AP Environmental Science

## Learning Outcomes

- Define **nonpoint source pollution** and distinguish it from **point source pollution**
- Explain how rainfall and snowmelt create polluted runoff across a **watershed**
- Identify major nonpoint pollutants from agriculture, cities, construction, and atmospheric deposition
- Trace cause-and-effect chains from nutrient runoff to **cultural eutrophication**, hypoxia, and fish kills
- Explain why **impervious surfaces** increase runoff and worsen water quality
- Propose **best management practices (BMPs)** that reduce nonpoint pollution in agricultural and urban settings
- Apply these ideas to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand nonpoint source pollution, with a focus on the following syllabus points:

- Know that **nonpoint source pollution** comes from many diffuse sources rather than one identifiable discharge point
- Identify the difference between a **point source** and a **nonpoint source**, including the idea of a single discharge versus broad runoff
- Explain how **runoff**, **stormwater runoff**, and **atmospheric deposition** move pollutants into surface water and groundwater
- Understand common nonpoint pollutants, including nutrients, sediment, pathogens, pesticides, oil, heavy metals, salts, and trash
- Explain how nutrient runoff causes **cultural eutrophication**, algal blooms, low dissolved oxygen, and fish kills
- Describe how **impervious surfaces** increase runoff, decrease infiltration, and reduce groundwater recharge
- Identify practical **BMPs** such as riparian buffers, no-till farming, cover crops, permeable pavement, rain gardens, and retention ponds
- Apply **watershed thinking** by linking upstream land use to downstream water quality

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between point source pollution and nonpoint source pollution?
2. How does rainfall turn fertilizer on a farm field into water pollution?
3. Why do parking lots and roads increase nonpoint source pollution?
4. Name two pollutants commonly carried by urban runoff.
5. How can a riparian buffer reduce nonpoint source pollution?

## Introduction

Nonpoint source pollution is one of the most tested ideas in AP Environmental Science because it links land use, the water cycle, habitatss, and pollution control. Students often know that it involves “runoff,” but lose marks because they stop at the definition and do not explain the transport process, the pollutant types, or the ecological effects.

This topic matters because many major water-quality problems do not begin at one factory pipe. They begin across a watershed. Rain falls on farm fields, lawns, roads, and construction sites, picks up pollutants, and carries them into streams, lakes, wetlands, estuaries, coastal waters, and groundwater. To answer AP questions well, you need to think from source to transport to impact to solution.

> **Key Term: nonpoint source pollution**
> Pollution that comes from many diffuse sources rather than one single identifiable discharge point, usually carried by rainfall or snowmelt runoff over land.

### What nonpoint source pollution is and how it differs from point source pollution

The simplest distinction is this: **point source pollution** comes from one identifiable location, while **nonpoint source pollution** comes from many spread-out locations.

A sewage outfall pipe emptying into a river is a point source. A whole suburban neighborhood contributing lawn fertilizer, pet waste, oil drips, and trash during a storm is a nonpoint source. The first has a specific outlet. The second is distributed across the land.

> **Key Term: point source pollution**
> Pollution released from a single identifiable source, such as a discharge pipe or outfall.

Point sources are usually easier to monitor and regulate. You can sample upstream and downstream of the pipe, measure what is entering the water, and require permits or penalties. Nonpoint sources are harder because each individual source may be small, but the total effect can be large.

This is why students should avoid saying “nonpoint means less serious.” It often means the opposite. Individual contributions may be small, but the cumulative load can be severe enough to create dead zones, unsafe swimming water, and contaminated groundwater.

A common AP shortcut is:

- **pipe or outfall = usually point source**
- **broad runoff across land = usually nonpoint source**

But there is one caveat. Stormwater is not automatically nonpoint. If diffuse runoff is collected into a storm drain, ditch, or pipe and then discharged through a discrete outlet, it may be treated as a regulated point source.

> **Key Term: runoff**
> Water that flows over land instead of infiltrating into the soil, often carrying pollutants with it.
>
> **Test Tip:** On AP questions, first ask: “Is this pollution coming from one discharge point or from many areas across land?” That usually tells you whether the answer is point or nonpoint.

### How polluted runoff forms across a watershed

To understand nonpoint source pollution, start with the water cycle. Rainfall or snowmelt moves over and through land. As it moves, it picks up both natural materials and human-made pollutants. It then carries them into nearby water bodies.

The key word is **transport**. Pollution on land is not automatically water pollution. It becomes water pollution when water moves it.

> **Key Term: watershed**
> An area of land that drains to a particular stream, river, lake, wetland, or other body of water.

A watershed approach matters because what happens upstream affects downstream water quality. If fertilizer is spread on land near a headwater stream, the nutrients do not stay there. They can move through the drainage system and affect lakes, estuaries, and coastal waters farther away.

