Keeping water away from waste is the single most effective way to prevent leachate from forming. Leachate is the contaminated liquid produced when water filters through garbage, dissolving chemicals, metals, and decomposition byproducts along the way. Since rainfall soaking into a landfill is the primary driver of leachate creation, every serious prevention strategy targets that interaction between moisture and waste.
How Leachate Forms
The process is straightforward: water enters a mass of solid waste, and the waste contaminates it. Some materials in garbage dissolve readily in water. Others become soluble only after bacteria break them down. Still others dissolve slowly as the liquid itself reacts with them over time. All three processes turn clean rainwater into a toxic soup.
Leachate doesn’t start flowing out of a waste mass immediately, though. Compacted waste behaves like soil. It absorbs and holds a certain amount of moisture, known as its field capacity. Only after the waste is fully saturated does excess liquid begin draining downward as leachate. This means that even small reductions in the water reaching the waste can have an outsized effect on leachate volume, because you’re keeping the waste below that saturation threshold.
Landfill Caps and Cover Systems
The most direct way to prevent leachate is to stop rain from reaching buried waste. Modern landfills accomplish this with a multilayered final cover system installed at closure. A typical design includes a thick layer of topsoil planted with vegetation, a barrier protection layer of soil (12 to 18 inches deep), a drainage layer, and a composite barrier made of a synthetic membrane bonded to an underlying clay layer. The membrane is as thin as 0.04 to 0.06 inches but is highly effective at blocking water.
Federal regulations require the final cover to be less permeable than the liner system underneath the landfill. This rule exists to prevent what engineers call the “bathtub effect,” where water seeps in through the top but can’t escape through the bottom, filling the landfill like a basin and eventually forcing contaminated liquid through the liner and into groundwater.
The vegetated topsoil layer does more than prevent erosion. Plants pull moisture from the soil and release it into the atmosphere through their leaves, a process called evapotranspiration. This natural water removal means less moisture ever reaches the barrier layers below.
Evapotranspiration Covers
Some landfills, particularly in dry climates, use an alternative approach that relies almost entirely on soil and vegetation to intercept rainfall before it penetrates the waste. These evapotranspiration (ET) covers use a thick layer of fine-grained soil planted with deep-rooted vegetation. The soil stores rainwater, and the plants extract it.
Performance data from several sites shows these covers can outperform conventional designs. At one California landfill, nearly three years of monitoring showed that most infiltrating water was removed by plant activity before reaching the bottom of the cover. Modeling projected that the ET cover would allow roughly 50 centimeters of water through over a decade, compared to 95 centimeters for a conventional cover. At a test site in Montana, a capillary barrier ET cover allowed just 0.5 millimeters of percolation over 32 months despite receiving 837 millimeters of rain.
The limitation is climate. ET covers work best in arid and semi-arid regions, primarily the western United States. In humid areas or during spring snowmelt when plants are dormant, the soil may not have enough storage capacity to hold all the incoming water.
Surface Water Drainage
Even the best cover system can be overwhelmed if stormwater from surrounding land flows onto the waste. Landfills use extensive drainage networks to intercept and redirect this runoff before it makes contact with refuse. These systems typically include:
- Diversion berms that redirect storm runoff around the landfill perimeter
- Drainage channels and V-ditches near active fill areas
- Overside drains that carry water from elevated fill areas down to drainage courses
- Sedimentation basins that capture and settle runoff before discharge
During active operations, each layer of refuse is sloped to promote drainage toward interim control facilities. Berms and ditches are positioned to prevent water from ponding against exposed waste. These systems are typically designed to handle runoff from a 25-year storm event, meaning they can manage the kind of heavy rainfall that occurs on average once every 25 years.
Minimizing the Active Working Face
While a landfill is still receiving waste, one area is always exposed to the elements: the active working face, where trucks dump and compactors spread fresh refuse. Rain falling on this exposed surface becomes leachate. Any runoff from the active face cannot be discharged to the environment and must be treated as leachate.
Keeping this exposed area as small as possible directly reduces the volume of rain that contacts waste. Operators achieve this by working in compact cells rather than spreading waste across a wide area. They also apply daily cover, usually six inches of soil or an approved alternative material, over each day’s waste to limit water infiltration until the next layer is placed. The smaller the working face relative to the total landfill footprint, the less leachate the site generates during its operational life.
Diverting Organic Waste
Food scraps and yard waste are the wettest, most biologically active materials in a typical landfill. As they decompose, they release water and generate soluble byproducts that make leachate more toxic. Diverting this organic fraction before it ever reaches the landfill attacks leachate formation from two angles: it reduces both the moisture entering the waste mass and the concentration of harmful compounds in whatever leachate does form.
Research comparing landfill cells with different percentages of food waste found that cells with less food waste produced leachate with significantly lower levels of ammonia and oxygen-depleting organic compounds. Ammonia concentrations dropped by roughly two orders of magnitude over the study period, and cells with 25% or 0% food waste started with lower contamination levels than those with higher food waste content. Composting or anaerobically digesting organic waste before landfilling, or routing it to separate facilities entirely, reduces the raw material that drives leachate contamination.
Waste Pre-Treatment
Mechanical biological treatment processes stabilize waste before it enters a landfill. These systems shred, screen, and biologically process mixed waste to break down the organic fraction and drive off moisture under controlled conditions. The goal is to produce a drier, more stable material that generates less leachate and fewer harmful byproducts once buried. Leachate from these treatment facilities is itself nearly 97 to 99% water with very low concentrations of nutrients, suggesting that much of the soluble contamination potential is removed during pre-treatment rather than left to leach slowly in a landfill over decades.
By reducing both the moisture content and the biological reactivity of waste before disposal, pre-treatment lowers the total volume of leachate a landfill produces and makes the leachate that does form less hazardous.