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Watershed, Water Quality in a

Watershed, Water Quality in a

A watershed is the connected series of rivulets, streams, rivers, and lakes that collects and directs water from a given area of land into a single watercourse. Watersheds are important as habitats for animals and plants, as a source of drinking and recreational water for many communities, and as a source of irrigation water for farms and ranches. Thus, the health of the watershed, as reflected in the quality of the water, is important to preserve or, if deterioration has occurred, to remediate.

Water quality for any given waterbody is defined as highest and best usethat is, the most beneficial use. Generally this refers to the ability to serve as drinking water (known as potability ) or to support aquatic life.

Prior to the 1800s, many watersheds in the United States were unaffected by human-made pollution , and contamination by natural sources(e.g., animal waste) usually was mediated by natural filtration and dilution. Today, high-quality watersheds are rare. Modern pressures from urban development and decreased air quality have made monitoring of water a necessity.

Indicators of Water Quality

A number of parameters known as indicators can be examined as a way of assessing water quality. Indicators are chemicals or living creatures whose presence in water suggests that there is a likely possibility that the water has been contaminated. A well-known example of a contaminant is the bacterium Escherichia coli.

Escherichia coli (commonly referred to as E. coli ) lives in the intestinal tract of humans and other warm-blooded animals. The microbe does not usually survive in other environments such as fresh water. High numbers of the bacteria in water, therefore, indicate the recent addition of intestinal waste to the water. Because the detection of E. coli involves easily performed and relatively inexpensive tests, it is a fundamental part of every fresh-water quality monitoring program.

A number of other indicators of water quality exist. One commonly used biological indicator is the macroinvertebrate (e.g., aquatic insect larvae). The more diverse and plentiful the macroinvertebrates are, the higher the water quality. Comparison with known standards can allow a determination regarding whether the number of invertebrates is normal or low.

Tests to measure biological indicators for water quality are performed at different sites in the watercourse and over a set period of time. Repeated test sites and test times are necessary because the number of bacteria or invertebrates can vary according to location. Thus, a low level of E. coli at one point in a stream does not necessarily guarantee that another segment of the stream is uncontaminated.

Inorganic, or nonliving indicators of water quality tend to be more stable and uniform throughout the watercourse. A common inorganic parameter that is used to judge watershed water quality is pH. Fresh water tends to have a pH between 6 and 7 (on the 14-point scale, where 1 is extremely acidic and 14 is extremely basic). Sudden changes in pH may indicate contamination, especially if the pH suddenly deviates from historic values.

Another inorganic measurement determines the level of nitrates in the water. Nitrate (NO32) is a chemical form of the element nitrogen. Another chemical form of nitrogen, nitrite (NO2), forms important compounds that permit the growth of algae and some plants. Too much nitrate leads to the explosive growth of algae, whose ultimate death and decay monopolizes the available oxygen in the water, potentially affecting oxygen-sensitive species.

The dissolved oxygen level is another chemical indicator of water quality. The level of oxygen in the water can be lowered by chemical conditions, a natural or artificial increase in water temperature, or the presence of organic material (i.e., sewage). A sustained deficiency in oxygen makes the watercourse less capable of supporting diverse life, and in extreme cases may create a nearly uninhabitable environment known as a "dead zone."

Other measurement tools indicate the presence of various chemicals in the water. For example, the conductivity test, which measures the ability of the water to conduct an electrical current, is an indicator of the presence and approximates the concentration of dissolved ions. A water density test determines the salt or total dissolved solids content (salinity) of the water. Density measurements are important in coastal areas, where salt water from the ocean can flow through ground water and pass into fresh-water streams or rivers.

The presence of phosphorus is typically monitored in freshwater bodies. Phosphorus is a component of fertilizers and can enter a watershed via runoff from lawns, golf courses, and agricultural land. Phosphorus is a nutrient that can stimulate the explosive growth of plants and algae.

Testing Methods

A variety of testing methods characterizes the physical aspects of the watershed. Examples include water temperature, air temperature, the maximum and minimum velocity of the watercourse, the type and arrangement of vegetation (also called the riparian zone), and the water's sediment load. Such measurements conducted over time can provide a warning of deteriorating quality of the stream or river.

Other tests can be performed, depending on the location of the watershed. For example, if the watershed is near a mine, then monitoring to detect the acid drainage that can flow from mine tailings may be warranted. Alternatively, if the watershed is near a nuclear power plant or uranium facility, testing for the presence of radioactive compounds is often warranted. Watersheds that incorporate urban areas often are monitored for the presence of petroleum compounds. Watersheds in rural areas are monitored for the presence of farm fertilizer and pesticides.