For example, imagine a storm over a farm watershed:

1. Fertilizer and manure are left on fields
2. Rain falls faster than the soil can absorb all the water
3. Water moves as surface runoff
4. Runoff carries nitrogen, phosphorus, soil particles, and pathogens
5. These enter ditches, streams, or ponds
6. The pollutants continue downstream
7. Water quality declines, often far from the original field

The same logic works in cities:

1. Oil drips from cars, road salt, metals from brake pads, and trash collect on roads and parking lots
2. Rain washes these pollutants off hard surfaces
3. Runoff moves rapidly into drains and streams
4. Receiving waters become more polluted and often warmer

Atmospheric deposition can also contribute. Pollutants released into the air, such as mercury or nitrogen compounds, may later return to land or water in rain or dry particles. Once deposited, they can enter runoff or directly affect water bodies.

> **Key Term: atmospheric deposition**
> Pollutants that fall from the atmosphere to land or water in wet form, such as rain, or dry particles.

### Major sources and pollutant types

Students do better when they connect each land use to specific pollutants rather than giving a generic list.

#### Agriculture

Agriculture is one of the most significant nonpoint sources of water pollution.

Common pollutants include:

- **nutrients** from fertilizers and manure, especially nitrogen and phosphorus
- **sediment** from soil erosion
- **pathogens** from animal waste
- **pesticides and herbicides**
- sometimes **salts** from irrigation-related processes

Why agriculture produces so much nonpoint pollution:

- large land area
- repeated fertilizer use
- exposed soil
- manure storage or spreading
- drainage channels that move runoff quickly

Students often forget that phosphorus behaves differently from nitrate. Nitrates are highly water soluble and often move readily in runoff or leach toward groundwater. Phosphates are less soluble and often attach to soil particles, so erosion can carry phosphorus into water.

#### Urban and suburban runoff

Cities and suburbs contribute a different pollutant mix:

- oil and grease
- heavy metals such as lead, copper, or zinc
- road salt
- trash and plastic debris
- pet waste and pathogens
- lawn fertilizers and pesticides
- sediment from construction sites

Urban runoff often moves faster than agricultural runoff because of hard surfaces.

> **Key Term: impervious surface**
> A hard surface, such as asphalt, concrete, or rooftops, that prevents infiltration and increases runoff.

Impervious surfaces worsen pollution in three ways:

- they reduce infiltration, so more water becomes runoff
- they speed delivery of pollutants to streams
- they reduce groundwater recharge

That means a paved watershed tends to produce flashier, more polluted stormwater than a vegetated watershed.

#### Construction, forestry, mining, and other land uses

Construction sites often expose bare soil. Even if the site has no fertilizer or manure, it can still be a major nonpoint source because rainfall moves sediment into water.

Forestry and logging can increase sediment runoff and, if streamside trees are removed, can raise stream temperature by reducing shade.

Mining can contribute sediment, acids, and heavy metals. Even when the original pollution source is on land, rainfall and drainage can spread it as nonpoint pollution.

Septic failures, marinas, and household activities also matter. A leaking septic system may introduce nutrients and pathogens into groundwater or nearby streams. Boating areas may contribute fuel, oil, or sewage.

> **Test Tip:** In FRQs, match the source to the pollutant. Farm field = nutrients/sediment/manure. Parking lot = oil/metals/trash. Construction site = sediment. Lawn runoff = fertilizer/pesticides.

### Why nonpoint pollution is so damaging to aquatic systems

Nonpoint source pollution is not just a transport issue. It changes aquatic freshwater habitatss in predictable ways, and AP questions often test those chains.

#### Nutrients and cultural eutrophication

The most important nutrient story is excess **nitrogen** and **phosphorus** entering water.

> **Key Term: cultural eutrophication**
> Human-caused nutrient enrichment of water that stimulates excessive algal or plant growth.

The chain usually works like this:

1. Fertilizer or manure runoff increases nitrogen and phosphorus
2. Algae and phytoplankton grow rapidly
3. An algal bloom forms
4. Algae die and become organic matter
5. Decomposers break down the dead material
6. Decomposers use dissolved oxygen
7. Dissolved oxygen falls
8. Hypoxia can develop
9. Fish and other aerobic organisms may die

This sequence is one of the highest-yield AP explanations in Unit 8.

> **Key Term: eutrophication**
> Nutrient enrichment of water that increases algal or plant growth and can lower dissolved oxygen.

You should also know the direct effects before fish kills even begin:

- more turbidity
- less light reaching submerged plants
- changes in species composition
- increased biological oxygen demand

#### Sediment and turbidity

Sediment is often treated by students as “just dirt,” but it is a major pollutant.

Its effects include:

- increased turbidity
- reduced light penetration
- smothering of eggs and benthic organisms
- burial of habitat
- transport of attached pollutants, especially phosphorus and some pesticides
  > **Key Term: turbidity**
  > The cloudiness of water caused by suspended particles.

