Three kinds of measurement: Water quality is the measurement of the chemical, physical and biological characteristics of water. Monitoring it shows where pollution comes from and whether a clean-up is working.
The points to remember
- Water quality = the chemical, physical and biological characteristics of water.
- Physical: temperature, turbidity, total suspended solids.
- Chemical: dissolved oxygen, pH, nitrates, phosphates, metals such as lead or mercury.
- Biological: the living things present, e.g. indicator species, bacteria counts.
- It varies with place, season, weather and time of day, so it is measured again and again.
- Results can be combined into one score, a water quality index; the data guide management.
| Kind | What is measured | How |
|---|---|---|
| Physical | Temperature | Thermometer or temperature probe |
| Physical | Turbidity, suspended solids | Secchi disc, turbidity tube; filter and weigh |
| Chemical | Dissolved oxygen, pH | Oxygen probe; pH probe or strips |
| Chemical | Nitrates, phosphates, metals | Test kits; laboratory spectrometry |
| Biological | Indicator species, bacteria | Kick sampling and a key; microscope counts |
Remember it as: Physical, chemical, biological: how it looks, what is in it, what lives in it.
Real example: in England the Environment Agency tests rivers at thousands of fixed sites, checking oxygen, ammonia, phosphate and the insects living there, and gives each river a status from high to bad.
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Physical measures show how warm and how cloudy the water is. High turbidity cuts the light that water plants need.
The points to remember
- Temperature: a thermometer or temperature probe, at the same depth each time.
- Turbidity: a Secchi disc lowered until it disappears (lakes), or a turbidity tube (rivers).
- Total suspended solids: filter a measured volume of water, dry the filter, weigh what it caught.
- Shallow Secchi depth, cloudy tube or heavy filter = more particles: soil, algae or sewage.
Lower the disc
A black-and-white disc on a rope marked in centimetres, from the shady side of the boat.
Watch it vanish
Note the depth at which you can no longer see it.
Repeat and average
Raise it until it reappears, note that depth, and use the mean of the two.
Real example: scientists have lowered a Secchi disc into Lake Tahoe, USA, since 1968. The depth at which it disappears tells them how clear the lake is.
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Chemical measures show what is dissolved in the water. The most important for life is dissolved oxygen, measured with a probe.
The points to remember
- Dissolved oxygen: an oxygen probe, or a chemical titration (the Winkler method).
- pH: a pH probe, or pH strips / universal indicator.
- Nitrates and phosphates: colour-change test kits compared with a chart, or a probe.
- Metals (lead, mercury): samples sent to a laboratory for spectrometry.
- BOD: measure oxygen, keep the sample dark for 5 days, measure again (taught on the next page).
- In an answer, always give what is measured AND how: 'pH, using a pH probe'.
Dissolved oxygen
- Oxygen probe or Winkler titration
- Low = fish suffocate
pH
- pH probe or strips
- Acid rain or mine water lowers it
Nitrate, phosphate
- Colour test kit or probe
- High = risk of algal blooms
What AND how: 'Test the pH' is only half an answer. 'Measure pH with a pH probe' scores. Each way of measuring is one point: the thing measured plus the tool.
Real example: after a storm, a school group on the River Wandle in London found nitrate levels rose sharply below a sewer overflow, using a simple colour test kit.
Living things show what the water has been like for weeks or months. An indicator species is the key.
The points to remember
- Indicator species: some animals live only in clean water, others tolerate pollution.
- Kick sampling: kick the riverbed for a set time, catching animals in a net held downstream.
- Identify and count them with a key or an app; note which indicator species are present.
- Turn the result into a biotic index: a score that is high for clean water, low for polluted.
- Advantage: it shows the real effect on living things, and the combined effect of all pollutants.
- Also cheap (no lab needed) and the score is easy to understand. Chemical tests name the pollutant.
Clean water
- Stonefly and mayfly larvae
- Need lots of dissolved oxygen
- High biotic index score
Polluted water
- Sludge worms and bloodworms
- Survive with little oxygen
- Low biotic index score
What does NOT score: 'Use fish' and 'measure biodiversity' alone are not a biotic-index method. 'It shows pollution over time' or 'it is quicker' are not accepted advantages: say it shows the effect on living things, or the combined effect of pollutants, or that it is cheap.
Real example: in many UK rivers, mayfly larvae disappear just below a sewage outfall and sludge worms take over, then mayflies return further downstream as the water recovers.
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A good monitoring plan answers one question: is this source harming the water? The design matters as much as the tool.
The points to remember
- Sample upstream and downstream of the source: the upstream site is the comparison.
- Sample before and after a release, or compare with historic records.
- Take samples along the river from the source, to see how far the pollution spreads.
- Sample over time (days, months, seasons) to see whether the water recovers.
- Repeat samples at each site and use the mean; keep the method the same each time.
- The data inform management: find the source, set limits, check that a clean-up works.
Upstream
- A site above the overflow pipe: the comparison
At the pipe
- Oxygen probe, nitrate kit, kick sample
Downstream
- Sites every 500 m to see how far it spreads
Over time
- Repeat monthly and after storms; compare means
Remember it as: Above and below, before and after, repeat and average.
Papers give you real measurements to read. Use the same four steps every time.
How to read a water-quality data set
- Read the units and what a high or low value means (deeper Secchi = clearer).
- Describe the trend: the overall change, with numbers and years quoted.
- Give a reason for the change, linked to a source of pollution.
- Say what the data cannot show: one measure, few years, one site.
Worked answer: Trend: the clarity fell from about 31 m in 1968 to about 18 m in 2017, then recovered a little to about 22 m in 2022. Reason: fine soil particles washed off roads and building sites, and algae fed by nutrients, make the water cloudier. Limit: three years only, one measure (turbidity), and the weather changes clarity from year to year.
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How this comes up: Paper 1: outline three ways in which water quality could be measured in a lake [3]. Each way: what, plus how. Only the first three count.
Lake Windermere, in England's Lake District, has had algal blooms in recent summers. Sewage works and farms drain into the streams that feed it.
Outline three ways in which the water quality of Lake Windermere could be measured.
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