Historically, local, regional, and federal governments conducted most watershed quality monitoring. Increasingly, however, citizens groups and committees are seeking funding to conduct tests. Governments generally support such civic efforts, and training is available in many areas for those desiring to learn proper sampling techniques. The quality of fresh water in watershed areas often is improved with civic awareness and involvement.

see also Acid Mine Drainage; Acid Rain; Chemicals from Agriculture; Clean Water Act; Fresh Water, Natural Composition of; Fresh Water, Natural Contaminants in; Lake Health, Assessing; Land Use and Water Quality; Nutrients in Lakes and Streams; Pollution of Lakes and Streams; Pollution Sources: Point and Nonpoint; Steam Health, Assessing.

Brenda Wilmoth Lerner

and Brian D. Hoyle


United States Department of the Interior. Wetlands and Groundwater in the US. Washington, D.C.: Library of Congress, 1994.

Internet Resources

Exploring the EnvironmentWater Quality: Methods for Monitoring. National Aeronautics and Space Administration. <>.

Monitoring and Assessing Water Quality. United States Environmental Protection Agency. <>.

Wetlands Monitoring and Assessing. United States Environmental Protection Agency.>.

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Water Quality

Water Quality

Next to a supply of air, nothing is so essential to life as a supply of high-quality water. We drink it, cook our food in it, use it as a source of energy, and lift a hundred pounds or so of it each time we stand up. Water carries nutrients in and removes waste materials from our bodies. Contaminated water also spreads numerous diseases.

We judge the quality of water by taste, smell, color, and lack of pathogenic organisms or harmful contaminants. Often bad taste, odor, or color indicates contamination. Most of the water we drink has been treated to remove harmful substances and has had chlorine, ozone, or chloramines added to kill bacteria. Ordinary water contains dissolved gases such as oxygen, nitrogen, carbon dioxide, and other atmospheric components, as well as harmless minerals.

Pollutants are usually present at very low concentrations, commonly measured and reported as parts per million (ppm) or parts per billion (ppb). A solution containing 2 grams (0.071 ounces) of lead in 1 million grams of water (1,000 liters, or 264.2 gallons) is a 2 ppm solution of lead in water. A 1 ppb solution of calcium in water 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 × 105 ounces) per liter.

It would be impractical and expensive to remove all impurities from water. The Safe Drinking Water Act of 1974 gives the Environmental Protection Agency (EPA) authority to set limits for dangerous contaminants. For each substance, the EPA sets Maximum Contaminant Level Goals (MCLGs), levels at which the substance could be consumed over a long period of time with no known adverse effects. The MCLG is the amount of contaminant that can safely be present in two liters of water drunk each day for seventy years by a person weighing 70 kilograms (154 pounds). In addition, the EPA sets Maximum Contaminant Levels (MCLs), the maximum permissible level of a contaminant in drinking water.

Removing all pollutants from water would be difficult and expensive, but concentrations below the MCL and MCLG are considered harmless. Lead damages kidneys, and chronic exposure to even tiny amounts may cause damage to the nervous system. The MCLG of lead is 0; the EPA maintains that no amount of lead should be consumed for an extended time. The MCL of lead is 0.015 ppm, but consumption of even low levels of lead in water is not recommended. Both the MCLG and MCL of mercury are set at 0.002 ppm.

Technicians use specialized analytical equipment to monitor pollutants. In the field, pH meters are used to measure acidity; very acidic or basic water may be corrosive. Turbidometers measure suspended solids, which may be harmless but often carry or hide pathogenic organisms. In laboratories, samples are subjected to gas chromatography to determine the presence of organic compounds such as vinyl chloride, high pressure liquid chromatographs measure pesticide traces, and absorption and emission spectroscopy are used to detect heavy metals . Such instruments are capable of detecting as little as one part per trillion of pollutants.

Biological tests are also commonly performed on drinking water. Biochemical oxidative demand (BOD) is a measure of the concentration of biodegradable organic matter. While coliform bacteria such as Eschericia coli are seldom dangerous themselves, they act as indicator bacteria. Water containing coliforms is likely to contain other, more dangerous pathogens.

The acceptable level of pollutants depends on the use intended. We need not flush toilets or water lawns with water pure enough to drink. River water commonly contains traces of animal wastes that are acceptable for irrigation but must be removed before human consumption. Ocean water too salty for consumption can be used for industrial cooling and may be purified by distillation or reverse osmosis to render it suitable for drinking.

Common pollutants include traces of human or animal waste; disease organisms; radioactive materials; toxic metals such as lead or mercury; agricultural chemicals such as pesticides, herbicides, or fertilizers; and high-temperature water discharged from industrial plants. Polluted water may be dangerous to drink, may harm crops, and may cause eutrophication.

