A water quality index (WQI) is a numerical score that integrates multiple water quality parameters into a single value to provide a simplified, overall assessment of water quality conditions.
A water quality index (WQI) aggregates measurements of several individual water quality parameters into a composite score, typically on a scale of 0 to 100, to communicate the overall condition of a water body in an easily understandable format. The most widely used WQI in the United States was developed by the National Sanitation Foundation (NSF-WQI) and incorporates nine parameters: dissolved oxygen, fecal coliform, pH, biochemical oxygen demand, temperature change, total phosphate, nitrate, turbidity, and total solids. Each parameter is assigned a quality score based on its measured value using a standardized sub-index curve, and the scores are combined using weighted aggregation. Scores generally classify water as excellent (91-100), good (71-90), medium (51-70), bad (26-50), or very bad (0-25). The Canadian Council of Ministers of the Environment Water Quality Index (CCME-WQI) uses a different approach based on scope, frequency, and amplitude of parameter exceedances relative to guidelines. While WQIs are valuable for public communication, trend analysis, and comparative assessments, they inevitably lose information through aggregation and should be supplemented with examination of individual parameter data for detailed water quality management.