E. Earth and Space Systems · Water Systems

Measuring Water Quality: Physical, Chemical and Biological Indicators

E2.6 A1.2 A1.4 A3.1About 50 minutes

Learning goals

I can describe physical, chemical and biological indicators of water quality.
I can explain what each indicator tells us about the health of a lake or stream.
I can interpret water-quality data and draw a conclusion.
I can follow safety procedures when testing water.

Words to know

water quality
How suitable water is for a use, such as drinking, swimming or supporting fish and other living things.

indicator
Something we can observe or measure that tells us about the condition of the water.

turbidity
How cloudy water is because of particles such as soil, algae or other small bits floating in it.

pH
A scale from 0 to 14 that measures how acidic or basic (alkaline) a solution is; 7 is neutral.

dissolved oxygen (DO)
Oxygen gas dissolved in water, which fish and other aquatic animals breathe. Measured in milligrams per litre (mg/L).

nutrients
Substances such as phosphorus and nitrogen that help plants and algae grow; too much causes algal blooms.

E. coli
A type of bacteria that lives in the intestines of people and warm-blooded animals; finding it in water means faecal (poop) contamination.

benthic macroinvertebrates
Small animals without backbones, big enough to see, that live on the bottom of streams and lakes, such as insect nymphs, snails and worms.

Remember from Grade 7

In Grade 7 you learned that water is called the universal solvent because so many substances dissolve in it, and you built a water filter. That is exactly why water quality matters: water easily picks up whatever it touches. Scientists check it using indicators.

Clear doesn't mean clean

A stream can look crystal clear and still contain bacteria or dissolved road salt. A brownish wetland can be perfectly healthy, coloured by natural tannins from leaves (like tea). To know if water is healthy, you have to measure.

A forest stream with naturally tea-coloured water over fallen leaves, and a glass jar of amber water on a mossy rock. — This brownish water is coloured by natural tannins from leaves. Colour alone can't tell you if water is healthy; you have to measure. AI-generated image

Type	Indicator	What it tells us	Healthy example
Physical	Temperature	Warm water holds less oxygen; cold-water fish like trout need cool water	Cool, shaded streams for trout
Physical	Turbidity (cloudiness)	Soil from erosion, algae or other particles; blocks light and can clog fish gills	Clear enough to see deep down
Physical	Colour and odour	Can signal pollution, algae or natural tannins	No oily sheen or sewage smell
Chemical	pH	Acidic or basic; most aquatic life does best between about 6.5 and 9	Around 6.5 to 9
Chemical	Dissolved oxygen	Whether fish and insects can breathe; low DO can cause fish kills	High DO in cold, flowing water
Chemical	Nutrients (phosphorus, nitrate)	Fertilizer, manure or sewage; too much feeds algal blooms	Low levels
Chemical	Chloride (salt) and conductivity	Road salt and other dissolved salts	Low levels
Biological	E. coli bacteria	Contamination from sewage or animal waste; risk of illness	None in drinking water; low at beaches
Biological	Benthic macroinvertebrates	Long-term health of a stream	Many mayfly, stonefly and caddisfly nymphs
Biological	Algae	Too many nutrients if there are large blooms	No large green scum

Common water-quality indicators.

Some important numbers

Drinking water in Ontario must have no detectable E. coli in a 100 mL sample.
For swimming, Health Canada's guideline is a geometric average of no more than 200 E. coli per 100 mL of water across several samples. Health units post warning signs at beaches that test above their limits, often after heavy rain.
pH: Canadian guidelines for protecting aquatic life suggest keeping pH between about 6.5 and 9.0.
Chloride: the Canadian guideline for protecting aquatic life from long-term exposure is 120 mg/L. Many urban streams in southern Ontario go above this in winter and spring because of road salt.
Dissolved oxygen: cold water holds more oxygen than warm water. Cold-water fish, such as brook trout, need more dissolved oxygen than warm-water fish, such as carp.

The pH scale runs from 0 (most acidic) to 14 (most basic), and 7 is neutral. The outlined box shows the range of about 6.5 to 9 where most aquatic life does best; a stream at pH 4.5 is far too acidic.

Living things as indicators

Benthic macroinvertebrates (bottom-dwelling insect nymphs, snails, worms and crayfish) are great indicators because they live in one spot for months and can't escape pollution.

Sensitive (need clean, oxygen-rich water): mayfly, stonefly and caddisfly nymphs. Scientists call them EPT after their scientific group names.
Somewhat tolerant: dragonfly nymphs, crayfish, scuds.
Tolerant (can live in polluted water): midge larvae, aquatic worms, leeches.

A stream with lots of EPT nymphs is usually healthy. A stream with only worms and midges is probably polluted. Groups such as the Ontario Benthos Biomonitoring Network help schools and volunteers sample streams in a standard way.

Just for fun

What do you call a stonefly nymph that moves into a polluted stream?

Nothing. It would never move there. It has standards.

Indicator	Upstream of town	Downstream of town
Temperature	9 °C	13 °C
Dissolved oxygen	11 mg/L	6 mg/L
Chloride	30 mg/L	340 mg/L
Mayfly, stonefly, caddisfly nymphs	many	none
Midge larvae and worms	few	many

Class stream data from April (sample data made up for this example).

