteristics, including biological, chemical, and physical descriptions of water clarity or contamination can be used to help determine water quality.
In many ways, the effects of poor-quality water on the spread of disease in communities formed the basis of public health. In mid-nineteenth century England, John Snow traced a cholera outbreak in London's East End to a single pump that was supplying water contaminated with human waste. He was able to halt the disease by removing the handle from the pump so people stopped using it. Until that time, people did not understand how cholera was spread, and they suspected other methods, such as transmission through air. His work was followed by that of Edwin Chadwick, who wrote Report on an Inquiry into the Sanitary Condition of the Labouring Population of Great Britain, which argued that problems such as inadequate waste disposal, overcrowding, poverty, and other issues in society were causing disease and the high economic burden of disease.
Water quality assessment usually involves the examination of a lake, river, bay, aquifer, or other water body for characteristics that are mandated by municipal, state, and/or federal legislation. The most important of these attributes are pollutants that deplete the oxygen content of the water or cause disease, nutrients that stimulate excessive plant growth, synthetic organic and inorganic chemicals, mineral substances, sediments, radioactive substances, and temperature.
After the 1950s, routine tests for water quality have evaluated temperature, turbidity, color, odor, total solids after evaporation, hardness (pH), and concentrations of carbon dioxide, iron, nitrogen, chloride, active chlorine, microorganisms, coliform bacteria, and amorphous matter. More parameters were added in later decades, in response to growing public concern over water quality: these included algal growth, chemical oxygen demand, and the presence of hydrocarbons, metals, and other toxic substances.
Water quality is evaluated through a set of samples taken at different depths of water (a water column), if the watercourse is sufficiently deep, or at selected sites in shallower watercourses. Because quality conditions change continually, each set of samples reflects the conditions at the time of sampling. Over time, samples collected from the same site can provide an indication of the temporal quality of the particular watercourse, which can be used to reveal conditions when water quality deteriorates (e.g., runoff following excessive rainfall).
To ensure that quality measurements are consistent at different times and locations, standards are legally set for various water uses and contaminant levels. As well, the testing protocols are standardized, allowing comparison of results obtained from different testing facilities, and permitting the auditing of testing laboratories to ensure that the tests are being done in a valid fashion. Two general types of standards are used around the globe to measure water quality. Water quality-based standards ensure that a water body is clean enough for its expected uses, which can include fishing, swimming, industrial use, or drinking. Technology-based standards are set for wastewater entering a water body so that overall water conditions remain acceptable. In the United States, the Environmental Protection Agency (EPA) is responsible for developing water quality standards and criteria for surface, ground, and marine waters.
Water quality standards are based upon criteria that designate acceptable levels of specific pollutants, water clarity, or oxygen content. Acceptable levels of these criteria are typically measured in parts per million (ppm) of each contaminant or nutrient. These measurements allow scientists to assess whether or not a body of water meets the prescribed water quality standards. If standards are not met, then local governing bodies or industries must develop and implement cleanup strategies that target specific criteria.
Both point and nonpoint sources of pollution affect water quality. Point sources are discrete locations that discharge pollutants, mainly industrial outflow pipes or sewage treatment plants. Nonpoint sources are more diffuse and include storm runoff and runoff from farming, logging, construction, and other land use activities.
In rivers, the most extensive causes of water quality impairment are usually siltation, nutrient concentration (nitrogen and phosphorus), fecal coliform bacteria, and low dissolved oxygen levels caused by high organic content (e.g., sewage, grass clippings, pasture and feedlot runoff). Agricultural runoff, including pesticides, fertilizers, and sediments, is the largest source of river pollution, followed by municipal sewage discharge. Estuaries, rich ecosystems of mixed salt and fresh water where rivers enter a sea or ocean, often have serious water quality problems. Estuarine contaminants are usually the same as those in rivers—high organic content and low oxygen, disease-causing pathogens, organic chemicals, and municipal sewage discharge.
In most lakes, water quality is affected primarily by nitrogen and phosphorus loading, siltation and low dissolved oxygen. Like rivers, lakes suffer from agricultural runoff, habitat modification, storm runoff, and municipal sewage effluent. Lakes especially suffer from eutrophication (the excessive growth of aquatic algae and other plants) caused by high levels of nutrients. This burst of growth, also called an algal bloom, feeds on concentrated nutrients. Thick mats of algae and other aquatic plants clog water systems, increase turbidity, and suffocate aquatic animals so that an entire ecosystem is disrupted.
