After a landfill closes, two things are actively collected from the site for decades: leachate (contaminated liquid that drains through the buried waste) and landfill gas (a mix of methane and carbon dioxide produced by decomposing trash). Federal regulations require this collection to continue for a minimum of 30 years, along with ongoing groundwater monitoring and surface inspections to catch any contamination escaping the site.
Leachate: The Contaminated Liquid
Leachate forms when rainwater or snowmelt filters down through layers of buried waste, picking up chemicals, heavy metals, and organic compounds along the way. Even after a landfill is sealed with a final cap, some moisture still reaches the waste mass, and the decomposition process itself generates liquid. This fluid has to be continuously captured and removed to prevent it from contaminating surrounding soil and groundwater.
Modern landfills are built with two layers of collection systems stacked on top of each other. The primary system sits directly beneath the waste: a network of drains and pipes embedded in a layer of sand or gravel, all resting on a thick plastic membrane about 0.08 inches thick backed by a layer of ultra-low-permeability clay. If any leachate slips past that barrier, a secondary system catches it, using another set of drains and pipes over a slightly thinner membrane underlain by a 24-inch clay layer. Pumps draw the collected leachate out of these systems for treatment.
Once collected, leachate goes through treatment before it can be safely discharged. Common approaches include biological treatment, where microorganisms break down organic pollutants, and membrane technologies like reverse osmosis and nanofiltration that physically filter out contaminants. Some sites pipe leachate to municipal wastewater plants, while others treat it on-site. In certain cases, collected landfill gas is even used to evaporate leachate, reducing the volume that needs further processing.
Landfill Gas: Methane and Carbon Dioxide
Buried waste decomposes without oxygen, producing a gas that’s roughly half methane and half carbon dioxide, with small amounts of other compounds mixed in. This gas builds pressure underground and will escape through cracks, seams, or weak spots in the landfill cover if it isn’t actively extracted. Uncontrolled releases create explosion risks, contribute to climate change (methane is a potent greenhouse gas), and produce odors.
The most common extraction method uses vertical wells drilled into the waste mass. These boreholes are typically 24 to 36 inches in diameter and reach depths of 40 to 140 feet below the landfill surface. Each well has a casing surrounded by permeable backfill material that allows gas to flow in, topped with a seal to prevent air from entering. Some sites also use horizontal collectors, pipes that can stretch more than 500 feet across and require at least 30 feet of waste above them to work properly without pulling in outside air. Blowers and compressors connected to the well network apply a vacuum that draws gas out of the waste and pushes it toward processing equipment.
Turning Landfill Gas Into Energy
Collected gas doesn’t just get destroyed. While basic flaring (burning it off) is the minimum requirement, many closed landfills convert their gas into usable energy through one of three main pathways.
- Electricity generation: Gas fuels engines, turbines, or fuel cells on-site to produce power that feeds into the electrical grid.
- Direct use: The gas is piped to nearby industrial facilities, greenhouses, or institutional buildings as a replacement for conventional natural gas. Some creative applications include firing pottery kilns and glass-blowing studios.
- Renewable natural gas: Through advanced treatment that strips out carbon dioxide, nitrogen, oxygen, and other impurities, landfill gas can be upgraded to pipeline-quality natural gas. This renewable natural gas can be injected into existing gas pipelines, compressed for use as vehicle fuel, or liquefied for transport to other locations.
Before any of these uses, the raw gas goes through at least basic treatment. A knockout pot and filter remove moisture, followed by additional processing depending on the end use. Gas destined for pipelines or vehicle fuel needs the most intensive cleaning, including removal of sulfur compounds and siloxanes that can damage equipment.
Groundwater Monitoring
A network of monitoring wells surrounds every closed landfill, and water samples are drawn from them on a regular schedule throughout the post-closure period. Each sample is tested for hazardous constituents and compared against background values, the baseline measurements of what the groundwater looked like before the landfill existed. If any parameter shows a statistically significant increase over background levels, the site operator must investigate and potentially take corrective action.
This monitoring is designed to catch leaks early. Even with double-lined collection systems, regulators treat containment as something that needs verification, not something to assume is working.
Surface Emissions Monitoring
The landfill’s final cap is inspected regularly for signs that gas is escaping. Traditionally, technicians walk the surface with handheld methane detectors held close to the ground. Newer methods use drones equipped with methane sensors that fly predetermined patterns across the landfill at 30-meter intervals, transmitting geo-located readings in real time. Any reading above 200 parts per million of methane triggers a closer ground-level survey of the area within at least a 15-meter radius of the detection point.
Drone-mounted cameras also scan for visual warning signs: distressed vegetation, cracks in the cover soil, or seeps where gas or liquid may be emerging. These indicators often reveal problems before instruments pick up elevated methane concentrations.
What the Final Cap Looks Like
Everything described above depends on the integrity of the cap that seals the landfill after closure. This cap is a multi-layer system, built from the bottom up, with each layer serving a specific purpose.
At the base sits a leveling layer that smooths out the surface of the waste. Above that, an optional gas vent layer allows decomposition gases to migrate toward the extraction wells rather than building pressure against the cap. Next comes a 12-inch layer of compacted low-permeability soil, followed by a thin but extremely impermeable plastic geomembrane that serves as the primary barrier against both water infiltration and gas escape. A drainage layer of gravel or sandy gravel (at least one foot thick) sits above the membrane to channel rainwater away and prevent it from pooling. Protective soil covers the drainage layer to shield it from frost and physical damage. Finally, a topsoil layer of sand-silt-loam supports a vegetative cover of grasses or other plants that stabilize the surface and reduce erosion.
How Long Post-Closure Care Lasts
Federal regulations set the standard post-closure care period at 30 years. During this time, the site owner is responsible for maintaining the cap, operating the leachate and gas collection systems, monitoring groundwater, and keeping records. Regulators have the authority to extend this period if monitoring shows the site still poses a risk, and some landfills require care well beyond the 30-year minimum. The period can also be shortened if the owner demonstrates that the site no longer threatens human health or the environment, though early release is uncommon.
The costs of post-closure care are substantial, which is why landfill operators are required to set aside financial assurance (essentially a trust fund) before the site ever closes. Pumping leachate, running gas extraction blowers, sampling monitoring wells, mowing vegetation, and repairing erosion damage all add up over three decades or more.