Metals such as lead, cadmium, chromium, and mercury are toxic even at low concentrations (the MCL for cadmium is 0.005 ppm). Aquatic microorganisms often concentrate toxic materials from soil or water and may

convert inorganic substances such as mercury to organic forms such as methylmercury. These organisms may be consumed by fish, which in turn are eaten by animals higher on the food chain, and eventually the toxic materials can find their way into human diets. Organic mercury is sometimes absorbed by the central nervous system. Mercurial wastes discharged into the bay at Minimata, Japan, resulted in birth defects and neurological disorders among many children. Even small amounts of lead or mercury may be converted by aquatic microorganisms into toxic organic mercury compounds such as methyl- or dimethylmercury, which, acting as neurotoxins, may be passed up the food chain, eventually causing damage to the central nervous system of humans.

Lead and copper ions in water pose health risks and contribute to the corrosion of pipes and fittings, as does water that is at a high or low pH. Lead solder was banned from pipes in 1986, but much old plumbing still contains a mixture of 50 percent lead and 50 percent tin solder in joints. In plumbing systems having pipes and fittings of two different metals, corrosion may lead to the failure of joints.

Hot water discharged by industries, such as at power plants, and nitrates and phosphates from feedlot runoff cause algae to grow rapidly, rendering water unfit for consumption by humans or farm animals. High-BOD organic matter in sewage, feedlot runoff, or excess fertilizer from farm fields accumulates in ponds and lakes. Oxidative processes then consume so much oxygen that fish and aquatic plants die.

Organic contaminants such as vinyl chloride or hydrocarbons, hormones from animal feed, and pesticides and herbicides often find their way into streams or aquifers. From these sources, the contaminants may make their way into water supplies.

Hard water contains metallic ions, such as magnesium or calcium ions, that interact with soap to form insoluble films or scum. Hardness is not hazardous to health but may form scale in boilers and clog water pipes. Excess calcium and magnesium can be removed by ion exchange water softeners.

see also Green Chemistry; Neurotoxins; Toxicity; Water; Water Pollution.

Dan M. Sullivan


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.

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Water Quality

Water quality

Water is the universal solvent. Many compounds that can dissolve in water are used as food sources by a variety of microbiological life forms. These microorganisms are themselves water-based and their constituent molecules are designed to function in aqueous environments. Thus, water can widely support the growth of microorganisms.

Some of this growth is advantageous. For example, the strains of yeast whose fermentative abilities make possible the brewing of beer, the production of wine, and the baking of bread. In addition, the growth of bacteria in polluted water is used as a means of decontaminating the water. The bacteria are able to use the pollutant compound as a food source. In contrast, some forms of microbial growth can detrimental to products being produced or dangerous to the health of people consuming the water. Ensuring the quality of water from a microbiological standpoint is thus of extreme importance.

The main concern surrounding water quality is the freedom of the water from microorganisms that can cause disease. Typically, these agents are associated with the intestinal tract of warm-blooded animals including humans. Examples of disease causing bacteria are those in the genera of Salmonella, Shigella, and Vibrio. As well certain types of the intestinal bacterium Escherichia coli can cause infections. Escherichia coli O157:H7 has become prominent in the past decade. Contamination of drinking water with O157:H7 can be devastating. An infamous example of this is the contamination of the municipal water supply of Walkerton, Ontario, Canada in the summer of 2000. Several thousand people became ill, and seven people died as a direct result of the O157:H7 infection.

The contamination of the well water in Walkerton occurred because of run-off from adjacent cattle farms. This route of water contamination is common. For this reason, the surveillance of wells for the presence of bacteria is often done more frequently following a heavy rain, or at times of the year when precipitation is marked.

The intestinal tract also harbors viruses that can contaminate water and cause disease. Some examples of these viruses are rotavirus, enteroviruses, and coxsackievirus.

A number of protozoan microorganisms are also problematic with respect to water quality. The two most prominent protozoans are in the genera Giardia and Cryptosporidium. These microorganisms are resident in the intestinal tract of animals such as beaver and deer. Their increasing prevalence in North America is a consequence of the increasing encroachment of civilized areas on natural areas.

Municipal drinking water is usually treated in order to minimize the risk of the contamination of the water with the above microbes. Similarly, the protection of water quality by the boiling of the water has long been known. Even today, socalled "boil water orders" are issued in municipalities when the water quality is suspect. The addition of disinfectant compounds, particularly chlorine or derivatives of chlorine, is a common means utilized to kill bacteria in water. Other treatments that kill bacteria include the use of a gaseous ozone, and irradiation of water with ultraviolet light to disrupt bacterial genetic material. In more recent decades, the filtering of water has been improved so that now filters exist that can exclude even particles as tiny as viruses from the treated (or "finished") water. The killing of the protozoan microorganisms has proved to be challenging, as both Giardia and Cryptosporidium form dormant and chemically resistant structures called cysts during their life cycles. The cyst forms are resistant to the killing action of chlorine and can pass through the filters typically used in water treatment plants. Contamination of the water supply of Milwaukee, Wisconsin with Cryptosporidium in 1993 sickened over 400,000 people and the deaths of at least 47 people were subsequently attributed to the contamination.

