Grade by Grade
E. Earth and Space Systems · Water Systems

How Human Activities Change Water Quality

E2.6 A1.1 A1.5 A3.2About 55 minutes
Learning goals
  • I can explain how human activities change water-quality indicators.
  • I can tell the difference between point-source and non-point-source pollution.
  • I can explain eutrophication and why algal blooms lower dissolved oxygen.
  • I can describe actions that improve water quality and research a local water-quality issue.

Words to know

point-source pollution
Pollution that enters water from one identifiable place, such as a factory pipe or a sewage outfall.
non-point-source pollution
Pollution that comes from many spread-out places, such as runoff from farms, lawns and roads.
eutrophication
The process where too many nutrients cause algae and plants to overgrow in a lake or river, which can lead to low oxygen.
algal bloom
A rapid overgrowth of algae (or cyanobacteria) that can turn water green and, in some cases, release toxins.
thermal pollution
Warming of water by human activities, such as hot runoff from pavement or warm water from industry.
sediment
Soil, sand and silt carried by water.
combined sewer overflow
In some older cities, sewage and stormwater share pipes; heavy rain can make them overflow into rivers and lakes.
buffer strip
A strip of trees, shrubs or grasses left along a stream to filter runoff and shade the water.

Remember from the last lesson

You learned the indicators of water quality. Now we ask: what changes them? For each human activity, think: which indicator would go up or down, and why?

A lake turns green

In August 2014, a bloom of toxic cyanobacteria (sometimes called blue-green algae) in western Lake Erie reached the water intake for Toledo, Ohio. Hundreds of thousands of people were told not to drink their tap water for about two days. Lake Erie's algal blooms are fed mainly by phosphorus washing off farm fields and from other sources across its watershed, which includes parts of southwestern Ontario.

A thick bright green algae bloom covers a lake near a rocky shore, with farm fields in the distance.
Algal blooms like this are fed by phosphorus, often washing off farm fields and lawns: a non-point source of pollution. AI-generated image

Point and non-point sources

  • Point source: one pipe or place you can point to: a factory outfall, a sewage treatment plant discharge, a leaking fuel tank. These are easier to find, regulate and fix.
  • Non-point source: many small sources spread over a whole watershed: fertilizer and manure from fields, lawn chemicals, road salt, oil dripping from cars, pet waste, soil from construction sites. Each is small, but together they add up. This is now the biggest challenge for many lakes and rivers.
Point sourceNon-point sourcesriverfactoryone outfall pipeyou can point toriverfarm fieldlawnroad (salt, oil)
A point source comes from one place, like a single pipe. Non-point sources are many small, spread-out sources (fields, lawns, roads) that each add a little pollution every time it rains.
Human activityIndicator changedHow
Fertilizer, manure, sewageNutrients ↑, algae ↑, dissolved oxygen ↓Eutrophication (see below)
Road saltChloride ↑, conductivity ↑Salt dissolves in meltwater and runs into streams and groundwater; harms sensitive aquatic life
Paving, removing shade trees, industrial cooling waterTemperature ↑, dissolved oxygen ↓Thermal pollution: warm runoff and less shade
Construction, ploughing, bare soilTurbidity ↑, sediment ↑Erosion washes soil into streams; it smothers fish eggs and insect habitat
Sewer overflows, leaky septic systems, animal wasteE. coli ↑Faecal bacteria reach beaches and streams, especially after heavy rain
Burning coal, smelting (past and present)pH ↓ (more acidic)Sulfur and nitrogen gases form acid rain
Plastics, litter, microbeads, synthetic fibresMicroplastics ↑Tiny plastic bits build up in water and are eaten by animals
Oil, chemicals, medicines poured down drainsToxic substances ↑Not all are removed by treatment plants
↑ means increases; ↓ means decreases.

Eutrophication step by step

  1. Extra nutrients (phosphorus and nitrogen) wash into a lake.
  2. Algae grow very fast: an algal bloom. The water turns green and cloudy (turbidity ↑).
  3. The algae die and sink.
  4. Decomposers (bacteria) break down the dead algae and use up dissolved oxygen.
  5. Oxygen drops, especially near the bottom. Fish and other animals may die (a fish kill), and "dead zones" can form.
  6. Some blooms (cyanobacteria) also release toxins that are dangerous for people and pets.

Scientists at the Experimental Lakes Area in northwestern Ontario proved, with whole-lake experiments in the 1970s, that phosphorus is the key nutrient behind algal blooms (you'll learn more in the lesson on discoveries and innovations).

1nutrientswash in2algaebloom3algae dieand sink4decomposersuse up oxygen5fish and otheranimals sufferPNgreen, cloudyoxygen ↓low oxygen
Eutrophication in five steps: extra phosphorus (P) and nitrogen (N) feed an algal bloom; when the algae die, decomposers use up the dissolved oxygen that fish and other animals need.

Good news: people can improve water quality

  • Sudbury's lakes. Decades of smelter emissions made acid rain that damaged many lakes near Sudbury. After big cuts in sulfur dioxide emissions starting in the 1970s, plus new smelter technology and community re-greening, many lakes have become less acidic, and fish and other life have come back.
  • Phosphorus limits on laundry detergents (early 1970s in Canada) and better sewage treatment helped Lake Erie recover in the 1970s and 1980s. Blooms have returned since the 2000s, mainly from farm runoff, so Canada and the U.S. agreed to new phosphorus reduction targets.
  • Microbeads in toiletries such as face scrubs and toothpaste have been banned in Canada since 2018.
  • Buffer strips of trees and plants along streams filter runoff, hold soil and shade the water.
  • Smarter salting: using brine before storms, calibrated spreaders and "only as much as needed" cuts salt use.
  • Rain gardens, rain barrels, permeable pavement and stormwater ponds slow and clean runoff.
  • At home: never pour paint, oil, chemicals or medicines down drains or storm sewers; pick up pet waste; use less fertilizer and salt.
Just for fun

Algae at a fertilizer buffet are like guests at an all-you-can-eat pizza place who never stop: they grow and grow until the food runs out, then leave behind a mess that uses up all the oxygen in the room.

