Soil around landfills gets polluted through several interconnected pathways: contaminated water seeping out of decomposing waste, gases migrating through the ground, and persistent chemicals slowly spreading outward over years and decades. Even well-engineered modern landfills can affect surrounding soil, though the most severe contamination comes from older, unlined sites and illegal dumps.
Leachate: The Primary Source of Contamination
When rain filters through layers of buried waste, it picks up a cocktail of dissolved chemicals. This liquid, called leachate, is the single biggest driver of soil pollution around landfills. As water moves through decomposing household products, electronics, batteries, cleaning supplies, and food waste, it dissolves heavy metals, organic chemicals, and salts. That toxic soup then migrates outward and downward into surrounding soil.
The concentrations can be significant. Soil near landfills has been found to contain lead at levels ranging from about 1 to 734 mg/kg, cadmium from 0.03 to 19.9 mg/kg, and mercury up to 1.54 mg/kg. For context, the EPA’s current residential screening level for lead in soil is 200 mg/kg in areas where children live and play, dropping to 100 mg/kg in communities already exposed to other lead sources like old paint or contaminated air. Some landfill-adjacent soils exceed those thresholds by a wide margin.
These metals don’t break down. Once lead or cadmium settles into soil, it stays there for centuries, binding to soil particles and slowly accumulating over time. The farther from the landfill boundary, the lower the concentrations tend to be, but the plume of contamination can extend hundreds of meters depending on soil type, rainfall, and the age of the site.
Landfill Gas Changes Soil Chemistry
Decomposing waste produces large volumes of methane and carbon dioxide. These gases don’t just rise straight up. They migrate laterally through the soil, sometimes traveling significant distances from the landfill edge. As they move, they fundamentally alter the chemistry of the soil they pass through.
Methane displaces oxygen in soil pores. Specialized bacteria in the soil consume methane, but this process uses up even more oxygen and generates additional carbon dioxide. The result is soil that becomes oxygen-starved in the root zone, where plants need it most. When carbon dioxide exceeds about 20% of the volume in soil pore spaces, vegetation starts to suffer. Roots can’t function properly, beneficial soil organisms die off, and the soil’s ability to support plant life deteriorates.
This gas migration also carries volatile organic compounds, the chemical vapors released by decomposing waste. Aromatic compounds like benzene and toluene, along with alkanes and halogenated chemicals, typically make up more than 70% of the volatile organics found in landfill cover soil. Some of these compounds are produced directly by the waste. Others form secondarily through chemical reactions within the soil itself as gases interact with soil minerals and microbes.
PFAS: The Long-Lasting Contaminant
A newer concern is PFAS contamination, sometimes called “forever chemicals” because they resist virtually all natural breakdown. These synthetic compounds, used in nonstick coatings, waterproof fabrics, and food packaging, end up in landfills in enormous quantities. As leachate carries them into surrounding soil, they persist indefinitely.
Background PFAS levels in soil (areas with no known contamination source) typically range from nondetect to about 5 micrograms per kilogram for the most common types. Near landfills and other contamination sources, individual PFAS compounds can reach concentrations exceeding 1,000 micrograms per kilogram. The contamination spreads through both leachate seeping into soil and through airborne deposition, as PFAS-laden particles settle on the ground surface around the site.
Why Liners Don’t Solve the Problem Entirely
Modern municipal landfills are required to have engineered liner systems, typically made from high-density polyethylene plastic combined with compacted clay. These barriers are designed to contain leachate and prevent it from reaching surrounding soil and groundwater. Under ideal conditions, these liners can last an estimated 400 years or more before reaching their half-life, the point at which 50% of the material has degraded.
But “ideal conditions” rarely describe a working landfill. Liners can be punctured during construction, stressed by the weight of waste above them, or degraded faster by heat generated from decomposing organic matter. Even intact liners allow some permeation at extremely low rates. And the waste inside the landfill will remain hazardous far longer than any liner will last. This means that every lined landfill is, on a long enough timeline, an unlined landfill.
Older landfills, those built before modern regulations took effect, often have no liner at all. Illegal dump sites, which are common worldwide, have zero containment. These are the sites where the most severe soil contamination occurs, with heavy metals and organic pollutants spreading freely into the surrounding landscape.
How Contaminated Soil Affects What Grows in It
Polluted soil doesn’t just sit there inertly. Plants growing in and around contaminated landfill sites absorb heavy metals through their roots. Research on crops grown in dumpsite-affected soil found that maize absorbed about 57% of the chromium present in the soil, 63% of the manganese, and 60% of the iron. Spinach showed similar uptake rates. Even metals that plants absorb less efficiently, like lead (about 13 to 20% uptake) and cadmium (around 7 to 8%), still accumulate in edible plant tissue at measurable levels.
This creates a direct pathway from landfill waste to the food chain. In areas where people garden or farm near old or unregulated dump sites, the crops themselves become carriers of contamination. Chromium concentrations in soil near one studied dumpsite exceeded 110 mg/kg, above the EU’s recommended limit of 100 mg/kg for agricultural soil, and the plants growing there reflected that excess.
Multiple Pathways Working Together
What makes landfill soil pollution so persistent is that these mechanisms reinforce each other. Leachate saturates the soil with metals and organic chemicals. Gas migration strips oxygen from the ground and introduces volatile compounds. PFAS and other synthetic chemicals resist any natural cleanup process. Meanwhile, the altered soil chemistry reduces populations of beneficial microorganisms that would otherwise help break down some organic contaminants.
The contamination also moves in multiple directions. Leachate travels downward with gravity and laterally with groundwater flow. Gases migrate upward and sideways through any available pore space. Volatile compounds can evaporate from soil surfaces and redeposit elsewhere. A single landfill creates an expanding zone of influence that can affect soil quality well beyond its visible footprint, with the severity depending on the site’s age, the types of waste buried there, local geology, and whether any containment system exists.