AP Environmental Science

## Learning Outcomes

- Understand how soil erosion lowers water quality in streams, lakes, and wetlands
- Identify the links between runoff, infiltration, sediment transport, and turbidity
- Explain how sediment can damage aquatic habitats and carry attached pollutants such as phosphorus
- Distinguish between turbidity and total suspended solids (TSS), and between soil loss and sediment yield
- Apply best management practices that reduce erosion and sediment runoff
- Use cause-and-effect chains that are common in AP Environmental Science questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand soil erosion and its effects on water systems, with a focus on the following syllabus points:

- Know that soil erosion moves topsoil by water, wind, or human disturbance, and that water erosion is especially important for water quality
- Identify how reduced infiltration increases runoff and how runoff increases sediment delivery to streams, rivers, lakes, and wetlands
- Explain how sediment runoff raises turbidity and often TSS, reducing light penetration and harming aquatic organisms
- Understand that sediment pollution is usually a **nonpoint source pollution** problem linked to agriculture, grazing, logging, construction, and urban stormwater
- Explain how phosphorus often attaches to soil particles, so erosion can increase nutrient loading and eutrophication
- Distinguish between **turbidity** and **TSS**, and between **soil loss** and **sediment yield**
- Recognise best management practices such as riparian buffers, no-till farming, cover crops, contour plowing, terracing, silt fences, and retention ponds
- Understand that erosion affects both water chemistry and physical habitat, not just one or the other

## 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. How does low vegetation cover increase sediment pollution in streams?
2. What is the difference between turbidity and total suspended solids?
3. Why can soil erosion increase eutrophication even when phosphorus is not dissolved in water?
4. What is the difference between soil loss and sediment yield?
5. Name two land-management practices that reduce erosion by increasing infiltration or slowing runoff.

## Introduction

Soil erosion matters in AP Environmental Science because it connects land systems to water systems. A question may begin with farming, logging, overgrazing, construction, or urbanization, but the answer often depends on what happens next: less vegetation means less infiltration, which means more runoff, which means more sediment entering water.

This topic is not just about losing soil from land. It is also about what that lost soil does after it moves. Once sediment reaches streams, lakes, wetlands, or estuaries, it can cloud water, block sunlight, bury habitats, transport nutrients and pesticides, and contribute to lower dissolved oxygen indirectly.

> **Key Term: soil erosion**
> The removal and movement of topsoil or weathered soil material by water, wind, or human disturbance.

### Why erosion changes water quality

Soil is more than loose material on the ground. Healthy soil stores water, supports roots, holds nutrients, and filters water as it moves downward. When erosion removes topsoil, the land loses part of that ability. The result is not only less productive land, but also dirtier water downstream.

A key chain to remember is:

vegetation removal -> less root support and less surface cover -> lower infiltration and more runoff -> more erosion -> more sediment in water -> lower water quality

This chain appears again and again in APES.

> **Key Term: infiltration**
> The process by which water soaks into the soil instead of flowing across the land surface.

When infiltration is high, rainwater enters soil, recharges groundwater, and moves more slowly. When infiltration is low, more water stays on the surface and flows downhill. That overland flow picks up detached soil particles and carries them into waterways.

Why would infiltration fall? Several reasons:

- bare soil is more exposed to raindrop impact
- compacted soil has fewer open pore spaces
- repeated tillage breaks down soil structure
- impervious surfaces such as asphalt and concrete block water entry completely
- overgrazing removes vegetation and compacts the ground with hooves
  > **Key Term: runoff**
  > Water that flows over land instead of soaking into the soil.

Runoff is especially important because it links erosion on land to pollution in water. In a forest, leaves soften rainfall and roots hold soil in place. In a plowed field or construction site, raindrops hit exposed soil directly, detach particles, and runoff transports them away.

> **Test Tip:** In FRQs, build the full sequence. Do not stop at “erosion increases.” Go further: erosion increases sediment runoff, which raises turbidity, reduces light penetration, and harms aquatic life.

