Sources of Pollution — AP® Environmental Science Notes

Before you can fix a pollution problem, you have to know where it comes from. Environmental scientists sort sources into two kinds, and the kind decides what solutions can work.

Point and nonpoint sources

A river flowing through a landscape. Point sources (a factory pipe, a sewage plant outfall and a smokestack) each send pollution in at one marked spot. Nonpoint sources (runoff arrows from crop fields, a suburb with lawns, a city street and a construction site) wash in along the whole length of the river. — **Figure 1.** Point sources enter at single spots; nonpoint sources wash in along the whole river. LevlPrep original diagram.

	Point source	Nonpoint source
Where it enters	one identifiable place	many diffuse places
Water examples	factory discharge pipe, sewage plant outfall, oil tanker spill	runoff from farm fields, lawns, streets, parking lots, construction sites
Air examples	smokestack, a single vent	exhaust from all the cars in a city, dust from plowed fields
Easy to measure?	yes, at the outlet	no, it arrives everywhere at once
Typical fix	permit limits, treatment at the outlet	changes in land use and practice

Why the difference matters

A point source can be found on a map, sampled at the end of a pipe, given a legal limit and inspected. The Clean Water Act (1972) did exactly that with permits for every pipe that discharges into United States waters, and point-source pollution fell sharply. A nonpoint source has no pipe. Fertilizer, sediment, oil, metals, pesticides and pet waste are picked up by rain across a whole watershed and enter streams along their entire length. Because thousands of landowners each add a little, nonpoint pollution is hard to measure and to assign. It is now the leading cause of water quality problems in the United States.

Worked example: what share is nonpoint? A river receives 2,600 metric tons of nitrogen a year from crop runoff, 700 from streets and lawns, 500 from pasture manure, 900 from a sewage plant and 300 from a food plant.

Nonpoint: $2{,}600 + 700 + 500$ $= 3{,}800$ metric tons. Total: $5{,}000$ metric tons. Setup: $\frac{3{,}800\,\text{t}}{5{,}000\,\text{t}} \times 100$ . Answer: $= 76\%$ .

So a permit on the sewage plant could cut at most 18% of the nitrogen. Cleaning up this river means changing what happens on the land.

Reducing nonpoint pollution

On farms: buffer strips of grass and trees along streams, cover crops, fertilizer applied only as needed, fences that keep livestock out of streams.
In towns: rain gardens, green roofs and permeable pavement so rain soaks in; street sweeping; pet waste rules.
On construction sites: silt fences and quick replanting to hold soil in place.

When a question asks for a solution, check which kind of source the scenario describes. A treatment plant upgrade fixes a pipe; it will not touch runoff.

Reading a storm graph

Nonpoint pollution often shows up as a spike after rain. Before a storm, a farm stream may carry 2 mg of nitrate per liter. During and just after the storm, runoff washes fertilizer off the fields and the level can climb to 9 mg per liter, then fall back over a few days as the runoff passes. A point source, by contrast, tends to release a steady flow whether it rains or not. So when a question shows pollution that rises with every storm, think nonpoint, and think of solutions that keep the pollutant on the land: plants that take it up, soil that soaks the water in, and fertilizer applied when rain is not forecast.