AP Environmental Science

## Learning Outcomes

- Understand why sediment can be a pollutant even though soil is a natural material
- Identify major human sources of sediment pollution in streams, rivers, lakes, wetlands, and estuaries
- Explain how suspended sediment and deposited sediment harm aquatic water lifes in different ways
- Distinguish between turbidity and total suspended solids (TSS)
- Describe how sediment affects photosynthesis, dissolved oxygen, fish reproduction, and benthic habitats
- Apply watershed thinking to explain how upstream land use affects downstream water quality
- Evaluate management strategies such as riparian buffers, erosion control, and stormwater best management practices

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand sediment impacts in waterways, with a focus on the following syllabus points:

- Know that sediments and suspended particulates are major water pollutants
- Identify major human sources of sediment pollution, including agriculture, construction, logging, mining, urban runoff, and riparian vegetation removal
- Explain the causal chain from erosion and runoff to turbidity, reduced light penetration, lower photosynthesis, and aquatic stress
- Understand the difference between suspended sediment in the water column and deposited sediment on the streambed
- Explain how sediment can smother benthic habitat, clog fish gills, bury eggs, and fill spaces between gravel
- Distinguish turbidity from total suspended solids (TSS), including their units and why they are related but not identical
- Understand that wetlands and riparian areas can trap sediment and improve water quality
- Recognise that sediment often carries attached pollutants such as phosphorus, pesticides, and heavy metals

## 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. Why can sediment be classified as a pollutant even though it is naturally present in rivers and streams?
2. What is the difference between turbidity and total suspended solids (TSS)?
3. How does increased sediment in water reduce photosynthesis and dissolved oxygen?
4. Why are fish eggs and benthic macroinvertebrates especially vulnerable to deposited sediment?
5. Name two land-use activities that commonly increase sediment pollution in waterways.

## Introduction

Sediment in waterways is one of the most common forms of water pollution tested in AP Environmental Science. At first, it can seem confusing because **sediment is natural**. Rivers always carry some soil, silt, sand, and organic particles. The exam expects you to understand that sediment becomes pollution when human activities increase sediment loads above normal background conditions.

This matters because sediment does more than make water look muddy. It changes light levels, habitat quality, oxygen conditions, and food webs. It can also carry other pollutants into aquatic systems. In many cases, sediment links Unit 4 ideas about erosion and watersheds with Unit 5 land use and Unit 8 pollution.

> **Key Term: sediment pollution**
> Excess soil, silt, sand, clay, or organic particles entering water bodies in amounts high enough to damage water quality or aquatic life.

## Introduction to sediment pollution and why it matters

Sediment pollution usually begins on land, not in the water itself. Soil erosion from farms, construction sites, logging roads, overgrazed land, mining areas, and urban surfaces moves loose particles into streams and rivers through runoff. Once these particles enter the water, they may stay suspended for a time or settle onto the bottom later.

That distinction is central to this topic. AP questions often separate effects in the water column from effects on the streambed. Suspended sediment mainly affects water clarity and organisms moving or feeding in the water. Deposited sediment mainly changes bottom habitat and reproduction sites.

> **Key Term: runoff**
> Water that flows over land instead of infiltrating into soil, often carrying pollutants into streams, rivers, or lakes.
>
> **Key Term: erosion**
> The movement of weathered soil or rock from one place to another by water, wind, or human activity.

Sediment is especially serious in watersheds with disturbed land. If vegetation is removed, roots no longer hold soil in place. If impervious surfaces such as roads and parking lots increase, infiltration drops and more water moves rapidly over the land. Faster runoff means more erosive force. That creates a simple exam chain:

land disturbance or impervious cover -> increased runoff -> increased erosion -> more sediment in waterways -> lower water quality and habitat quality

> **Test Tip:** In FRQs, do not stop at “erosion increases sediment.” Extend the chain to at least one biological effect, such as lower photosynthesis, reduced dissolved oxygen, or smothered fish eggs.

### Suspended sediment, turbidity, and TSS

When sediment remains in the water column, it is called suspended sediment. These particles scatter light, making the water look cloudy. This cloudiness is measured as turbidity.

> **Key Term: turbidity**
> A measure of water cloudiness or clarity caused by suspended particles that scatter light, commonly reported in **NTU**.

Turbidity is not the same thing as sediment mass. It is an optical measure, not a weight measure. Very fine clay particles can create high turbidity even if the total mass of sediment is not extremely high. Dark particles, organic matter, and algae can also affect the reading. That is why the relationship between turbidity and actual sediment concentration is often site-specific.

A second measurement is total suspended solids.

> **Key Term: total suspended solids (TSS)**
> The mass concentration of suspended particles in water, commonly reported in **mg/L**.

TSS tells you how much material is suspended in the water by mass. Turbidity tells you how cloudy the water is. They often rise together after storms, but they are not interchangeable.

A simple comparison helps:

- **Turbidity:** optical cloudiness, measured in NTU
- **TSS:** particle mass per volume, measured in mg/L

Students often mix these up because both increase when runoff adds sediment to streams. But if an exam asks for “water clarity,” turbidity is the better term. If it asks for “mass of suspended particles,” TSS is the better term.

