You cannot manage what you have not measured. This page is the practical half of water pollution: which parameters to record, what equipment to use, how to work out biochemical oxygen demand, and how to read what the invertebrates in a river are telling you.
📚 What you need to know
Water quality is measured through chemical, physical and biological characteristics.
Key parameters: dissolved oxygen, pH, temperature, nitrate and phosphate, metals, total suspended solids and turbidity.
A water quality index (WQI) combines several measurements into one score, which makes results easy to compare and easy to explain to the public.
BOD is the mass of oxygen used by decomposers to break down organic matter in a sample — measured as the drop in dissolved oxygen over 5 days at 20 °C.
Indicator species give a picture of pollution over weeks, not just the moment you dipped the probe.
Always sample upstream and downstream of a suspected source, and repeat at regular intervals.
Three kinds of characteristic
Chemical — what is dissolved in the water: oxygen, nutrients, metals, pollutants, and pH.
Physical — how the water behaves: temperature, clarity, turbidity, flow.
Biological — what is living in it: bacteria, algae, invertebrates, invasive species.
Any one reading on its own is weak evidence. Low oxygen might mean sewage, or it might just mean a hot afternoon. That is why scientists combine several parameters into a water quality index: a single score, usually where a higher number means better water, that can be tracked over time and compared between sites.
A WQI is useful because a councillor understands “62 out of 100” instantly. Its weakness is the same thing — combining parameters hides which one is causing the problem.
The parameters, and what a bad reading means
Parameter
What it tells you
What a poor reading means
Dissolved oxygen (DO)
How much oxygen is available for aquatic organisms to respire
Low DO causes hypoxia; fish and insects suffocate or leave, and only tolerant species remain
pH
How acidic or alkaline the water is
Unusual values point to acid deposition, mine drainage or industrial discharge, and stress eggs and larvae
Temperature
Controls metabolic rate and how much oxygen the water can hold
Warm water holds less oxygen while organisms need more, so thermal pollution squeezes from both sides
Nitrate and phosphate
The level of nutrient enrichment
High values warn that an algal bloom and eutrophication are likely
Metals
Contamination from mining, industry or waste disposal
Metals build up in tissue and pass to organisms in higher trophic levels, including people
Total suspended solids (TSS)
The mass of solid particles carried in the water
Signals erosion or a discharge; particles smother gravel beds and clog gills
Turbidity
How cloudy the water is, measured by how light behaves in it
Less light reaches submerged plants, so photosynthesis and visibility both fall
How each measurement is actually taken
Abiotic factor
Method
Dissolved oxygen
Oxygen meter with a probe placed in the water
pH
pH meter with a probe, calibrated with buffer solutions before use
Temperature
Digital thermometer or temperature probe, held at a set depth
Nitrate and phosphate
Colorimetric test kit — a reagent reacts with the sample, and the colour produced matches a concentration on a chart
Total suspended solids
Filter a known volume through a pre-weighed filter paper, dry it, weigh it again; the gain in mass divided by the volume gives the concentration
Turbidity
Secchi disc lowered on a marked rope until it disappears from view; the depth is recorded
The rope is marked in metres, so the reading is simply the depth at which the black and white pattern stops being visible.
Turbidity or TSS? Turbidity measures the effect — how light scatters in cloudy water. TSS measures the cause — the actual mass of particles per litre, in mg/L. Related, but not the same thing, and examiners test the difference.
Biochemical oxygen demand (BOD)
BOD is the cleverest measurement in this topic, because it measures a pollutant you cannot see by watching what it does to something you can measure.
Definition to learn
BOD = the mass of dissolved oxygen needed by decomposers to break down the organic matter in a given volume of water
Add organic matter to a river and the bacteria that feed on it multiply. Bacteria respire aerobically, so more bacteria means more oxygen taken out of the water. Measure how much oxygen disappears and you have an indirect measure of how much organic pollution went in. High BOD means heavy organic pollution.
🧩 The BOD test, step by step
Collect a water sample in a sealed bottle with no trapped air.
Measure the initial dissolved oxygen concentration straight away.
Store the bottle in the dark for 5 days at 20 °C. Dark stops algae adding oxygen by photosynthesis; a fixed temperature keeps respiration rates comparable.
Measure the dissolved oxygen again.
BOD = initial DO − final DO, in mg/L.
