AP Environmental Science

## Learning Outcomes

- Explain how modern sanitary landfills reduce pollution compared with open dumps
- Describe how **leachate** forms and why it threatens groundwater and surface water
- Understand how **landfill gas** forms, including why methane is a climate and safety concern
- Identify the main design features of landfills and match each feature to its function
- Explain why landfills create long-term land use and monitoring problems even after closure
- Apply landfill concepts to AP-style questions about impacts, mitigation, and site suitability

## AP Environmental Science Syllabus

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

- Know that **sanitary landfills** are engineered waste-disposal sites, unlike open dumps
- Identify how **leachate** forms when water percolates through waste and carries contaminants
- Explain how landfill gas forms by **anaerobic decomposition** and why methane is a major concern
- Understand the functions of **composite liners**, leachate collection systems, daily cover, and final caps
- Know that groundwater contamination is monitored using **upgradient** and **downgradient** wells
- Understand that modern landfills reduce risk but do not eliminate risks such as leaks, gas escape, and long-term pollution
- Recognise landfills as a major source of human-related methane emissions and a major use of land space
- Know that post-closure care commonly lasts about **30 years**

## 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 leachate, and how does it form in a landfill?
2. Why is methane produced in landfills, and why is it dangerous?
3. What is the difference between daily cover and a final cap?
4. How do upgradient and downgradient wells help detect landfill pollution?
5. Why can a sanitary landfill still cause environmental problems even after it closes?

## Introduction

Landfills are still one of the most common methods of disposing of municipal solid waste. For AP Environmental Science, you need more than a simple idea that “trash goes into the ground.” You need to know how landfills are designed, what risks they create, and why those risks continue for decades.

The exam often tests landfill impacts through cause-and-effect chains. Rain falls on a landfill, water moves through waste, contaminated liquid forms, gases are produced by decomposition, and pollutants can then affect soil, groundwater, nearby surface water, air quality, climate, and human health. Strong answers explain both the impact and the mechanism.

> **Key Term: sanitary landfill**
> An engineered waste-disposal site where waste is compacted, covered, and isolated from the environment to reduce pollution, pests, odors, and fire risk.

### Why landfill impacts matter

Landfills solve one problem by concentrating waste in one place, but that creates three major impact areas: **water contamination**, **air and climate effects**, and **land use with long-term management**. These three themes come up again and again in AP questions.

Older dumps were often just piles or pits of waste with few protections. Modern municipal solid waste landfills are much safer because they use liners, collection systems, covers, and monitoring. However, safer does not mean harmless. A landfill can still leak, release gas, take up large areas of land, and require decades of care after closure.

> **Key Term: municipal solid waste landfill (MSWLF)**
> A landfill designed for everyday trash from homes, businesses, and communities, regulated to reduce environmental harm.

A common exam mistake is to describe landfills only as ugly or smelly. Those are real nuisance effects, but APES usually wants deeper impacts: contamination of aquifers, methane emissions, and land demand.

> **Test Tip:** If a question asks for a landfill impact on water, do not answer only with “methane.” Methane is mainly an air and climate issue. For water questions, focus on leachate, liner failure, runoff, or groundwater contamination.

### Leachate and water contamination

The most tested landfill water issue is **leachate**. This forms when water moves through buried waste and dissolves or carries chemicals, metals, and other pollutants with it. The water source is usually rainwater, though other liquids can contribute.

> **Key Term: leachate**
> Contaminated liquid formed when water percolates through landfill waste and picks up dissolved chemicals and suspended pollutants.

The mechanism matters. First, precipitation lands on the landfill. Next, some water infiltrates downward through gaps in the cover and through the waste. As it moves, it contacts food waste, metals, cleaning products, batteries, plastics, and other discarded materials. Then it emerges as a polluted liquid. If that leachate escapes the landfill, it can contaminate soil and groundwater. If contaminated groundwater later discharges into streams or lakes, surface water is affected too.

