Professionals prefer to give no answer rather than an imperfect answer that could lead to misunderstanding. Yet the layman usually prefers an imperfect answer to no answer at all. While the use of an index may not be the best way to understand large-scale water quality conditions, it is for many the only way. Professionals must understand the need for an imperfect answer and laymen must understand and accept the answer’s limitations.

Water Quality Index (WQI) is an attempt at an imperfect answer to non-technical questions about water quality.  It is a unit-less number ranging from 1 to 100; a higher number is indicative of better water quality. Multiple constituents are combined and results aggregated over time to produce a single score for each sampling site.  In general, sites scoring 80 and above met expectations for water quality and are of "lowest concern," scores 40 to 80 indicate "marginal concern," and water quality at sites with scores below 40 did not meet expectations and are of "highest concern" (see chart below).

Indexes by design contain less information than the raw data that they summarize; many uses of water quality data cannot be met with an index. An index is most useful for comparative purposes (what sites have particularly poor water quality) and for general questions (how is water quality in my stream).

Indexes are less suited to specific questions. Site-specific decisions should be based on an analysis of the original water quality data. In short, an index is a useful tool for communicating water quality information to the lay public and to legislative decision makers; it is not a complex predictive model for technical and scientific application. This index was developed as a tool to summarize and report our routine stream monitoring data to the public.  Besides being general in nature (imprecise), there are at least two reasons that an index may fail to accurately communicate water quality information. First, most indexes are based on a preidentified set of water quality constituents.  For example, a particular site may receive a good WQI score, and yet have water quality impaired by constituents not included in the index.  Second, aggregation of data may either mask (or over-emphasize) short-term water quality problems.  A satisfactory WQI at a particular site does not necessarily mean that water quality was always satisfactory. A good score should, however, indicate that poor water quality (for evaluated constituents, at least) was not chronic during the period included in the index.

The basic methodology used to determine WQI scores was originally developed by the Environmental Protection Agency (EPA). The methodology appears to be based on or similar to the well-known National Sanitation Foundation index, which uses curves to relate concentrations or measurements of various constituents to index scores and then aggregates scores to a single number.

In an effort to develop a system to compare water quality in various parts of the country, over 100 water quality experts were called upon to help create a standard Water Quality Index (WQI). The index is basically a mathematical means of calculating a single value from multiple test results. The index result represents the level of water quality in a given water basin, such as a lake, river, or stream.

The WQI, which was developed in the early 1970s, can be used to monitor water quality changes in a particular water supply over time, or it can be used to compare a water supply's quality with other water supplies in the region or from around the world. The results can also be used to determine if a particular stretch of water is considered to be "healthy."

To determine the WQI, the following nine water quality parameters are measured:

After the nine water quality tests are completed and the results recorded, a "Q" value is calculated for each parameter, and the overall WQI for the sampling site is then calculated.

A worksheet can be downloaded here (requires Adobe Reader).

It is important to monitor water quality over a period of time in order to detect changes in the water's ecosystem. The Water Quality Index can give an indication of the health of the watershed at various points and can be used to keep track of and analyze changes over time.

The Water Quality Index uses a scale from 0 to 100 to rate the quality of the water, with 100 being the highest possible score. Once the overall WQI score is known, it can be compared against the following scale to determine how healthy the water is on a given day.

Water supplies with ratings falling in the good or excellent range would able to support a high diversity of aquatic life. In addition, the water would also be suitable for all forms of recreation, including those involving direct contact with the water. Water supplies achieving only an average rating generally have less diversity of aquatic organisms and frequently have increased algae growth.

Water supplies falling into the fair range are only able to support a low diversity of aquatic life and are probably experiencing problems with pollution. Water supplies that fall into the poor category may only be able to support a limited number of aquatic life forms, and it is expected that these waters have abundant quality problems. A water supply with a poor quality rating would not normally be considered acceptable for activities involving direct contact with the water, such as swimming.