Another simple field measure of water clarity is Secchi depth. A greater Secchi depth means clearer water and lower turbidity.

Why does suspended sediment matter biologically? First, it blocks sunlight. If less light penetrates the water, algae and aquatic plants photosynthesize less. That can reduce primary productivity.

The core APES chain is:

erosion/runoff -> suspended sediment -> higher turbidity -> lower light penetration -> lower photosynthesis -> lower primary productivity -> lower dissolved oxygen -> stress on aquatic organisms

The photosynthesis equation is:

6CO2 + 6H2O + light -> C6H12O6 + 6O2

Less available light means this process slows down. In shallow lakes, ponds, wetlands, and stream margins, submerged vegetation may die back if turbidity stays high. Once plants die, decomposers use oxygen to break them down, which can further reduce dissolved oxygen.

Suspended sediment can also harm animals directly. Fish gills may become irritated or clogged. Visual predators such as many fish may have trouble seeing prey. Organisms that feed by filtering or scraping may have feeding structures coated by fine particles.

> **Test Tip:** If a graph or prompt mentions muddy water after storms, link it to turbidity first, then to reduced light and lower photosynthesis. That is often the quickest path to an AP-scoring explanation.

### Wetlands, downstream transport, and sediment trapping

Sediment effects do not stop at the first stream reach where erosion occurs. In a watershed, sediment is transported downstream through connected channels, floodplains, wetlands, and estuaries. This means land disturbance in one area can degrade habitats far away from the original source.

Wetlands are especially important in this process. Under natural conditions, wetlands can slow water movement and trap suspended particles before they move farther downstream. This improves water clarity and protects aquatic habitats in connected rivers, lakes, and estuaries.

However, wetlands can also be harmed by excessive human-caused sediment inputs. If too much sediment enters a wetland from agriculture, development, grazing, or channel disturbance, the wetland may become buried or its normal hydrology may be altered. The reference material also notes that dams and levees can interrupt the downstream movement of nutrient-rich sediments, affecting floodplains and wetland food webs, while development and grazing can destabilize banks and increase sediment pollution.

This creates an important APES idea: wetlands can act as natural filters, but they are not unlimited. Excessive sediment loading can overwhelm their ability to protect water quality and habitat.

## How deposited sediment changes habitat and food webs

Suspended sediment does not stay suspended forever. When water velocity slows, particles settle out. This process is sedimentation or deposition.

> **Key Term: sedimentation (deposition)**
> The settling of suspended particles onto the bottom of a water body when water velocity decreases.

This often happens behind dams, in reservoirs, on floodplains, in wetlands, in stream pools, and where streams widen or slow down. Deposited sediment can drastically alter habitat.

One of the most damaging effects is burial of benthic habitat. Benthic organisms live on or in the bottom sediments of streams, rivers, ponds, and lakes. Many aquatic insects, worms, mollusks, and microbial communities depend on the spaces between gravel and cobbles, called interstitial spaces.

> **Key Term: interstitial spaces**
> The gaps between particles such as gravel and cobble in streambeds, important for oxygen flow and habitat.

When fine sediment fills those spaces, habitat quality drops. Water can no longer move as easily through the gravel. Less oxygen reaches buried organisms and eggs. This can lower survival rates even if the water above the bed still contains oxygen.

A related idea is embeddedness.

> **Key Term: embeddedness**
> The degree to which larger streambed particles such as gravel or cobble are surrounded or covered by fine sediment.

High embeddedness means spawning gravel and benthic habitat are clogged with fine material. This is especially harmful to fish that lay eggs in gravel beds. Clean gravel allows water flow around eggs and embryos. Fine sediment blocks that flow, reducing oxygen delivery and increasing mortality.

Even without naming a specific fish species, AP answers should explain this clearly:

- adult fish spawn in gravel
- fine sediment infiltrates the gravel
- oxygen flow around eggs decreases
- egg and fry survival falls

This is one reason sediment pollution is strongly tied to reproduction, not just general water quality.

Benthic macroinvertebrates are also vulnerable. These include insect larvae such as mayflies and caddisflies in cleaner systems. If fine sediment buries rocky substrates and fills pore spaces, these organisms lose shelter, feeding surfaces, and oxygenated habitat. Because they are key prey for fish and other animals, the damage spreads through the food web.

Sediment can also change the physical structure of streams. Excess deposition can make pools shallower, alter channel shape, and increase flooding risk. In wetlands, sediments may either be beneficial at natural rates or harmful in excess. Wetlands normally trap sediment and reduce siltation downstream, but too much incoming sediment from human land use can overwhelm wetland function.

The same logic applies in coastal systems. Sediment runoff is one cause of coral reef degradation because it reduces water clarity and can smother bottom habitats.

## Human sources and watershed pathways

The most tested sediment sources are human land uses that increase erosion or runoff. Agriculture is a major one. Tillage breaks soil apart, reduces protective plant cover, and can increase surface runoff. The textbook also notes that sediments can transport nitrogen and phosphorus from fields into lakes and streams, linking sediment pollution to eutrophication.