So if a question gives a construction site or eroded farm field, do not jump straight to eutrophication unless nutrients are mentioned. Sediment's main pathway is usually turbidity and habitat burial.

#### Pathogens, toxins, and groundwater contamination

Animal waste, septic leaks, and sewage contamination can introduce pathogens, making water unsafe for recreation or drinking. Pesticides and heavy metals can harm aquatic organisms directly and may bioaccumulate.

Groundwater is especially vulnerable because it does not flush quickly. Once pollutants infiltrate, cleanup is slow and difficult.

### How to reduce nonpoint source pollution

Because nonpoint pollution comes from many places, control usually focuses on land management and slowing runoff before it reaches water.

> **Key Term: best management practices (BMPs)**
> Practical methods used to reduce runoff, erosion, and pollutant movement from land into water.

#### Agricultural BMPs

Common agricultural BMPs include:

- **no-till farming** to reduce soil disturbance and erosion
- **cover crops** to hold soil and take up leftover nutrients
- **contour plowing** to slow runoff on slopes
- careful **fertilizer timing and application rates**
- **manure management**
- **riparian buffers** along streams
  > **Key Term: riparian buffer**
  > A vegetated strip along a stream or river that traps sediment, absorbs nutrients, and slows runoff.

A riparian buffer works because vegetation slows flowing water, increases infiltration, and captures soil particles before they enter the stream. It also can cool streams by providing shade.

#### Urban BMPs and green infrastructure

Cities often use:

- **permeable pavement**
- **rain gardens**
- **bioswales**
- **detention basins**
- **retention ponds**
- **constructed wetlands**
- **green roofs**
- street cleaning and better waste control

Permeable pavement reduces runoff by allowing water to infiltrate instead of running straight into drains. Rain gardens and bioswales capture and filter stormwater. Detention and retention systems slow water movement and allow sediment to settle.

Students often confuse detention and retention:

- **detention basin** = temporary storage
- **retention pond** = long-term or permanent storage

Wetlands also help naturally. They trap sediment, filter pollutants, slow water, and support infiltration.

#### Watershed-based planning

The broadest solution is to manage the whole watershed rather than one site at a time. Since pollution travels downstream, isolated fixes may help locally but fail overall.

This is why nonpoint control often uses multiple BMPs across farms, neighborhoods, roads, and stream corridors. One rain garden alone does little; many linked practices across a watershed can greatly reduce polluted runoff.

### How to use this topic in AP free-response answers

A strong AP answer does four things:

1. identifies the source
2. names the pollutant
3. explains transport by runoff or infiltration
4. states the ecological effect

For example:

- “Fertilizer spread on a farm field is washed by rainfall into nearby streams as nonpoint source runoff. The added nitrates and phosphates stimulate algal growth, which leads to eutrophication and lower dissolved oxygen after decomposition.”

That earns more than simply writing “runoff causes pollution.”

If the question asks for a solution, tie the BMP directly to the mechanism:

- “A riparian buffer would reduce nonpoint source pollution by slowing runoff, trapping sediment, and absorbing excess nutrients before they reach the stream.”

That is much stronger than “plant vegetation near the stream.”

> **Test Tip:** In FRQs, build a chain: source → pollutant → transport → impact → solution. This structure helps you avoid vague answers and usually matches the scoring points.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Nonpoint source pollution comes from many diffuse sources rather than a single identifiable discharge point
- Point source pollution comes from a specific outlet such as a pipe or outfall
- Runoff is the main transport pathway for many forms of nonpoint pollution
- Watersheds connect upstream land use to downstream water quality
- Major nonpoint pollutants include nutrients, sediment, pathogens, pesticides, oil, heavy metals, salts, and trash
- Agricultural runoff commonly carries fertilizer, manure, pesticides, and eroded soil
- Urban runoff commonly carries oil, metals, road salt, lawn chemicals, pet waste, and debris
- Impervious surfaces increase runoff, decrease infiltration, and reduce groundwater recharge
- Excess nitrogen and phosphorus can cause cultural eutrophication, algal blooms, low dissolved oxygen, and fish kills
- Sediment pollution increases turbidity, reduces light penetration, and can smother aquatic habitat
- Atmospheric deposition can add pollutants such as mercury and nitrogen compounds to land and water
- BMPs such as riparian buffers, no-till farming, cover crops, permeable pavement, rain gardens, and retention ponds reduce nonpoint pollution

## Key Terms and Concepts

- nonpoint source pollution
- point source pollution
- runoff
- watershed
- atmospheric deposition
- impervious surface
- cultural eutrophication
- eutrophication
- turbidity
- best management practices (BMPs)
- riparian buffer


Sources of Pollution - Nonpoint source pollution