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 use—that 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.
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 (NO 3 2− ) is a chemical form of the element nitrogen. Another chemical form of nitrogen, nitrite (NO 2 − ), 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.
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 ;
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.
Exploring the Environment—Water Quality: Methods for Monitoring. National Aeronautics and Space Administration. <http://www.cotf.edu/ete/modules/waterq/methods.html> .
Monitoring and Assessing Water Quality. United States Environmental Protection Agency. <http://www.epa.gov/owow/monitoring/> .
Wetlands Monitoring and Assessing. United States Environmental Protection Agency. <http://www.epa.gov/owow/wetlands/monitor/> .