Construction is another major source because exposed soil is left loose and vulnerable to rain. Without plant cover, stormwater can move large sediment loads into drains and nearby streams. Urbanization adds a double effect: more disturbed soil during building and more impervious surfaces afterward.

Logging increases sediment by removing vegetation and increasing erosion from roads and disturbed slopes. Mining can deposit spoils into floodplains and expose loose material that washes into water. Grazing can compact soil, remove streambank vegetation, and destabilize banks. Recreation such as boating or off-road vehicles can also stir up bottom sediments in wetlands and shallow waters.

A good watershed explanation uses upstream and downstream connections. Waterways do not isolate pollution. What happens on hillslopes, roads, fields, and streambanks upstream affects receiving waters downstream.

Important sources to remember:

- agriculture
- construction
- logging
- mining
- urban stormwater
- grazing
- riparian vegetation removal
- channel disturbance

Riparian zones matter because vegetation along streambanks slows runoff, traps sediment, stabilizes banks, and increases infiltration.

> **Key Term: riparian buffer**
> A strip of vegetation next to a stream, river, lake, or wetland that slows runoff, traps sediment, and stabilizes soil.

Without riparian vegetation, banks erode more easily and sediment enters the water directly. This is why planting native vegetation on stream banks is a repeated solution in APES.

Sediment also interacts with other pollutants. Fine particles often carry attached phosphorus, pesticides, and heavy metals. That means sediment is not only a physical pollutant but also a transport medium for chemical pollution. In an FRQ, that earns stronger analysis because it shows multiple effects from one stressor.

## Monitoring, standards, and management strategies

Sediment pollution is measured in several ways. The main ones you need are turbidity and TSS. Water quality tests may also include dissolved oxygen and pH to see ecological consequences, and Secchi depth for transparency in still waters.

The policy point students often miss is that there is no single nationwide sediment or turbidity number that applies to all waters. EPA provides frameworks for states and Tribes, including the **2006 Suspended and Bedded Sediments (SABS) framework**, but sediment criteria are often state-specific or narrative.

That matters because exam questions may refer to standards as “NTU above background.” Background means the natural turbidity level of that water under normal conditions. Human-caused increases above that background are what regulators may limit.

You do not need to memorize one national threshold, because there is not one. Instead, know the principle:

- standards may be numeric or narrative
- many are based on departure from natural background conditions
- sediment impacts depend on magnitude, duration, frequency, and timing

Management should begin with erosion prevention, not just trapping sediment after it moves. That is a common environmental science principle: stopping pollution at the source is usually more effective than cleaning it up later.

> **Key Term: best management practices (BMPs)**
> Practical methods used to reduce pollution, such as erosion control, vegetated buffers, sediment basins, and permeable surfaces.

Key BMPs include:

- preserving vegetation
- riparian buffers
- contouring and cover crops on farmland
- no-till or reduced tillage
- silt fences at construction sites
- sediment traps and sediment basins
- stabilized construction entrances
- quick revegetation of disturbed soils
- permeable pavement and green infrastructure
- stormwater retention and infiltration systems

Wetlands are also natural sediment controls. As sediment flows through wetlands, it becomes trapped, which reduces turbidity and protects downstream habitats. Living shoreline methods using plant roots can also stabilize soil and reduce erosion.

When evaluating solutions, think in terms of source, pathway, and receiving water:

- source: reduce disturbed soil
- pathway: slow runoff and increase infiltration
- receiving water: protect stream channels, wetlands, and habitat

If asked to design or judge a solution, connect it to mechanism. For example, a riparian buffer works because plant roots stabilize banks and vegetation slows runoff, allowing sediment to settle before reaching the stream.

> **Test Tip:** In a management FRQ, avoid vague answers like “make stricter laws.” Name a specific BMP and explain how it reduces runoff, erosion, or sediment delivery to the waterway.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Sediment becomes a pollutant when human activity increases sediment loads above natural background levels
- Major sources of sediment pollution include agriculture, construction, logging, mining, grazing, and urban stormwater
- Runoff and erosion are the main processes that carry sediment from land into waterways
- Suspended sediment increases turbidity and reduces light penetration in the water column
- Turbidity measures cloudiness in **NTU**, while total suspended solids (TSS) measures particle mass in **mg/L**
- Higher turbidity reduces photosynthesis, which can lower primary productivity and dissolved oxygen
- Deposited sediment smothers benthic habitat and can alter stream channel structure
- Fine sediment filling gravel spaces increases embeddedness and reduces oxygen flow to fish eggs
- Sediment can carry attached pollutants such as phosphorus, pesticides, and heavy metals
- Riparian buffers, erosion control, sediment basins, and permeable surfaces are important BMPs for reducing sediment pollution
- Wetlands can trap sediment naturally and help protect downstream water quality
- There is no single nationwide sediment standard for all waters; many criteria are state-specific or based on increases above background

## Key Terms and Concepts

- sediment pollution
- runoff
- erosion
- turbidity
- total suspended solids (TSS)
- sedimentation (deposition)
- interstitial spaces
- embeddedness
- riparian buffer
- best management practices (BMPs)


Human Impacts on Ecosystems - Sediment impacts in waterways