WORKED EXAMPLE
Sample A has an initial DO of 9.2 mg/L, falling to 3.4 mg/L after five days. Sample B falls from 8.8 mg/L to 7.9 mg/L. Calculate both BOD values and comment. [3]
Step 1: apply the definitionBOD (A) = 9.2 − 3.4 = 5.8 mg/LBOD (B) = 8.8 − 7.9 = 0.9 mg/LStep 2: compare
Sample A used about six times more oxygen over the same five days.
Step 3: interpretA large amount of organic matter in A — most likely sewage or farm slurry — supporting a big population of decomposers. B is close to clean water.A = 5.8 mg/L, B = 0.9 mg/LAlways give the unit. A bare number rarely scores.
What happens downstream of a discharge
This is the pattern that ties the whole page together. Sewage enters a river at one point. Oxygen does not crash instantly and it does not stay down forever — it dips, bottoms out some distance downstream, then slowly recovers as the organic matter is used up and the moving water dissolves oxygen back in.
Sample one point and you get a number. Sample along the river and you get the story: where the pollution enters, how bad it gets, and how far the recovery takes.
Indicator species: letting the river do the monitoring
A probe tells you about the second you dipped it in. Invertebrates cannot move far and live for months, so the community you find tells you what conditions have been like for a long time. That makes them a cheap, powerful check on chemical data.
Clean water, high oxygen — mayfly nymphs and stonefly larvae. Finding them is good news.
Polluted water, low oxygen — bloodworms and Tubifex worms, which tolerate conditions that kill almost everything else.
Nothing at all — the worst sign, suggesting toxic pollution rather than just organic enrichment.
Remember what “tolerant” means. Bloodworms do not like sewage — they simply survive where competitors cannot, so they end up dominating.
Sampling so the data means something
🧩 Good monitoring design
Sample upstream and downstream of the suspected source. The upstream site is your control.
Repeat over time. Rainfall dilutes; hot weather lowers oxygen. One visit proves very little.
Keep the method identical at every site: same depth, same time of day, same equipment, calibrated.
Take replicates at each site and use the mean, so one odd reading does not mislead you.
Combine chemical and biological data. Agreement between the two is much stronger evidence than either alone.
WORKED EXAMPLE
Upstream of a factory a student finds stonefly larvae and DO of 8.9 mg/L. Two hundred metres downstream she finds only bloodworms and DO of 3.1 mg/L. Explain what has happened, and suggest one improvement to her method. [4]
Step 1: describe the changeDO falls by 8.9 − 3.1 = 5.8 mg/L and the clean-water indicator species is replaced by a tolerant one.
Step 2: explain the cause
An organic or warm discharge from the factory has raised BOD. Decomposer bacteria multiply and their aerobic respiration strips oxygen from the water.
Step 3: explain the species changeStonefly larvae need high oxygen, so they die or leave. Bloodworms tolerate low oxygen, so with no competition they dominate.Step 4: improve the method
Take replicate samples at several distances downstream, on more than one day, so the pattern is not a one-off.
Organic discharge → high BOD → low DO → tolerant species only
💡 Exam tip
Learn the BOD conditions exactly: 5 days, 20 °C, in the dark. Each detail can be a mark.
BOD is a subtraction, not a measurement. Show the working, keep the unit mg/L.
Say high BOD = high organic pollution = low dissolved oxygen. That chain answers half the questions on this page.
When you describe a graph, quote figures from the axes. “Falls from 9.5 to 3.7 mg/L” beats “goes down”.
If asked to evaluate a monitoring method, weigh cost, speed, training needed and reliability — not just accuracy.
A biological survey and a chemical reading answer different questions: history versus this instant. Use both.
⚠ Common mix-up
Turbidity is not TSS. Turbidity is the optical effect; TSS is the mass of particles per litre.
High BOD does not mean high oxygen. It means high oxygen demand, so the measured DO ends up low.
Hypoxia is not anoxia. Hypoxia is low oxygen; anoxia is oxygen effectively gone.
A high WQI is good. Students often flip this because most pollution numbers are bad when they are high.
Finding tolerant species does not prove toxicity. It points at organic enrichment. Finding nothing at all is the toxic warning.
Forgetting the control site. Without an upstream reading you cannot show the factory changed anything.
Up next: Eutrophication and Its Consequences — the full chain from fertiliser to dead zone, and the feedback loop that makes it accelerate.
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