### Sediment as a pollutant: turbidity, TSS, and habitat damage

Once soil enters water, it becomes sediment pollution. This is often a **nonpoint source** problem because it comes from many diffuse locations rather than a single discharge pipe.

> **Key Term: nonpoint source pollution**
> Pollution that comes from many diffuse sources rather than one easily identified point source.

Sediment pollution affects water in physical ways first. Water becomes cloudier, which reduces light penetration. That matters because aquatic plants and algae need light for photosynthesis. Less light means lower photosynthesis, which can reduce primary productivity in the water body.

> **Key Term: turbidity**
> A measure of water cloudiness caused by suspended particles that scatter light.

Turbidity is not exactly the same as the amount of sediment, even though the two are often related. Turbidity is an optical measure. It tells you how cloudy the water looks or how much light is scattered. It does not directly tell you the mass of particles.

> **Key Term: total suspended solids (TSS)**
> The concentration of suspended particles in water, usually measured as a mass per volume, often in mg/L.

This distinction matters. Two water samples can have similar turbidity but different TSS if their particle sizes or types differ. Fine clay and organic matter can make water very cloudy even when mass is not especially high. Coarser particles may increase TSS but settle more quickly.

Students often confuse these.

- **Turbidity** = optical cloudiness
- **TSS** = measured mass concentration of suspended solids

Sediment also damages habitats directly. It can:

- clog fish gills
- bury stream bottoms
- fill spaces between gravel
- smother fish eggs and spawning grounds
- cover benthic organisms living on the bottom
- reduce feeding success for predators that hunt by sight

This is why sediment pollution is not “just dirt.” It changes habitat structure. A stream with high sediment load may still contain water, but it may no longer support the same organisms.

In addition, warmer, muddier water can hold less dissolved oxygen than cooler water. Still, be careful with wording: erosion does not always lower dissolved oxygen directly. Often the stronger claim is that sediment raises turbidity and harms habitat, while oxygen decline happens later through warming or eutrophication.

> **Test Tip:** If a question asks for a water-quality effect of erosion, the safest direct answers are increased turbidity, increased suspended sediment, reduced light penetration, and smothered habitat.

### Why phosphorus makes erosion more dangerous

Sediment pollution often becomes nutrient pollution too. That is because phosphorus commonly attaches to soil particles. When soil erodes, phosphorus can move with it into lakes, rivers, ponds, and wetlands.

> **Key Term: eutrophication**
> The enrichment of water by nutrients, especially nitrogen or phosphorus, causing excessive algal or plant growth.

A common student mistake is assuming only dissolved nutrients matter. In reality, particulate phosphorus can be very important. If runoff carries eroded soil into water, it may also carry phosphorus adsorbed to that soil.

The chain looks like this:

exposed soil -> erosion -> sediment runoff -> phosphorus attached to sediment enters water -> excess nutrient supply -> algal bloom -> decomposition of algae -> higher biological oxygen demand -> lower dissolved oxygen

Notice the steps. Phosphorus itself does not instantly kill fish. It stimulates plant or algal growth. When that biomass dies, decomposers use oxygen to break it down.

> **Key Term: biological oxygen demand (BOD)**
> The amount of oxygen used by decomposers to break down organic matter in water.

Higher BOD usually means less dissolved oxygen is available for fish and invertebrates. This is why nutrient-rich sediment can trigger fish kills indirectly.

Sediment-bound pollutants may also include:

- pesticides
- some heavy metals
- hydrocarbons
- organic matter

So erosion can worsen chemical pollution at the same time as physical pollution.

A lake is often more vulnerable than a fast stream because slow-moving water allows sediment and nutrients to accumulate. In lakes and reservoirs, increased nutrient loading is more likely to produce strong eutrophication effects.

### Soil loss, sediment yield, and measuring erosion

Another distinction students miss is the difference between soil loss and sediment yield.

> **Key Term: sediment yield**
> The portion of eroded soil that actually reaches a stream, lake, or watershed outlet.