Water quality testing often involves the use of a test that measures the turbidity of the water. Turbidity gives an indication of the amount of particulate material in the water. If the water is contaminated with particles as small as bacteria and viruses, the turbidity of the water will increase. Thus, the turbidity test can be a quick means of assessing if water quality is deteriorating and whether further action should be taken to enhance the quality of the water supply.

Water quality is also addressed in many countries by regulations that require the sampling and testing of drinking water for microorganisms. Testing is typically for an "indicator" of fecal pollution of the water. Escherichia coli is often the most suitable indicator organism. The bacterium is present in the intestinal tract in greater numbers than the disease-causing bacteria and viruses. Thus, the chances of detecting the indicator organism is better than detecting the actual pathogen. Additionally, the indicator does not usually multiply in the water (except in tropical countries), so its presence is indicative of recent fecal pollution. Finally, Escherichia coli can be detected using tests that are inexpensive and easy to perform.

Because the prevention of water borne disease rests on the adequate treatment of the water, underdeveloped regions of the world continue to experience the majority of water borne diseases. For example, in India the prevalence of cholera is so great that the disease is considered to be epidemic. But, as exemplified by communities like Walkerton and Milwaukee, even developed countries having an extensive water treatment infrastructure can experience problems if the treatment barriers are breached by the microorganisms.

See also Bacteria and bacterial infection; Bioremediation; Epidemics and pandemics; Water purification

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Water Quality


"Water quality" is a technical term that is based upon the characteristics of water in relation to guideline values of what is suitable for human consumption and for all usual domestic purposes, including personal hygiene. Components of water quality include microbial, biological, chemical, and physical aspects.

Microbial Aspects. Drinking water should not include microorganisms that are known to be pathogenic. It should also not contain bacteria that would indicate excremental pollution, the primary indicator of which are coliform bacteria that are present in the feces of warm-blooded organisms. Chlorine is the usual disinfectant, as it is readily available and inexpensive. Unfortunately, it is not fully effective, as currently used, against all organisms.

Biological Aspects. Parasitic protozoa and helminths are also indicators of water quality. Species of protozoa can be introduced into water supply through human or animal fecal contamination. Most common among the pathogenic protozoans are Entamoeba and Giardia. Coliforms are not appropriate direct indicators because of the greater resistance of these protozoans to inactivation by disinfection. Drinking water sources that are not likely to be contaminated by fecal matter should be used where possible due to the lack of good indicators for the presence or absence of pathogenic protozoa. A single mature larva or fertilized egg of parasitic roundworms and flatworms can cause infection when transmitted to humans through drinking water. The measures currently available for the detection of helminths in drinking water are not suitable for routine use.

Chemical Aspects. Chemical contamination of water sources may be due to certain industries and agricultural practices, or from natural sources. When toxic chemicals are present in drinking water, there is the potential that they may cause either acute or chronic health effects. Chronic health effects are more common than acute effects because the levels of chemicals in drinking water are seldom high enough to cause acute health effects. Since there is limited evidence relating chronic human health conditions to specific drinking-water contaminants, laboratory animal studies and human data from clinical reports are used to predict adverse effects.

Physical Aspects. The turbidity, color, taste, and odor of water can be monitored. Turbidity should always be low, especially where disinfection is practiced. High turbidity can inhibit the effects of disinfection against microorganisms and enable bacterial growth. Drinking water should be colorless, since drinking-water coloration may be due to the presence of colored organic matter. Organic substances also cause water odor, though odors may result from many factors, including biological activity and industrial pollution. Taste problems relating to water could be indicators of changes in water sources or treatment process. Inorganic compounds such as magnesium, calcium, sodium, copper, iron, and zinc are generally detected by the taste of water, and contamination with the oxygenated fuel additive MTBE has affected the taste of some water.

Mark G. Robson

(see also: Ambient Water Quality; Clean Water Act; Drinking Water; E. Coli; Pathogenic Organisms; Water Treatment; Waterborne Diseases )


Shelton, T. (1991). Interpreting Drinking Water Quality AnalysisWhat Do the Numbers Mean? New Brunswick, NJ: Rutgers Cooperative Extension.

World Health Organization (1985). Guidelines for Drinking Water Quality, Vol. 3: Drinking Water Quality Control in Small Community Supplies. Geneva: Author.

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