Watershed pollution and water quality testing
GeographyWater & OceansAges 15–16Virtual lab
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A river flows past forest, farmland, a town, a sewage treatment works, a factory and a power plant. Choose the weather (dry spell, normal or after a storm), the water temperature, the amount of fertiliser and a buffer strip along the banks, the level of sewage treatment, an acidic factory discharge and how much the cooling water is warmed, then test six sites A–F from upstream to downstream: temperature, dissolved oxygen (mg/L and % saturation), pH, nitrate, phosphate, turbidity and a kick sample of macroinvertebrates for a pollution tolerance index. See the difference between point sources (pipes) and nonpoint sources (runoff from fields and streets), the oxygen sag below the sewage outfall and how warm water holds less oxygen.
Lesson: Water pollution in a watershed: point and nonpoint sources, dissolved oxygen, thermal pollution and water quality testing
What it shows
What enters a river along its course changes the water downstream. Point sources leave through a single pipe, such as a sewage works, a factory or a power plant's cooling water; nonpoint sources come from a wide area, such as rain running off fertilised fields and streets. Bacteria use up oxygen as they break down sewage, so dissolved oxygen sags below the outfall and recovers slowly as air dissolves back in. Warm water holds less oxygen, so thermal pollution makes this worse. Macroinvertebrates show the water quality over weeks. The river model mixes each input and applies first-order decay, with illustrative values.
How to use
Set Weather, Upstream water temperature and the pollution sources: Fertiliser on the fields, Riverside buffer strip, Sewage treatment works, Factory discharge and Cooling water warming. Tap a site A–F on the map or choose Test site, then press Test this site, or Test all six sites. Pick the Graph and tick Show the model along the river to compare.
Parameters you can change
- Weather Dry spell, Normal, After a storm
- Upstream water temperature 4–28 °C
- Fertiliser on the fields None, Low, Medium, High
- Riverside buffer strip
- Sewage treatment works No treatment, Primary (settling), Secondary (biological), Tertiary (N and P removal)
- Factory discharge None, Acidic waste, Neutralised
- Cooling water warming 0–12 °C
- Test site A, B, C, D, E, F
- Graph Dissolved oxygen, Temperature, pH, Nitrate, Phosphate, Turbidity, Pollution tolerance index
- Show the model along the river
Questions to explore
- Why is dissolved oxygen lowest some distance below the sewage outfall rather than at the pipe itself?
- Why do nitrate and turbidity rise after a storm while the sewage is diluted more?
- What does finding stonefly and mayfly nymphs tell you about the water at a site?
