Publication Summary

The Water Quality Index (WQI) is intended as a tool to summarize and report Ecology's Freshwater Monitoring Unit's routine stream monitoring data. The WQI is a unitless number ranging from 1 to 100; a higher number is indicative of better water quality. Scores are determined for temperature, pH, fecal coliform bacteria, dissolved oxygen, total suspended sediment, turbidity, total phosphorus, and total nitrogen. Constituent scores are then combined and results aggregated over time to produce a single yearly score for each sample station. This report presents the methodology behind the WQI and the results of a trend analysis on monthly WQI scores at our long-term monitoring stations.

The Water Quality Index (WQI) presented here is a unitless number ranging from 1 to 100. A higher number is indicative of better water quality. For temperature, pH, fecal coliform bacteria and dissolved oxygen, the index expresses results relative to levels required to maintain uses according to criteria specified in WAC 173-201A. For nutrient and sediment measures, where standards are not specific, results are expressed relative to expected conditions in a given Ecoregion. 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." A spreadsheet-version for calculating the WQI is available from the author.

Monthly WQI scores are suitable for statistical trend analysis. Prior to adjusting for flow, statistically significant (p < 0.05) improving trends in overall (aggregated constituents) WQI scores were indicated at four stations and declining trends at one station out of 62 evaluated. Adjusting for flow increased the trend slope at nearly three quarters of the stations and resulted in statistically significant improving trends at nine stations and no declining trends. That is, trends in flow were apparently masking improving trends in overall WQI scores at many stations.