What Do We Mean by Water Quality?

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Water quality, believe it or not, is a statement not easily defined. Like many things in nature, water changes as the seasons change. Generally water quality is referenced in a technical manner, meaning the overall 'scientific' quality of the water. But water quality can also mean something else.

For scientific and legal purposes the following definition is most often used: Water quality is the ability of a water body to support all appropriate beneficial uses.

Beneficial uses are the ways in which water is used by humans and wildlife; drinking water and fish habitat are two examples. If water supports a beneficial use, water quality is said to be good or unimpaired. If water does not support a beneficial use, water quality is said to be poor or impaired.

A key concept is that different beneficial uses have different needs. Most people believe good water quality means the water is pure and clean. This is partly true, especially when you are using water for drinking. However, fish and wildlife have lots of other requirements. Fish must get all of their oxygen and food from water, and therefore need water that has enough oxygen and nutrients. Thus, good water quality implies that harmful substances (pollutants) are absent from the water, and needed substances (oxygen, nutrients) are present.

Water quality can often be defined in terms of the chemical, physical, and biological content of water. The water quality of rivers and lakes changes with the seasons and geographic areas, even when there is no pollution present. Oddly enough, there is no single measure that constitutes good water quality. For instance, water suitable for drinking can be used for irrigation, but water used for irrigation may not meet drinking water guidelines.

Water quality guidelines provide basic scientific information about water quality parameters and ecologically relevant toxicological threshold values to protect specific water uses. Now that we can define water quality in general terms, we need to have parameters we can measure to describe the water quality of a river, stream, or lake. Parameters that are measured include physical, chemical, and biologic properties. Physical measurements are those that include water temperature, depth, flow velocity, flow rate, and turbidity. These are all useful in analyzing how pollutants are transported and mixed in the water environment, and can be related to habitat requirements for fish and other aquatic wildlife. For instance, many fish have very specific temperature requirements, and cannot tolerate water that is either too cold or too hot.

Chemical measurements include a wide range of chemicals and chemical properties. Most water chemistry tests measure concentration, defined as milligrams of chemical per liter of water (mg/l). Even the purest water contains countless chemicals, and it would be impossible to measure all of them. Water quality studies therefore focus on the chemicals that are most important for the problem at hand. In agricultural areas, studies measure chemicals found in manure, fertilizers, and pesticides. In an industrial area studies focus on measuring chemicals used by the nearby industries.

Water quality as you now know can be measured in a variety of ways. While this article is by no means an all-inclusive list of the parameters of water quality measurement, it does describe that water is a changeable substance that involves constant diligence and research to most effectively treat it and make it safe. Next time you have a drink of water, remember that the water you are drinking once had a very different appearance and overall quality.

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