AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/8/8-1)
Unit 8 · Topic 8.1
8.1 Sources of Pollution
Pollution sources are sorted by whether you can point to where they come from. Point sources come from one identifiable spot, like a pipe, while nonpoint sources are spread across a landscape and carried by runoff, which makes them far harder to measure and control.
Key terms
- point source
- nonpoint source
- runoff
- pollutant
Point sources
A point source of pollution is a single, identifiable place where pollution enters the environment. If you could walk up and put your finger on the spot, it is a point source.
Examples include a factory's discharge pipe into a river, the outflow pipe of a sewage treatment plant, a power plant's smokestack, a leaking underground storage tank and a single animal feedlot's waste lagoon. Because the source is known, it can be monitored, permitted and fined. In the United States, the Clean Water Act requires permits for point sources that discharge into rivers, lakes and coastal waters, which is a big reason many point sources are much cleaner than they were in the 1970s.
Nonpoint sources
A nonpoint source is pollution that comes from many diffuse places over a large area, so there is no single spot to fix. It is usually carried by runoff, the rain or snowmelt that flows over land instead of soaking in. As the water moves, it picks up whatever is on the ground and carries it to streams, lakes and the ocean.
Examples include fertilizer and pesticides from many farm fields, oil, metals and road salt washed off streets and parking lots, pet waste and lawn chemicals from neighborhoods, sediment from construction sites and plowed land, and acid deposition falling across a whole region.
Nonpoint pollution is now the leading cause of water quality problems in the United States. It is hard to control because it comes from millions of small sources owned by different people, and it changes with every rainstorm.
Not just water
The point and nonpoint labels apply to air and land pollution too. A single smokestack or a leaking chemical storage tank is a point source. Exhaust from millions of cars spread across a city, dust blowing off plowed fields, and pesticide drifting from many farms are nonpoint sources.
Some pollution changes category as it travels. SO₂ leaves a power plant's stack (a point source), but when it falls as acid rain over an entire region, the lakes receiving it can't trace it back to one pipe. From the lake's point of view, that acid arrives as nonpoint pollution.
Controlling each type
| Feature | Point source | Nonpoint source |
|---|---|---|
| Where it comes from | One identifiable location | Many scattered locations |
| How to measure it | Sample the pipe or stack | Hard; must sample whole watersheds |
| Typical controls | Permits, treatment, emission limits, fines | Land-use practices: buffer strips, less fertilizer, permeable pavement, rain gardens |
| Example | Discharge pipe from a paper mill | Fertilizer runoff from farms across a watershed |
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Classifying sources
Classify each as a point or nonpoint source: (a) a pipe from a chemical plant into a river; (b) road salt washing off highways across a state; (c) runoff from hundreds of lawns in a suburb; (d) a single coal plant's smokestack.
Show the solutionHide the solution
- Step 1: Ask whether you can identify one specific spot where the pollutant is released.
- Step 2: (a) One pipe: point source.
- Step 3: (b) Salt washes off thousands of miles of road: nonpoint source.
- Step 4: (c) Hundreds of lawns, no single outlet: nonpoint source.
- Step 5: (d) One smokestack: point source.
Answer: (a) Point. (b) Nonpoint. (c) Nonpoint. (d) Point.
- Example 2
Trap: one farm versus many farms
A student says fertilizer runoff is always nonpoint pollution, while a large hog farm's manure lagoon that leaks into a stream is also nonpoint because it is 'agricultural.' Is the student right?
Show the solutionHide the solution
- Step 1: The type of activity doesn't decide the category; the shape of the source does.
- Step 2: Fertilizer running off many fields across a watershed has no single outlet, so it is nonpoint.
- Step 3: A specific lagoon leaking at a specific spot is identifiable, so it is a point source, even though it is on a farm.
Answer: Partly. Widespread fertilizer runoff is nonpoint, but a single leaking manure lagoon is a point source.
Common mistakes
- Classifying by industry instead of by source shape. A farm can have a point source (one leaking lagoon) and nonpoint sources (runoff from fields).
- Calling all air pollution nonpoint. A smokestack is a point source of air pollution.
- Proposing end-of-pipe treatment for nonpoint pollution. There is no pipe; solutions must change how land is used.
On the exam
- You will often be asked to identify a source as point or nonpoint and explain why. Use the words 'single, identifiable location' or 'many diffuse sources carried by runoff.'
- When asked to reduce nonpoint pollution, give a land-based practice such as vegetated buffer strips or reduced fertilizer use.
Connected topics
Videos
Check yourself
4 questions on 8.1 Sources of Pollution. Pick an answer to see if you got it, and why.
Which of the following is a point source of water pollution?
Nonpoint source pollution is generally harder to regulate than point source pollution because it
| Site | What lies just upstream of the site | Nitrate load (kg per day) |
|---|---|---|
| 1 | Forested headwaters | 50 |
| 2 | 30 km of cropland | 900 |
| 3 | City wastewater treatment plant outfall | 1,050 |
| 4 | 20 km more of cropland | 1,600 |
Hypothetical monitoring data from one river
Between which two sites was the most nitrate added to the river, and what kind of source was it?
If the city's treatment plant removed all of the nitrate from its discharge, by about what percentage would the nitrate load at Site 4 fall?
0 of 4 answered