A water quality index (WQI) combines measurements of several physical, chemical and sometimes biological water-quality variables into one numerical score or category. It simplifies complex data so that water quality can be compared between locations or monitored over time.
How Does a WQI Work?
Researchers measure selected variables, convert each result into a standardized, sometimes weighted sub-index, and combine the sub-indices into an overall score.
Because variables and scales differ between WQI models, interpret the key supplied with the data. A higher score often means better quality, but this is not universal.
| Variable | What it may indicate |
|---|---|
| Dissolved oxygen | Oxygen available for aquatic organisms; low concentrations may indicate decomposition or pollution |
| Biochemical oxygen demand (BOD) | Oxygen required by microorganisms to decompose organic matter; high BOD usually indicates greater organic pollution |
| Nitrate and phosphate concentrations | Nutrient pollution and risk of eutrophication |
| pH | Acidity or alkalinity and suitability for aquatic organisms |
| Turbidity | Suspended material that reduces light penetration |
| Faecal bacteria | Possible sewage contamination and health risks |
For example, WQI monitoring along India’s River Ganga can evaluate changes and pollution-management strategies.
A WQI is easy to communicate to governments and communities. However, it can hide a dangerous individual pollutant, depends on selected variables and weightings, and may miss short-term events. Different indices can therefore classify the same water body differently.
Common misconception: a WQI is not a direct measurement made by one instrument. It is an index calculated from several separate measurements.
IB Exam Technique
In subtopic 4.4 Water pollution, interpreting WQI data is relevant to both SL and HL, while calculating an overall WQI is HL only. In an exam, describe the trend using figures, interpret what the score means using the stated scale, and evaluate limitations rather than assuming the index gives a complete picture of ecosystem health.