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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. In 2009, Ecology modified the WQI to reflect revised state water quality rules for the protection of native fish and aquatic resources reflected in supplemental temperature criteria for many of the Puget Sound basins. In addition to modifications for revised state criteria, the WQI was further modified in 2009 by Ecology to more directly reflect conditions in Puget Sound lowland streams. King County is using Ecology’s this Puget Sound lowland stream version of the WQI. For purposes of year-to-year comparison, results from previous years were recalculated using the new Puget Sound Lowland Stream WQI.

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."