4.4.3—Water quality measurement
- Syllabus
- First assessment 2026
- Objective
- 4.4.3
- Level
- HL
Match the Test to the Pollution Pathway
Choose a measurement from the suspected mechanism, then compare matched sites or times with replication.
Organic waste suggests BOD and dissolved oxygen; fertilizer suggests nitrate/phosphate tests; heated effluent suggests temperature; sediment suggests turbidity and total suspended solids; industry may require metal-specific tests. Use oxygen and pH probes, a thermometer, a Secchi disc and nitrate/phosphate tests as appropriate.
Upstream DO is 9 mg/L and downstream is 4 after a sewage outlet; pair the result with BOD and flow data.
Matched upstream/downstream sites, repeated at comparable times, help separate the suspected input from weather, flow and seasonal variation.
A WQI can summarize, but one average cannot replace mechanism-specific measurements.