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The Water Quality Index (WQI) is a general method used to report water quality data for many parameters as a single number or score. Using a WQI helps to compare water quality for different sampling locations over time. WQIs have been widely used in many fields of water quality, from drinking water to watershed management, since the 1800s. Horton (1965) developed the first parameter based numerical WQI in 1965 to evaluate the health of rivers. Horton’s method integrated ten commonly monitored water quality parameters. 

WQI methods follow three steps. First, water quality data is collected. Next, concentrations or measurements of parameters are converted to subindex scores on a common scale. Scoring metrics are typically based on water quality standards and technical guidelines. Finally, scores are aggregated into a single number. 

WQI_ScoringMetrics_ConceptualDiagram Conceptual diagram showing the process of converting water quality data to WQI scores.

Washington Department of Ecology (Ecology) adapted an index developed by the Environmental Protection Agency (EPA) Region 10. Ecology adjusted the scoring metrics to reflect local water quality standards and/or guidelines. They also modified the nutrient scoring metrics to better reflect conditions in Puget Sound lowland streams.  

King County is using Ecology’s Puget Sound lowland stream version of the WQI. The WQI developed by Ecology ranges from 0-100: the higher the number, the higher the water quality. It includes eight parameters: temperature, dissolved oxygen, pH, fecal coliform bacteria, total nitrogen, total phosphorus, total suspended sediment, and turbidity (Hallock, 2002). The scoring metrics for parameters are based on regulatory standards. Those levels are defined by criteria in Washington’s Water Quality Standards, WAC 173-201A. Nutrient and sediment measures have no set standard. Therefore, results for those metrics are expressed relative to guidelines for this ecoregion. Results from the eight parameters are aggregated over time to produce a single score for each monitoring site. In general, stations scoring 80 and above did not fail water quality standards or guidelines and are of "low concern". Scores 40 to 80 indicate “moderate concern”, and water quality at stations with scores below 40 are of "high concern."  

As noted above, the WQI assesses and presents water quality scores based on conditions required to maintain designated uses for each site evaluated. Designated uses are the goals for a water body (e.g., recreation, drinking, fishing) assigned by the state that the water body should support. Designated uses and relevant standards vary by water body. Thus, a given water temperature at one site may result in a lower score than the same water temperature at another site.

Environmental Factors Influencing WQI Scores

Many factors affect stream water quality.  In King County, development is a significant cause of water quality impacts - primarily through stormwater runoff. 

In urban streams, things like combined sewer overflows (CSO's), pet waste, and bird droppings can lead to bacteria in the water. In agricultural and suburban areas, problems can come from poorly managed livestock manure and septic systems that don't work properly. In wetlands, animal waste and stagnant water conditions can cause bacteria levels to go up. 

High phosphorus concentrations are found in fecal waste and elevated concentrations are often linked to similar sources as bacteria. Also, elevated phosphorus concentrations are linked to areas undergoing development, since phosphorus in the soil can wash away from construction sites.  

Low oxygen levels in the water can be caused by things like low water flow, high temperatures (warmer water holds less oxygen), and high levels of organic matter (which uses up oxygen as it decomposes). 

The WQI method shows that we need a coordinated approach to solve problems with streamflow management. When there's not enough water flowing in the streams or there's too much stormwater runoff, the water quality parameter scores in the WQI get worse.