Not all eroded soil reaches the water. Some is redeposited on the slope, trapped by vegetation, or settled in low spots before leaving the site. So total soil erosion from a field is usually greater than sediment yield at the stream.

This matters in exam questions. If a farmer loses a large amount of soil from a hillside, the stream may receive less than that full amount because some sediment is deposited before getting there.

A related APES concept is the Revised Universal Soil Loss Equation.

> **Key Term: RUSLE**
> A model used to estimate average annual soil loss from sheet and rill erosion on hillslopes: A = R × K × LS × C × P.

You do not need to memorise advanced detail beyond the basic use, but you should know what it does and does not estimate.

- It estimates **soil loss at the source**
- It mainly applies to **sheet and rill erosion**
- It does **not** directly predict stream turbidity, TSS, or full watershed sediment load
- It does **not** usually estimate gully or streambank erosion

This is a classic distinction examiners like.

### Preventing erosion and protecting water quality

The most effective strategies work by doing three things:

- keeping soil covered
- increasing infiltration
- slowing runoff before it reaches water

Agricultural best management practices include:

- **cover crops** to shield bare soil and add root structure
- **no-till farming** to reduce disturbance and preserve soil aggregates
- **contour plowing** to slow water moving down slopes
- **terracing** to shorten slope length and reduce runoff velocity
- **crop rotation** to improve soil condition and reduce nutrient depletion
- **strip cropping** or intercropping to provide more root networks across the field
- **windbreaks** and vegetation to reduce erosion by wind and stabilize soil

Riparian protection is also very important.

> **Key Term: riparian buffer**
> A strip of vegetation along a stream or river that slows runoff, traps sediment, stabilizes banks, and can absorb some nutrients.

Riparian buffers work because they intercept runoff before it enters the stream. Water slows down, sediment settles, roots stabilize soil, and some nutrients are taken up by plants. They are especially useful against sediment-bound phosphorus.

Construction and urban controls include:

- silt fences
- sediment basins
- vegetated cover on disturbed slopes
- retention or detention ponds
- reduced exposed soil area
- permeable surfaces where possible

Urbanization creates extra risk because impervious surfaces sharply reduce infiltration. Water that once soaked into soil now rushes into drains and channels, increasing stream erosion and sediment delivery.

Wetlands can also help. As runoff flows through wetlands, sediment tends to settle out, reducing turbidity farther downstream. Wetland plants slow water, trap material, and help protect water quality.

A final cause-and-effect example to remember:

clear-cutting or construction -> loss of vegetation -> greater runoff -> more sediment in streams -> higher turbidity -> less underwater light -> reduced photosynthesis -> lower habitat quality for fish and invertebrates

That single chain can answer many APES questions.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Soil erosion removes topsoil and often increases sediment pollution in nearby water
- Lower infiltration usually means higher runoff and greater erosion risk
- Vegetation reduces erosion by intercepting rain and holding soil with roots
- Sediment runoff is usually a nonpoint source pollution problem
- Turbidity measures water cloudiness, while TSS measures the mass of suspended particles
- Sediment reduces light penetration and can lower photosynthesis in aquatic systems
- Sediment can clog fish gills and smother eggs, benthic organisms, and spawning habitat
- Phosphorus often attaches to soil particles, so erosion can increase nutrient loading
- Sediment-bound phosphorus can contribute to eutrophication and later oxygen depletion
- Soil loss is not the same as sediment yield because some eroded material is deposited before reaching water
- RUSLE estimates soil loss from sheet and rill erosion, not full stream sediment load
- Riparian buffers, cover crops, no-till, contour plowing, terracing, and construction controls help reduce sediment runoff

## Key Terms and Concepts

- soil erosion
- infiltration
- runoff
- nonpoint source pollution
- turbidity
- total suspended solids (TSS)
- eutrophication
- biological oxygen demand (BOD)
- sediment yield
- RUSLE
- riparian buffer


Soil Formation and Erosion - Soil erosion and water quality