Water quality degradation affects the stability of aquatic ecosystems as well as human health. Much is still unknown about the effects of specific pollutants on ecosystem health. However, fish and other aquatic organisms exposed to elevated levels of pollutants may have lowered reproduction and growth rates, diseases, and, in severe cases, high death rates.
Contaminated drinking water can cause a number of short-term and long-term health problems, and the number of possible contaminants, such as chemicals and minerals, is lengthy. Several waterborne substances can cause illness if people drink water containing them. Some of the most common diseases are:
Drinking water quality is not the only cause of disease, however. The bacteria Legionella is natural in ground and sea water. Under certain conditions, it can transmit from water to air. Systems such as faucets, showerheads, cooling towers, and nebulizers can help transmit the bacteria from water to air. Human inhalation of contaminated aerosols leads to infection and Legionnaire's disease. Some diseases are spread by people washing their hands in water or are water-related diseases. An example is malaria, which can be spread by mosquitoes that breed in standing water.
Further, pollution of water can be a public health hazard that causes numerous problems later on. For example, if wastewater is dumped into the rivers or lakes and is not treated properly, the polluted water affects aquatic populations and can make fish unsafe to eat. If quantities of certain nutrients, such as nitrogen, become too high in water used to irrigate crops, the water can damage the crops. Too much nitrogen also kills algae, which in turn affects the water body's ecosystem.
In the United States, public water systems supply nearly 90% of drinking water to residents, or about 286 million Americans. The EPA regulates the private and public entities that run the systems. The Centers for Disease Control and Prevention (CDC) estimates that up to 15% of Americans get their drinking water from private wells, which neither the EPA nor any federal agency regulates. Globally, safe community drinking water is more problematic. Diarrheal disease accounts for about 4% of all disease and could kill nearly 1.8 million people a year, according to the World Health Organization (WHO). WHO estimates that 88% of the diarrheal disease is caused by unsafe water supplies, poor sanitation, and insufficient hygiene.
The CDC estimates that the costs of the three most common waterborne diseases, cryptosporidiosis, giardiasis, and Legionnaire's disease, are more than $500 million a year in hospitalizations and other medical care.
In the United States and many other countries, national water quality criteria have been developed for most conventional, toxic, and nonconventional pollutants. Conventional pollutants include suspended solids, biochemical oxygen demand (BOD), pH (acidity or alkalinity), presence and/or quantity of fecal coliform bacteria, oil, and grease. Toxic priority pollutants include metals and organic chemicals. Nonconventional pollutants are any other contaminants that harm humans or marine resources and require regulation.
Drinking water must meet especially high standards to be safe for human consumption, and its quality is strictly monitored in most developed countries, including the United States. Most governments have a legal responsibility to maintain acceptable drinking water quality. Indeed, in the United States, the Clean Water Act was conceived based on the belief that clean water was a national necessity.
Until 1974, efforts to maintain acceptable levels of drinking water quality in the United States were limited to preventing the spread of contagious diseases. The Safe Drinking Water Act of 1974 expanded the government's regulatory role to cover all substances that may adversely affect human health or a water body's odor or appearance. This act established national regulations for acceptable levels of various contaminants.
Even ocean waters, with their massive volume of water, are vulnerable. One example began on April 20, 2010, when the Deepwater Horizon oil drilling platform exploded and sank in the Gulf of Mexico, triggering the rupture of a deep water oil pipeline. Estimates were that up to 60,000 barrels of oil (2,500,000 US gallons) per day flowed into the Gulf before the rupture was capped on July 15. Some beaches were soiled and wildlife affected. But as of July 2010, the ultimate environmental fate of the massive amount of oil that remained underwater was unknown. Fears were that the oil would be pushed ashore during a tropical storm or hurricane, which could be disastrous for the coastline of the Gulf states.
See also Algal bloom ; Cholera ; Cryptosporidiosis ; Giardiasis ; Legionnaire's disease, Malaria ; Safe Drinking Water Act 1974 ; Sanitation .
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Chapra, Steven C. Surface Water-Quality Modeling. Long Grove, IL: Waveland Press, 2008.
Craig, Robin Kundis. The Clean Water Act and the Constitution. Washington, DC: Environmental Law Institute, 2009.
Virgil, Kenneth M. Clean Water: An Introduction to Water Quality and Pollution Control. Corvallis: Oregon State University Press, 2003.
Centers for Disease Control and Prevention. “Water Quality.” http://www.cdc.gov/healthyplaces/healthtopics/water.htm (accessed October 14, 2012).
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Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Washington, DC, 20460, (202) 272-0167, http://water.epa.gov .
Marci L. Bortman
Revised by Teresa G. Odle