The quick answer
- Leachate is landfill liquid. Federal rules define it as liquid that has passed through or come out of solid waste and carries dissolved, suspended or mixed-in material picked up from that waste.
- Most of it starts as rain. Rain and snowmelt soak down through the trash, joined by moisture squeezed out of the waste as it is compacted and breaks down.
- It can pollute water, because it picks up organic matter, ammonia, salts, metals and trace chemicals on the way down.
- Modern landfills trap it. A liner under the waste and a network of drains and pipes on top of the liner collect the liquid so it can be pumped out.
- Then it is treated, usually by sending it to a wastewater treatment plant or by treating it on site before discharge under a permit.
Where leachate comes from
Think of a landfill as a giant sponge stacked in layers. When rain falls on the working face or on soil cover, some of it runs off, some evaporates and some soaks in. The water that soaks in trickles down through paper, food scraps, yard waste and everything else, dissolving and carrying material as it goes. By the time it reaches the bottom it is no longer clean water. It is leachate.
Rain is the big driver, so the amount of leachate depends heavily on climate and on how much of the site is open. A wet region makes more than a dry one, and an active area with fresh trash exposed makes more than a section that has been capped. The waste adds liquid of its own too: food scraps and yard waste hold water, and some of it is pressed out as heavy equipment compacts each layer and as the material decays.
What is in leachate
Leachate varies from site to site and changes as a landfill ages, but it usually carries:
- Dissolved organic matter from rotting food, paper and yard waste. Treatment plants measure this as oxygen demand, because microbes use up oxygen breaking it down.
- Ammonia, a common product of decomposing waste.
- Dissolved salts and minerals such as chloride.
- Metals from batteries, electronics, paint, treated wood and other discarded items.
- Trace organic chemicals from household products, solvents and plastics.
The EPA’s effluent guidelines for nonhazardous landfills that discharge leachate directly to rivers or lakes set limits for oxygen demand (BOD5), suspended solids, ammonia, zinc, several organic compounds and pH, which gives a good picture of what operators have to remove. EPA has also found PFAS, the so-called forever chemicals, in landfill leachate and has said it intends to revise those guidelines to address PFAS.
Fresh leachate is often dark and strong-smelling. If you have noticed odors around a landfill, the gas side of the story is in why landfills smell.
Liners: keeping leachate out of groundwater
The main risk from leachate is that it seeps into soil and groundwater. That is why US municipal solid waste landfills built under the federal rules in 40 CFR Part 258 sit on a liner. The standard design spelled out in the rules is a composite liner with two parts working together:
- A plastic membrane at least 30 mils thick, or at least 60 mils if it is high-density polyethylene (HDPE), the material most landfills use.
- Compacted soil at least two feet thick, usually clay, packed so tightly that water moves through it extremely slowly.
The membrane has to sit in direct, even contact with the soil, so a small hole in the plastic meets clay instead of an open gap. States with approved programs can allow other designs if the owner shows that groundwater at a monitoring point near the landfill, no farther than 150 meters from the waste, will stay within federal limits. Some states and landfills go further than the minimum, with double liners and a leak detection layer between them.
How leachate is collected
A liner alone would just turn the landfill into a bathtub. So the rules pair it with a leachate collection system designed to keep less than 30 centimeters, about a foot, of liquid sitting on the liner. A typical setup looks like this:
- A drainage layer of gravel or coarse sand spread over the liner, often with a plastic drainage net.
- Perforated pipes laid in that layer on a slope, so liquid flows toward low points.
- Sumps and pumps at the low points that lift the leachate out to a holding tank, pond or pipeline.
- Cleanouts so crews can flush the pipes when they clog with sediment or mineral buildup.
Some landfills pump part of the collected leachate back over the waste, called recirculation, to help it break down faster. Federal rules allow that only at landfill units built with a composite liner and leachate collection system.
How leachate is treated
Once it is collected, leachate goes one of two main ways:
- To a public wastewater treatment plant. Many landfills send leachate by sewer line or tanker truck to a municipal plant. The landfill has to follow the national pretreatment rules, and the city can set local limits or require some treatment on site first so the leachate does not upset the plant.
- On-site treatment and discharge. Larger sites may run their own treatment plant and release the cleaned water to a river or stream under a Clean Water Act permit. For nonhazardous landfills, EPA based its discharge limits on equalization tanks, activated sludge biological treatment and filtration, though operators can use any technology that meets the limits.
Collecting, hauling and treating leachate is one of the ongoing costs of running a landfill. For what dumps typically charge at the scale house, see landfill prices.
Leachate after a landfill closes
Leachate does not stop on closing day. The final cover, a cap of compacted earth and topsoil, is designed to keep rain from soaking in, so the flow drops over time, but buried waste can keep draining for years. Under federal post-closure rules, the owner must keep maintaining and operating the leachate collection system, generally for 30 years, unless the state agrees that the leachate no longer threatens health or the environment. Groundwater monitoring wells around the site keep checking for leaks during that time. The full closure process is in what happens when a landfill is full.
Old dumps without liners
The federal liner and collection standards for municipal landfills date from rules published in 1991. Many small town dumps closed before then and were never lined, so rain could carry leachate straight into the ground beneath them. Some of those sites are now monitored or cleaned up under state programs. If you are looking at land near an old dump, ask the state environmental agency what records it has on that site and on any nearby groundwater testing.
What it means for your dump runs
Every landfill’s leachate is a mix of what people throw away, which is why sites are strict about some items:
- Liquids. Federal rules bar most bulk liquid waste from municipal landfills, and many sites go further. Leftover paint, for example, often has to be dried out or taken to a collection program; see where to dump paint.
- Motor oil and antifreeze belong in recycling, not the trash; see where to recycle motor oil.
- Chemicals, pesticides, pool chemicals and batteries usually go to a household hazardous waste program; see where to dump household hazardous waste.
- Find the right site on the Dumpmapper map or the dump near me page, then call ahead to confirm what the site takes.
Leachate FAQ
What is leachate in a landfill?
It is the liquid that drains through or out of the waste in a landfill. It starts mostly as rain and snowmelt and picks up dissolved and suspended material from the trash on the way down.
Is landfill leachate dangerous?
It can be. Leachate can carry organic matter, ammonia, salts, metals and trace chemicals, including PFAS, so it has to be kept out of groundwater and streams and treated before it is released.
How do landfills stop leachate from reaching groundwater?
Modern municipal landfills sit on a liner, typically a plastic membrane over at least two feet of compacted clay, with drains and pipes on top that collect the liquid so it can be pumped out.
Where does landfill leachate go?
Most of it is pumped out and sent by sewer line or tanker truck to a wastewater treatment plant, or treated at the landfill and discharged under a permit. Some landfills recirculate part of it over the waste.
Does a closed landfill still make leachate?
Yes, for years. The final cap cuts down how much rain gets in, but owners generally must keep operating the leachate collection system during a 30-year post-closure period unless the state agrees it is no longer needed.
Why can't I throw liquids in the trash at the dump?
Federal rules keep most bulk liquid waste out of municipal landfills, and many sites set stricter rules. Liquids add to leachate, so items like paint, motor oil and chemicals usually go to drying, recycling or hazardous waste programs.
The bottom line
Leachate is the liquid that seeps through landfill waste, mostly rainwater that picks up organic matter, ammonia, salts, metals and trace chemicals on the way down. Modern landfills catch it with a composite liner of plastic and compacted clay, drain it through gravel and perforated pipes to sumps, and pump it out for treatment at a wastewater plant or on site, and they keep doing that for decades after the landfill closes.