This is why site choice matters so much. A landfill built over a shallow water table or near wetlands or flood plains has a higher risk of pollution spread. Low-permeability soils such as clay help because they slow water movement. A deeper water table gives more separation between waste and groundwater.

Modern landfills reduce this risk with a **composite liner** and a **leachate collection and removal system**.

> **Key Term: composite liner**
> A landfill barrier system, usually a geomembrane over compacted clay, designed to reduce the movement of leachate into soil and groundwater.

The clay layer is useful because clay has very low permeability. The synthetic liner adds another barrier. Above the liner sits the leachate collection system, which removes contaminated liquid before it builds up.

If the liner were perfect forever, groundwater contamination would be impossible. But liners age, seams can fail, waste can shift, and systems need maintenance. That is why APES stresses that modern landfills reduce risk rather than remove it.

### How landfills are monitored and managed

Because leaks can happen, groundwater monitoring is required at many landfill sites. Wells are placed around the landfill to compare water quality before and after groundwater passes the site.

> **Key Term: upgradient well**
> A monitoring well placed where groundwater approaches the landfill, used to measure background water quality.
>
> **Key Term: downgradient well**
> A monitoring well placed where groundwater leaves the landfill area, used to detect contamination from the landfill.

The logic is simple. Upgradient wells show what the groundwater was like before contact with the landfill. Downgradient wells show whether pollutants have been added after water moved past the site. If downgradient water is worse than upgradient water, that suggests a release from the landfill.

Students often reverse these terms. Think of gradient as the direction groundwater flows. “Up” is before the landfill. “Down” is after.

Landfills are also managed day by day. Waste is compacted to reduce volume, and then covered.

> **Key Term: daily cover**
> Material placed over exposed waste at the end of each day to reduce odors, pests, litter spread, and fire risk.

Daily cover is not the same as the cap placed when a landfill closes. Daily cover is routine operational control. It reduces access for rats, insects, and scavengers, and it helps keep trash from blowing away. It also lowers the chance of surface fires.

Once a landfill cell is full, it is sealed with a final cover.

> **Key Term: final cap**
> An engineered cover placed on a closed landfill to reduce water infiltration, erosion, gas escape, and direct exposure of waste.

Caps matter because less water infiltration means less leachate. They also reduce erosion and limit direct contact between humans, wildlife, and buried waste. Textbook descriptions of hazardous waste caps often include an upper topsoil layer, a compacted soil barrier, and a low-permeability synthetic layer over compacted clay.

Even after closure, care does not end. Landfills commonly need about 30 years of post-closure monitoring and maintenance, though the period can be adjusted by regulators. During that time, managers maintain the cap, inspect the leachate system, monitor groundwater, and manage gas emissions.

> **Test Tip:** If an FRQ asks how modern landfills reduce impacts, pair each feature with its job: liner—blocks leakage; leachate collection—removes contaminated liquid; daily cover—reduces pests and odors; cap—reduces infiltration; monitoring wells—detect contamination.

### Methane, landfill gas, and climate impacts

The major air impact of landfills is **landfill gas**. This gas forms because microbes break down organic waste without oxygen.

> **Key Term: landfill gas (LFG)**
> Gas produced by anaerobic decomposition in landfills, typically containing large amounts of methane and carbon dioxide.

Food scraps, paper, yard waste, and other biodegradable materials are decomposed by anaerobic bacteria. In simplified form, organic matter breaks down into methane, carbon dioxide, water, and trace gases. A common example reaction is:

C₆H₁₂O₆ → 3CO₂ + 3CH₄

Landfill gas is often roughly half methane and half carbon dioxide, though exact percentages vary. The methane is the most important part for APES because it creates two major problems.

First, methane is a strong greenhouse gas. Municipal solid waste landfills were one of the largest human-related methane sources in the United States, accounting for about **14.4%** in 2022. That makes landfill methane an important climate issue, not just a local pollution issue.