Worked example

Let's build a cause-and-effect chain

A new subdivision is built beside a trout stream. Trees along the bank are cut, and the area is paved. Predict how three indicators will change, and explain.

  1. Temperature ↑: without shade trees, the sun warms the stream, and hot runoff from pavement flows in.
  2. Dissolved oxygen ↓: warmer water holds less oxygen.
  3. Turbidity ↑: during construction, bare soil washes into the stream.
  4. Chloride ↑ (in winter): road and driveway salt runs off the new pavement.
  5. Biological effect: trout and sensitive EPT nymphs may decline, while tolerant species increase.
  6. Solutions: keep a buffer strip of trees, use silt fences during construction, build a stormwater pond or rain gardens, and limit salt.

Temperature and turbidity rise, dissolved oxygen falls, and in winter chloride rises. Sensitive species like trout suffer unless the developer uses buffers, silt fences and stormwater controls.

Hands-on activity

Two jars, two weeks: modelling eutrophication (A1.2), plus a research task (A1.1, A1.5)

Part A: experiment. You need: 2 identical clear jars, water from a pond (collected by an adult) or an aquarium (or tap water left out overnight with a pinch of garden soil), a small amount of houseplant fertilizer, labels, a sunny windowsill.

  1. Fill both jars with the same water. Label them A (control) and B (fertilizer).
  2. A grown-up adds a small pinch of fertilizer to jar B only.
  3. Place both on the same sunny windowsill. Observe every 2 days for 2 weeks. Record colour, cloudiness and smell. Take photos if you can.
  4. Conclusion: which jar turned greener? How is this like Lake Erie?

Part B: research a local issue. Choose one: your local beach's E. coli postings, your conservation authority's watershed report card (most publish one every few years), road salt in your area, or a local algal bloom. Use at least two reliable sources (government, conservation authority, university). Write a one-page fact sheet: the problem, the evidence (indicators), the causes, and two solutions.

What to look for: jar B usually turns greener and cloudier. The fact sheet should name sources and use indicator vocabulary.

Safety: an adult handles fertilizer and keeps it away from younger children and pets; label the jars "not for drinking"; wash hands; afterwards, pour the water onto garden soil, not down the drain.

Your turn

1

What is the difference between point-source and non-point-source pollution? Give an example of each.

2

List the steps of eutrophication.

3

How does road salt affect water quality?

4

What is thermal pollution? Give one cause.

5

Why do beaches sometimes close after heavy rain?

6

Name three ways people can improve water quality.

7

Which nutrient was found to be the key cause of algal blooms in lakes?

More practice: warm-up

1

What is an algal bloom?

2

What causes sediment pollution?

3

Since what year have microbeads in toiletries been banned in Canada?

4

What is a buffer strip?

5

Is a sewage plant pipe a point or non-point source?

More practice: on your own

1

Why is non-point-source pollution harder to control than point-source pollution?

2

Explain why an algal bloom can make dissolved oxygen drop even though algae produce oxygen during photosynthesis.

3

How did Sudbury's lakes recover from acid rain?

4

A stream's chloride level is highest in March. Explain why.

5

Give two reasons Lake Erie is especially prone to algal blooms. (Hint: think about depth and land use.)

Challenge

1

A farmer says, "I can't stop runoff; rain is rain." Suggest three practical actions and explain how each protects the water.

2

Explain how one action (cutting all the trees along a stream) can change at least four indicators.

3

In your eutrophication model, what is the independent variable, the dependent variable and one limitation of the model?

Check your understanding

Quiz

1.

Which is a NON-point source of pollution?

2.

During eutrophication, why does dissolved oxygen drop?

3.

Which indicator rises most because of road salt?

4.

Cutting shade trees along a stream most directly causes...

5.

Which nutrient did Experimental Lakes Area research show was key to algal blooms?

6.

Which action would BEST reduce E. coli at a city beach?

For parents and teachers

Big idea (E2.6, part 2): human activities change water-quality indicators: nutrients drive eutrophication and low oxygen; salt raises chloride; pavement and tree removal cause thermal pollution; erosion raises turbidity; sewage raises E. coli; emissions cause acid rain. Point sources are easier to regulate than non-point sources. Solutions work (Sudbury, early Lake Erie recovery, microbead ban). The research task practises A1.1 and A1.5; links to geography and health (A3.2).

How to help: look up your conservation authority's watershed report card together. Talk about what goes down your drains and storm drains.

Common difficulties:

  • Thinking algae add oxygen, so blooms are good. Decomposition of dead algae uses more.
  • Thinking pollution is mostly from factories. Today much of it comes from everyday runoff.

Safety: adults handle fertilizer; jars labelled and kept away from small children and pets.

Sources: Toledo 2014 drinking water advisory (widely reported; about two days); Experimental Lakes Area (IISD); Canada's Microbeads in Toiletries Regulations (2018); Great Lakes Water Quality Agreement phosphorus targets for Lake Erie.