Landfills are the dominant destination for municipal solid waste in the United States. Understanding their benefits and environmental risks helps explain why careful siting, management, and monitoring are essential.

What landfills do (and why they are used)

Landfills are designated sites where solid waste is concentrated for long-term disposal. Their continued use reflects practical benefits:

• High capacity: Landfills can accept large volumes of mixed waste over long time periods.

• Centralised containment: Waste is confined to a defined area rather than scattered across the landscape, reducing widespread litter and direct contact hazards.

• Lower immediate cost: Compared with many alternative disposal methods, landfilling is often less expensive per tonne in the short term.

• Land reuse potential (long term): Once filled and closed, some sites can be converted to limited-use spaces (for example, open space), though this depends on settlement and gas management.

These benefits explain why most solid waste is disposed of in landfills, even as communities also pursue reduction and diversion strategies.

Environmental risks: groundwater contamination from leachate

A core environmental concern is that landfills can contaminate groundwater through leachate.

Leachate forms when precipitation, surface runoff, or moisture already in the trash moves through the landfill. The resulting liquid can contain:

• Dissolved organic matter that increases oxygen demand if it reaches surface waters

• Inorganic ions and salts

• Toxic substances depending on what was discarded (e.g., household chemicals)

Groundwater is especially vulnerable when landfills are placed over permeable soils, near a high water table, or where cracks and preferential flow paths allow faster downward movement.

Hydrogeologic cross-section illustrating how infiltration through refuse can create a local water-table mound beneath a landfill and drive leachate movement outward and downward. The figure highlights that groundwater flow patterns (not just the presence of contaminants) control where pollution travels and where it may emerge as springs or seepage. This provides a mechanistic explanation for why landfill siting and long-term groundwater monitoring are critical. Source

Because groundwater can move slowly, contamination may persist for years and spread beyond the landfill boundary before it is detected.

Conceptual cross-section of a landfill-related groundwater contamination plume, with contour lines representing decreasing contaminant concentration away from the source. It illustrates how leachate-driven contamination migrates with groundwater flow and can extend well beyond the landfill footprint. This kind of diagram helps explain why monitoring wells must be placed downgradient to track plume movement over time. Source

Why groundwater impacts matter

Groundwater pollution is difficult to reverse because contaminants can:

• Disperse through an aquifer and reach private wells

• Require costly treatment or replacement water supplies

• Create long-lasting plumes that demand monitoring over decades

Environmental risks: harmful gases released during decomposition

Landfills can also release harmful gases during decomposition.

Flowchart showing the major components of a basic landfill gas (LFG) system: gas extraction from the waste mass (via wells and piping), movement of gas with blowers/vacuum, and routing to control devices such as a flare or to beneficial-use treatment. It connects the biology (anaerobic decomposition generating methane) to the engineered controls used to reduce explosion hazards and greenhouse-gas emissions. The diagram also emphasizes that “collection” and “treatment/use” are separate steps in managing landfill methane. Source

As buried waste breaks down, microbes generate gases whose composition depends on oxygen availability. In deeper, oxygen-poor zones, decomposition tends to be anaerobic and produces significant methane along with carbon dioxide and trace gases.

Even when methane is the most discussed gas, landfill emissions can include:

• Volatile organic compounds (VOCs) that contribute to odours and may pose health risks

• Hydrogen sulfide and other reduced sulfur compounds associated with strong odours

• Carbon dioxide (CO₂), a greenhouse gas that also contributes to asphyxiation risk in confined spaces

Why gas emissions are an environmental and safety issue

• Climate forcing: Methane has much higher heat-trapping ability than CO₂ over short time scales, so uncontrolled releases increase climate impact.

• Fire and explosion hazards: Methane can migrate and accumulate, creating flammability risks.

• Local air quality and nuisance impacts: Odours and certain trace emissions can affect nearby communities.

Additional environmental concerns closely tied to landfill operation

While leachate and gas are the headline risks in AP Environmental Science, landfills can also create secondary impacts:

• Habitat loss and fragmentation from land conversion

• Vectors and pests (e.g., rodents, insects, scavenging birds) if waste is exposed

• Noise, traffic, and dust from hauling and daily operations

• Settling and subsidence as materials compact and decompose, complicating future land use

Key takeaways for AP Environmental Science

• Landfills provide a central, high-capacity disposal option, which is why they remain widely used.

• The major environmental risks are groundwater contamination via leachate and air impacts from decomposition gases, especially methane.

• These risks make site selection and long-term monitoring critical parts of landfill management.