AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/8/8-6)
Unit 8 · Topic 8.6
8.6 Thermal Pollution
Thermal pollution is heat added to a body of water, usually when power plants and factories return cooling water warmer than they took it. Warm water holds less dissolved oxygen while making aquatic animals need more, so it can stress or kill fish and other life.
Key terms
- thermal pollution
- dissolved oxygen
- thermal shock
- cooling water
Where it comes from
- Power plants and factories that pump water from a river, lake or bay to cool their equipment, then release it several degrees warmer. Nuclear and fossil fuel plants are the biggest users of cooling water.
- Urban runoff, as rain flowing over hot pavement and rooftops in summer warms up before reaching streams.
- Removing trees along streambanks, which takes away shade.
- Shallow reservoirs behind dams, where the water warms in the sun.
Why warm water is a problem
Gases dissolve better in cold water than in warm water. So as water warms, it holds less dissolved oxygen. At the same time, fish and other cold-blooded aquatic animals speed up their metabolism in warmer water, so they need more oxygen. Less oxygen available plus more oxygen needed is a double squeeze.
| Water temperature | Maximum dissolved oxygen (fresh water, sea level) |
|---|---|
| 0 °C | about 14.6 mg/L |
| 10 °C | about 11.3 mg/L |
| 20 °C | about 9.1 mg/L |
| 30 °C | about 7.6 mg/L |
A closer look at power plant cooling
Older plants often use once-through cooling: water is pulled from a river or bay, passed once through the plant's condenser, and returned several degrees warmer. Besides heating the water, the huge intake can trap fish against screens or suck in fish eggs and larvae, killing them.
Newer plants more often use closed-cycle cooling, where the same water is cooled in towers or ponds and reused. These systems take in far less water and return far less heat, though they lose some water to evaporation.
Effects on aquatic life
Thermal shock is a sudden change in temperature that organisms can't adjust to, which can kill fish quickly. It happens when hot water is released, and also when a plant suddenly shuts down and the warm water it was supplying disappears.
Longer-term warming can push species past their range of tolerance (topic 2.4). Cold-water fish like trout and salmon are especially sensitive. Warm water can also disrupt spawning, favor warm-water species and parasites, and speed up algal growth, which can add to eutrophication.
Reducing thermal pollution
Plants can release heat to the air instead of the water by using cooling towers, or hold water in cooling ponds until it cools before returning it. Other fixes include using the waste heat for something useful (cogeneration, topic 6.3), planting trees along streambanks for shade, and reducing runoff from hot urban surfaces.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
How much oxygen does warming remove?
Using the table of maximum dissolved oxygen in fresh water, calculate the percent decrease in maximum DO when a river warms from 10 °C to 30 °C because of power plant discharge.
Show the solutionHide the solution
- Step 1: Read the table: 11.3 mg/L at 10 °C and 7.6 mg/L at 30 °C.
- Step 2: Percent change = (7.6 − 11.3) ÷ 11.3 × 100 = −3.7 ÷ 11.3 × 100 ≈ −32.7%.
- Step 3: So the water can hold about a third less oxygen, even before considering the fish's higher oxygen demand.
Answer: About a 33% decrease in maximum dissolved oxygen.
- Example 2
Trap: cooling towers do not make radiation or smoke
A town worries that the new cooling towers at a power plant will release pollution. Explain what the towers release and why they reduce harm to the river.
Show the solutionHide the solution
- Step 1: Cooling towers let the hot water from the plant transfer its heat to the air, mostly by evaporating some of the water.
- Step 2: The white plume is water vapor condensing into droplets, like a cloud, not smoke.
- Step 3: Because the heat goes into the air instead of the river, the water returned to the river is much cooler, which protects its dissolved oxygen levels.
Answer: The towers release water vapor and heat to the air; that keeps the heat out of the river and reduces thermal pollution.
Common mistakes
- Saying warm water holds more oxygen. Gas solubility falls as temperature rises.
- Forgetting the second half of the problem: warmer water also raises the oxygen demand of aquatic animals.
- Thinking thermal pollution only means hot water. A sudden drop in temperature can also cause thermal shock.
On the exam
- Questions often show a temperature and dissolved oxygen table. Describe the inverse relationship and connect it to fish survival.
- A typical prompt asks for one way to reduce thermal pollution from a power plant. Cooling towers or cooling ponds are the standard answers.
Connected topics
Videos
Check yourself
4 questions on 8.6 Thermal Pollution. Pick an answer to see if you got it, and why.
| Water temperature (°C) | Maximum dissolved oxygen (mg/L) |
|---|---|
| 10 | 11.3 |
| 20 | 9.1 |
| 30 | 7.6 |
Approximate oxygen solubility in fresh water at sea level, from standard solubility tables
A power plant releases cooling water that raises a river's temperature from 20 °C to 30 °C. Based on the data, which effect is most likely?
| Location | Water temperature (°C) | Dissolved oxygen (mg/L) | Trout counted per 100 m |
|---|---|---|---|
| Upstream of plant | 16 | 9.8 | 22 |
| At outlet | 27 | 7.9 | 0 |
| 200 m downstream | 24 | 8.4 | 2 |
| 1,000 m downstream | 19 | 9.2 | 15 |
| 3,000 m downstream | 17 | 9.6 | 20 |
Hypothetical survey of a river in summer
Which statement best explains why trout are scarce near the outlet?
One winter, the plant shuts down suddenly, and the water near the outlet drops from 27 °C to 6 °C within a day. Many fish that had gathered there die. This is best described as
Which change would most directly reduce the thermal pollution from this plant?
0 of 4 answered