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Political decision-makers, non-technical water managers, and the general public usually have neither the time nor the training to study and understand a traditional, technical review of water quality data. A number of indexes have been developed to summarize water quality data in an easily expressible and easily understood format. Water quality professionals are frequently resistant to the automated, uncritical summarization represented by indexes, and there are good reasons to use the results of any index with caution (see the next section on "Uses and Limitations"). "[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" (Ott, 1978).
Ecology's stream monitoring Water Quality Index (WQI) is an attempt at an imperfect answer to non-technical questions about water quality. It is a unitless number ranging from 1 to 100; a higher number is indicative of better water quality relative to expectations. For temperature, pH, fecal coliform bacteria and dissolved oxygen, the index expresses results relative to levels required to maintain beneficial uses (based on criteria in Washington’s Water Quality Standards, WAC 173-201A). For nutrient and sediment measures, where standards are not specific, results are expressed relative to expected conditions in a given region. Multiple constituents are combined and results aggregated over time to produce a single score for each sample station. In general, stations 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 stations with scores below 40 did not meet expectations and are of "highest concern."
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 stations have particularly poor water quality compared to expectations?) 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" (McClelland, 1974).
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 pre-identified set of water quality constituents. A particular station may receive a good WQI score, and yet have water quality impaired by constituents not included in the index. Second, aggregation of data may mask short-term water quality problems. A satisfactory WQI at a particular station 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.
For management purposes, a more useful index is not one that merely ranks stations by relative water quality, but rather one that indicates whether water quality is less than expected or necessary to support beneficial uses. Our WQI follows this approach. There are disadvantages as well, however. For one thing, this approach requires subjective determinations of the beneficial uses that a particular stream segment should support, the level of water quality required to support those uses, and how critical a variation from that level of quality is. For several key parameters, the first two of these determinations are already codified in Washington’s Administrative Code (WAC 173-201A). Nevertheless, a number of subjective decisions were made in developing our WQI. Another disadvantage is that, by design, the WQI indicates how well water quality at a station meets expectations, not how good the absolute quality is. Comparing WQIs for different stations does not indicate which station has the better absolute water quality unless expectations for both stations were the same.
The methodology used to determine WQI scores was originally developed by the Environmental Protection Agency (EPA), Region 10. Initial development was documented only in the "gray" literature, but the methodology is similar to and perhaps based on the well-known National Sanitation Foundation index. This index uses curves to relate concentrations or measurements of various constituents to index scores and then aggregates scores to a single number (Brown, et al., 1970). The EPA curves were "a synthesis of national criteria, state standards, information in the technical literature, and professional judgment" (Peterson and Bogue, 1989). Washington's index is based largely on these curves, adjusted to reflect local water quality standards criteria. Procedures may be modified in the future as criteria change and other information becomes available. We welcome comments on methodology (do the details make sense?), final outcome (are scores what they should be?), and presentation.
If you wish to determine index scores based on your own data, a spreadsheet version (DRAFT) is available for download here.
Questions or comments on content may be sent to Dave Hallock.
Questions or comments on presentation may be sent to Steve Barrett.
Copyright © Washington State Department of Ecology. See http://www.ecy.wa.gov/copyright.html.