Chloride from the class data in the table above. Upstream is well below the long-term guideline (dashed line); downstream is almost three times the guideline.

Worked example

Let's interpret stream data

A class tested a creek above and below a town in April (see the table just above). What do the results suggest?

Temperature is higher downstream: warm runoff from pavement and less shade can warm a stream.
Dissolved oxygen is much lower downstream, which fits the warmer water (warm water holds less oxygen) and possibly decomposing waste.
Chloride jumped from 30 to 340 mg/L, well above the long-term guideline of 120 mg/L. In April, the most likely source is road salt washing in with snowmelt.
Biological indicators agree: sensitive EPT nymphs disappear and tolerant species increase.
Conclusion: water quality is worse below the town. Several indicators point the same way, which makes the conclusion stronger. To be more certain, the class should test again in other seasons.

Water quality drops downstream of the town, likely because of road salt and warm runoff. The biological indicators back up the chemical and physical ones.

Hands-on activity

Test water from home (A1.2, A1.4)

You need: 3 clear jars or glasses, tap water, rainwater or melted snow, water with a spoonful of garden soil stirred in, a white plate or paper, a coin, a ruler, pH test strips (from a pharmacy or pool supply store, if available) or red cabbage indicator (see below), labels, and rubber gloves if you collect water outdoors.

Turbidity tube: put the coin under each jar on white paper. Pour water in slowly until you can no longer see the coin clearly. Measure the water depth. The deeper you can still see the coin, the clearer the water.

pH: dip a test strip in each sample and compare with the colour chart. Or make red cabbage indicator: a grown-up chops a few cabbage leaves and pours hot tap water over them, lets it cool for 30 minutes, and strains it. Add a few spoonfuls of the purple liquid to each sample. Pink-red means acidic, purple about neutral, blue-green basic. Test lemon juice and baking-soda water too, for comparison.

Record your results in a table and rank the samples from clearest to cloudiest. Which indicators can't you test at home? (Bacteria, dissolved oxygen, nutrients: they need lab equipment.)

Safety (A1.4): label every jar. Never drink or taste samples. If you collect water from a creek or pond, go with an adult, stay back from steep banks and fast or deep water, wear gloves, and wash your hands with soap afterwards. Never collect water from ice-covered ponds or after a heavy storm.

Careers connection (A3.1)

Water quality technicians, environmental scientists, public health inspectors, laboratory technologists and water treatment operators measure these indicators every day. Ontario's drinking water system operators must be certified and must keep taking and testing samples.

Your turn

Name one physical, one chemical and one biological indicator of water quality.

Why does warm water often have less dissolved oxygen?

What does finding E. coli in water tell us?

Why are mayfly, stonefly and caddisfly nymphs useful indicators?

What is turbidity, and what can cause high turbidity?

Why should you never judge water quality just by looking at it?

What pH range is generally healthy for aquatic life?

More practice: warm-up

What does pH 7 mean?

Name a tolerant macroinvertebrate.

What does DO stand for?

How much E. coli is allowed in Ontario drinking water?

What is the main source of chloride in Ontario streams in winter?

More practice: on your own

Why do beaches often get posted as unsafe for swimming right after a heavy rainstorm?

A stream has pH 4.5. What might this mean for aquatic life?

A sample has 650 mg/L of chloride. How many times the long-term guideline of 120 mg/L is that?

Why do scientists use several indicators instead of just one?

A creek sample has many stonefly nymphs but slightly high turbidity after rain. Is the stream likely healthy? Explain.

Challenge

Explain why biological indicators can reveal a pollution event that chemical tests miss.

Design a fair investigation to find out whether a parking lot is affecting a creek. Include where and when you would sample and which indicators you would measure.

Lake water at a beach tested 120, 180, 260, 150 and 210 E. coli per 100 mL on five days. Find the ordinary mean. Should you worry? (Note: health units use a slightly different average called a geometric mean, which gives about 178 here.)

Check your understanding

Quiz

Which is a BIOLOGICAL indicator of water quality?

Finding E. coli in water means...

Turbidity measures...

Which stream is MOST likely to be polluted?

Why does warm water hold less dissolved oxygen?

Most aquatic life does best in a pH range of about...

For parents and teachers

Big idea (E2.6, part 1): water quality is measured with physical (temperature, turbidity), chemical (pH, dissolved oxygen, nutrients, chloride) and biological (E. coli, benthic macroinvertebrates, algae) indicators, and stronger conclusions come from several indicators agreeing. Home tests practise investigation (A1.2) and safety (A1.4); careers tip links to A3.1.

How to help: check your local health unit's beach water testing page together in summer. If you can, join a community stream monitoring event with a conservation authority or naturalist club.

Common difficulties:

"Clear water is clean water." Bacteria and dissolved salt are invisible.
Mixing up the three types of indicators.
Reading data tables: compare upstream and downstream row by row.

Safety: red cabbage indicator needs hot water: an adult prepares it. Natural water samples are never tasted; collect only with an adult, away from steep banks, fast or deep water and ice.

Sources: Ontario drinking water standard (E. coli not detectable per 100 mL); Health Canada Guidelines for Canadian Recreational Water Quality (geometric mean of 200 E. coli per 100 mL); CCME Canadian Water Quality Guidelines for the Protection of Aquatic Life (pH 6.5 to 9.0; chloride 120 mg/L long-term).