A landfill’s core purpose is to isolate waste from the surrounding environment, keeping it contained so it doesn’t contaminate groundwater, soil, or air. About 50% of all municipal solid waste in the United States ends up in a landfill, totaling roughly 146 million tons per year. Far from being simple holes in the ground, modern landfills are engineered containment systems designed to protect public health and minimize environmental damage for decades after they stop accepting trash.
How Landfills Differ From Old Dumps
Before modern regulations, communities disposed of trash in open dumps. These were unmanaged sites where old cars, dead animals, household garbage, and even industrial chemicals piled up together. Rain carried contaminants into streams and groundwater. Fires burned constantly, sometimes started deliberately by operators trying to free up space. Rats and flies thrived. The smell on a hot summer day was overpowering, and litter blew freely into surrounding areas.
The first major reform was the “sanitary landfill,” which introduced a simple but critical rule: cover the garbage with soil at the end of each day. That single change reduced pests, fires, odors, and windblown litter. Modern landfills go far beyond daily cover. They function more like massive sealed containers, with engineered barriers separating waste from the ground beneath it, systems to collect contaminated liquids, and networks to capture gases produced by decomposing trash.
Containing Waste and Protecting Groundwater
The most important feature of a modern landfill is its liner system. At the bottom of the landfill, before any waste is placed, engineers install layers of protection to keep liquids from seeping into the earth. A typical composite liner includes a sheet of high-density plastic (often about 60 thousandths of an inch thick) bonded to a layer of compacted clay roughly one foot deep. Some designs use a thin layer of processed bentonite clay sandwiched between sheets of fabric, creating an additional seal just 0.2 to 0.3 inches thick.
As rain filters through buried waste, it picks up chemicals, heavy metals, and organic compounds, forming a toxic liquid called leachate. Without containment, leachate soaks directly into groundwater. Modern landfills collect this liquid through drainage systems installed above the liner, then pipe it to treatment plants where pollutants are removed and clean water is produced. This is one of the starkest differences between a regulated landfill and an old dump: the liquid never touches the natural ground.
Controlling Gas, Odor, and Pests
Bacteria breaking down organic material inside a landfill produce a mix of gases, primarily methane and carbon dioxide. Methane is flammable and a potent greenhouse gas, so landfills are required to capture it. Collection systems use a network of vertical wells or horizontal trenches connected to blowers that pull gas to a central processing point. From there, the gas is either burned off safely or put to productive use.
Federal regulations also require operators to cover all disposed waste at the end of each operating day. The minimum standard is six inches of soil, though many landfills now use alternative materials like spray-on foams, reusable tarps, construction debris, or composted green material. Daily cover serves several purposes at once: it blocks odors, discourages rats and birds, reduces the risk of fire, and prevents litter from blowing off-site. Operators also maintain litter fences around the perimeter, keep roads clean of mud and dust, and use water trucks on unpaved surfaces during dry weather.
Turning Landfill Gas Into Energy
Rather than simply burning off captured methane, many landfills convert it into a resource. About 63% of landfill gas energy projects in the U.S. generate electricity. Another 17% use the gas directly as a fuel substitute, replacing natural gas, coal, or fuel oil in nearby industrial operations. The remaining 20% process it into renewable natural gas that can be injected into pipelines or used as vehicle fuel. The EPA tracks more than 2,600 municipal solid waste landfills across the country, many of which are candidates for these energy recovery projects.
What Happens When a Landfill Is Full
Landfills don’t just get abandoned when they reach capacity. Operators install a final cover system, typically layers of soil and synthetic material designed to keep rainwater out, which slows leachate production and gas generation. Groundwater monitoring, gas collection, and leachate management continue for 30 years or more after closure.
Because the ground above a closed landfill is unsuitable for buildings or agriculture, communities often repurpose these sites in creative ways. In Milwaukee, eight acres of a closed, city-owned landfill near the airport were converted into a 2.25-megawatt solar array in 2020. The project turned otherwise unusable land into a profitable renewable energy source. Other closed landfills have become parks, golf courses, or wildlife habitat areas.
The Scale of Landfilling in the U.S.
The United States generated roughly 292 million tons of municipal solid waste in 2018, a 93% increase since 1980. Of that total, 50% went to landfills, 32% was recycled or composted (diverting about 94 million tons), and 12% was burned in facilities that recover energy from combustion. The amount of material diverted through recycling and composting has grown to about 2.8 times what it was in 1990, but landfills still handle the largest single share of the country’s trash.
There were approximately 1,908 active municipal solid waste landfills in the continental U.S. as of the last comprehensive count. Each is managed at the state level under federal standards set by the Resource Conservation and Recovery Act, which establishes criteria for landfill siting, design, operation, and closure. These regulations are what separate a modern landfill from the unregulated dumps of the past, ensuring that today’s waste doesn’t become tomorrow’s environmental crisis.