Second, methane is flammable. If it migrates through soil into nearby structures, it can create fire or explosion hazards. This is why gas collection systems are used.

> **Key Term: methane (CH4)**
> A flammable greenhouse gas produced by anaerobic decomposition of organic matter in landfills and other oxygen-poor environments.

A useful AP distinction is this: carbon dioxide is also released, but methane usually matters more in short-term climate impact because it traps much more heat per molecule over shorter timescales. That is why capturing methane is such a major landfill mitigation strategy.

Operators can install gas wells and pipes to collect landfill gas. Once collected, it can be flared or used for energy recovery.

- **Flaring** burns methane and converts it mainly into carbon dioxide and water vapor
- **Energy recovery** uses the gas to generate electricity, heat, steam, or renewable natural gas

Both options reduce methane release. Energy recovery has the extra benefit of replacing some fossil fuel use. However, not all landfill gas is captured. Fugitive methane still escapes from many sites, especially older ones.

Food waste is especially significant here. A large share of landfill methane comes from decomposing food waste, so diverting food scraps to composting or anaerobic digestion can lower emissions.

### Land use, long-term risks, and why landfills are still controversial

A third major landfill impact is land use. Landfills take up large areas, and suitable sites are limited. They need to be away from faults, flood plains, wetlands, and drinking water sources, and they need geology that lowers contamination risk. As population grows and waste production continues, finding new sites becomes harder and more expensive.

This is one reason **NIMBY** attitudes are common.

> **Key Term: NIMBY**
> “Not in my backyard” — opposition to placing facilities such as landfills near where people live.

People may accept that a landfill is necessary but still oppose having it nearby because of odor, truck traffic, lower property values, fire concerns, gas migration, and fear of pollution. From an APES viewpoint, this is not just social conflict. It affects where landfills can be located, which can increase transportation distance and costs, and increase fuel use from hauling waste farther.

There is also the issue of time. Trash does not disappear quickly. Some organic waste decomposes fast, but plastics, metals, and glass can persist for decades to extremely long periods. That means the landfill remains an active management problem long after it is closed.

Closed landfills can sometimes be reused as parks, golf courses, or commercial space, but this does not erase the risks. Gas may still move underground, and the cap must remain intact. Subsidence can also occur as buried waste settles over time, which can damage whatever is built on top.

Modern sanitary landfills are still usually safer than open dumping or ocean dumping. They centralize waste and use engineered barriers. Yet they should be seen as damage control, not a perfect solution. The most effective long-term strategy is to reduce the amount of waste reaching landfills in the first place through source reduction, recycling, composting, and diversion of organic waste.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Sanitary landfills are engineered waste sites designed to reduce environmental harm compared with open dumps
- Leachate forms when water percolates through waste and carries pollutants
- Leachate can contaminate soil, groundwater, and nearby surface water if containment fails
- Composite liners and leachate collection systems reduce groundwater contamination risk
- Upgradient wells measure background groundwater quality, while downgradient wells detect releases from the landfill
- Daily cover reduces odors, pests, litter spread, and fire risk during landfill operation
- Final caps reduce water infiltration, erosion, gas escape, and exposure after closure
- Anaerobic decomposition of organic waste produces landfill gas, including methane and carbon dioxide
- Methane is both a strong greenhouse gas and a fire or explosion hazard
- Landfills occupy large areas and require long-term post-closure care, often for about 30 years
- Modern landfills reduce risks, but leaks, gas escape, and long-term pollution are still possible
- Diverting organic waste, especially food waste, reduces landfill methane emissions

## Key Terms and Concepts

- sanitary landfill
- municipal solid waste landfill (MSWLF)
- leachate
- composite liner
- upgradient well
- downgradient well
- daily cover
- final cap
- landfill gas (LFG)
- methane (CH4)
- NIMBY


Solid Waste Disposal - Landfill disposal impacts