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Current Theme: Watershed
Water Quality Index

How can you tell if water is healthy? One way is to calculate a water quality index.

A water quality index provides a single number (like a grade) that expresses overall water quality at a certain location and time based on several water quality parameters. The objective of an index is to turn complex water quality data into information that is understandable and useable by the public. This type of index is similar to the index developed for air quality that shows if itís a red or blue air quality day. The use of an index to "grade" water quality is a controversial issue among water quality scientists. A single number cannot tell the whole story of water quality; there are many other water quality parameters that are not included in the index. The index presented here is not specifically aimed at human health or aquatic life regulations. However, a water index based on some very important parameters can provide a simple indicator of water quality. It gives the public a general idea the possible problems with the water in the region.

BASIN has researched several types of water quality indices and has found one that is used by many communities for characterizing overall water quality. This index was originally developed by the National Sanitiation Foundation (NSF) (Brown and others, 1970). BASIN may revise the NSF index in the future to better suit Boulder area water specifically.

WQI for all months and stations on Boulder Creek from 2000 to the present.

WQI for Boulder Creek High Flow Conditions

WQI for Boulder Creek Low Flow Conditions

Read about the National Sanitation Foundation's water quality index.
Boulder Creek Water Quality Index


Brown Robert M., McClelland Nina I., Deininger Rolf A., and Tozer Ronald G. 1970. "A water quality index- do we dare?" Water and Sewage Works. October. p. 339-343.

Mitchell Mark K. and Stapp William B. 2000. Field Manual for Water Quality Monitoring. Twelfth Edition.

For examples of how the NSF WQI has been used on the internet, see the following sites:

Kansas Collaborative Research Network (KanCRN) website, "Water Quality Index: Calculating a WQI."

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BASIN is supported by the US EPA, the City of Boulder, the Keep it Clean Partnership, BCWI and BCN

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Last Page Update - Tuesday December 27, 2005