Water Quality Index for Streams and Rivers

The Water Quality Index (WQI) attempts to integrate a series of key water quality parameters into a single number that can be used to compare different sampling locations over time. Originally the WQI was developed by the Environmental Protection Agency (EPA) Region 10 and was based on curves that relate concentrations or measurements of eight constituents to index scores and then aggregates scores into a single number (Hallock, 2002). The EPA curves were a synthesis of national criteria, state standards, and technical guidelines. Washington Department of Ecology (Ecology) adapted this index for use in Washington State by adjusting the curves to reflect local water quality standards and/or guidelines. King County is using Ecology’s version of the WQI. This WQI may be modified in the future as criteria change and other information becomes available.

The WQI is a unit-less number ranging from 10 to 100: the higher the number, the higher the water quality. For temperature, pH, fecal coliform bacteria, turbidity, and dissolved oxygen, the index expresses results relative to levels required to maintain beneficial uses according to criteria in Washington’s Water Quality Standards, WAC 173-201A. For nutrient and sediment measures where there are no standard, results are expressed relative to guidelines for this eco-region. Results from these eight metrics (dissolved oxygen, pH, total phosphorus, total nitrogen, turbidity, total suspended solids, temperature, and fecal coliform bacteria) are aggregated over time to produce a single score for each sample station. 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.”