⏳ Estimated Reading Time: 3 - 5 minutes

Identify the differences between point and nonpoint sources of pollution.

The Land-Water Connection

Earth’s systems are interconnected and nowhere is this more apparent than the land-water connection. When water on Earth becomes polluted, it often comes from the land. Most of the pollutants in water can be traced to the land.

The Mississippi River is an excellent example of a river that has a large number of pollutants from the land. Pollutants from the land, along the entire Mississippi River, flow into the river, which dumps into the Gulf of Mexico. In fact, 1/3 of the topsoil in the United States has been lost to the Gulf of Mexico.

An aerial view of a coastal area with a river flowing through it, sediment from the land flows into the ocean. — **Runoff pollution from 31 states collects and flows out into the Gulf from the Mississippi River. You can see the sediment flowing into the Gulf in this picture from NASA.**

This soil often has high levels of pesticides and fertilizers, which has caused a dead zone in the Mississippi Delta. Inorganic nutrients are necessary for plant growth but too much of a good thing (nitrogen and phosphates, again) can cause algal blooms and cultural eutrophication. These algal blooms eventually die when they run out of their limiting nutrients, which increases the numbers of bacterial and fungal decomposers, which use up the majority of the dissolved oxygen (DO) in the area, which causes hypoxic, areas of low oxygen, dead zones. We will discuss the details of eutrophication later in this module.

Point v. Nonpoint Sources

Point sources of pollution are places that directly discharge pollution. They are easy to identify and monitor. Some examples of point sources are factories, sewage, power plants, coal mines, and oil well discharge. A way to remember point sources is that they are places you can physically POINT to with your finger.

Today, most water resources have recovered from waste disposal practices of the past, but many remain polluted to a certain extent due to nonpoint source pollution. Nonpoint sources of pollution are poorly defined and scattered. Nonpoint source pollution occurs when pollutants run off from agricultural fields, urban areas, roadways, and other impervious surfaces such as parking lots where pollutants tend to collect. This makes them difficult to monitor and identify. Some other examples of nonpoint sources of pollution are storm-water drainage and atmospheric deposition.

An illustrative diagram that categorizes sources of water pollution into point and nonpoint sources. — **Many point and nonpoint sources of pollution are labeled for you. Look at the cows - is their waste point or nonpoint pollution?**

Essential Knowledge

A point source refers to a single, identifiable source of pollutant, such as a smokestack or waste discharge pipe.

Nonpoint sources of pollution are diffused and can therefore be difficult to identify, such as pesticide spraying or urban runoff.

AP Exam Tip

You should know the difference between point and nonpoint sources.

A way to remember point sources is that they are places you can physically POINT to with your finger.

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