“The river looks dirty” is worth no marks. Water quality questions want measurements: what you measure, how you measure it, and what the number tells you about the ecosystem. Get comfortable with a handful of parameters and you can handle almost any data question in this topic.
📚 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.
Turbidity measures the effect (how cloudy the water looks). TSS measures the cause (the actual mass of particles).
Biochemical oxygen demand (BOD) is the dissolved oxygen used up by aerobic organisms breaking down organic matter in a sample over a set time. High BOD means a lot of organic pollution.
A water quality index (WQI) combines several parameters into one score, which makes results easy to compare and easy to explain.
Always sample upstream and downstream of a suspected source, and repeat over time — a single reading proves nothing.
The three kinds of characteristic
Chemical — what is dissolved in the water: oxygen, nitrate, phosphate, metals, pH.
Physical — how the water behaves: temperature, clarity, turbidity, suspended solids.
Biological — what is living in it: bacteria, invertebrates, algae, invasive species.
Notice that these overlap in useful ways. Warm water (physical) holds less oxygen (chemical), which changes which invertebrates can survive (biological). Examiners love that kind of chain.
The parameters and what they tell you
Parameter
How it is measured
What a bad value means
Dissolved oxygen (DO)
Oxygen meter with a probe placed in the water
Low DO means hypoxia — fish and sensitive invertebrates suffocate or leave
pH
pH meter with a probe, or a calibrated colour test
Unusual pH suggests acid deposition or an industrial discharge; affects breeding and growth
Temperature
Digital thermometer or temperature probe
Warm water holds less oxygen and speeds up metabolism, so demand rises as supply falls
Nitrate and phosphate
Colorimetric test kits — the sample changes colour and the shade is matched to a concentration
High values signal fertiliser run-off or sewage and warn of eutrophication
Metals
Laboratory chemical analysis of a collected sample
Mercury, lead, cadmium and arsenic accumulate in tissue and magnify up the food chain
Total suspended solids
Filter a known volume through pre-weighed paper, dry it, weigh again; the gain in mass is the solids
High TSS smothers gravel beds, clogs gills and blocks light
Turbidity
Secchi disc lowered until it disappears, or a turbidity meter
Cloudy water cuts light for plants and makes hunting harder for sight predators
Turbidity vs TSS — the one-line version: turbidity is the effect on light, measured optically; TSS is the cause, measured as mass per volume (mg per litre). Related, but not the same measurement.
The Secchi disc
The cheapest water quality tool in the world: a black and white disc on a marked rope. You lower it until you can no longer see it, and record the depth. Deep reading means clear water. Shallow reading means high turbidity.
It is subjective (your eyes decide) and useless in shallow or fast water, but it costs almost nothing and works anywhere, which is why it is still standard practice.
Biochemical oxygen demand (BOD)
BOD is the parameter students find hardest, mostly because it is measured backwards. You are not measuring the pollution. You are measuring how much oxygen the decomposers use up while eating the pollution — and using that as a stand-in for how much organic matter was there.
🧩 The BOD test, step by step
Collect a water sample in a sealed bottle with no air space.
Measure the starting dissolved oxygen concentration.
Keep the bottle in the dark at a constant 20 °C for 5 days. Dark, so algae cannot add oxygen by photosynthesis.
Measure the dissolved oxygen again.
BOD = starting DO − final DO, in mg per litre.
Formula
BOD = initial dissolved oxygen − final dissolved oxygen (mg per litre)
WORKED EXAMPLE
A sample taken below a dairy farm has an initial DO of 9.2 mg per litre. After 5 days in the dark at 20 °C the DO is 3.4 mg per litre. Calculate the BOD, and express the oxygen used as a percentage of the starting value. [3]
Step 1: subtractBOD = 9.2 − 3.4BOD = 5.8 mg per litreStep 2: as a percentage of the start(5.8 ÷ 9.2) × 100 = 63.0 %Step 3: say what it means
Nearly two thirds of the oxygen was consumed, so the sample carries a heavy organic load — consistent with slurry or manure entering the water.
Units cost marks. mg per litre, every time. And always add the interpretation sentence.
Remember the direction of the relationship: high BOD = dirty water. It feels backwards because a high number sounds good. It is not. A high demand means a lot of organic matter waiting to be broken down.
The oxygen sag curve
This is the classic graph for organic pollution in a river, and it turns up again and again in Paper 2. It shows dissolved oxygen along a river below a sewage outfall.
Reading left to right: clean water, a sharp fall as decomposers use oxygen faster than the river can replace it, a minimum, then slow recovery as the organic load runs out and turbulence redissolves oxygen from the air.
Designing a sampling programme
A lot of Paper 2 marks come from method, not content. If you are asked how you would investigate a suspected pollution source:
Sample upstream of the suspected source as a control, and downstream at several distances.
Take repeat samples at each site so you can spot anomalies and calculate a mean.
Sample at regular intervals over time — weekly, monthly, seasonally. Rainfall, temperature and farming activity all shift the readings.
Keep the method identical at every site: same depth, same time of day, same equipment, calibrated the same way.
Record conditions that could confound the result: recent rain, water flow rate, air temperature.
💡 Exam tip
Learn one sentence for each parameter: what it measures and why organisms care. That covers most short-answer questions.
For any graph of DO along a river, look for the dip and recovery shape and name it as an oxygen sag curve.
When comparing sites, always mention the upstream control. Without it you cannot attribute the change to the source.
BOD needs the exact conditions to be credible: 5 days, 20 °C, in the dark, sealed.
If asked to evaluate a method, comment on accuracy, cost, training needed and whether it gives a number or just an impression.
⚠️ Common mix-up
Thinking high BOD means high oxygen. It means high oxygen demand, so the water ends up low in oxygen.
Treating turbidity and TSS as the same reading. Optical effect vs. measured mass.
Forgetting the dark part of the BOD test. Leave the bottle in light and algae add oxygen, which ruins the result.
Placing the oxygen minimum right at the outfall. It sits downstream, because the bacterial population takes time to build.
Relying on one sample. One reading is an anecdote, not evidence — and examiners reward you for saying so.
Up next: Eutrophication and Its Consequences — the process that ties nitrate, algae, BOD and dead fish into one chain.
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